Guiding AP mounts with PHD


Roland Christen
 

Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%.  This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.

Rolando


Dan
 

Very interesting. I always hear slow the exposure in bad seeing. Or else your “chasing seeing.” Constantly bouncing around. 
But this makes sense, find the threshold of your seeing and guide based on that. 
I can’t wait to try this. 

Dan Pelzel


On Jan 31, 2018, at 7:51 PM, chris1011@... [ap-gto] <ap-gto@...> wrote:

 

Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%.  This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.

Rolando


Roland Christen
 

The threshold method is another way to avoid chasing the seeing. Let it bobble around in its envelope, but once outside the envelope, then make a correction quickly before it has a chance to go too far.

Rolando



-----Original Message-----
From: Dan Pelzel dandadrumman@... [ap-gto]
To: ap-gto
Sent: Wed, Jan 31, 2018 8:03 pm
Subject: Re: [ap-gto] Guiding AP mounts with PHD



Very interesting. I always hear slow the exposure in bad seeing. Or else your “chasing seeing.” Constantly bouncing around. 
But this makes sense, find the threshold of your seeing and guide based on that. 
I can’t wait to try this. 

Dan Pelzel


On Jan 31, 2018, at 7:51 PM, chris1011@... [ap-gto] <ap-gto@...> wrote:

 
Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%.  This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.

Rolando



Bill Long
 

Great info. A few questions:


- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would have been 0.8 pixels. Did you guys just do the math to figure out the correct pixel value? 

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can be configured per axis. 


The rest of this looks very interesting and as soon as the weather gets better I plan to test this and see how my guiding performs. 




From: ap-gto@... on behalf of chris1011@... [ap-gto]
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD
 
 

Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%.  This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.

Rolando


Worsel
 

PHD2 can display data in either " or pixels.  

You could display the data in pixels, determine the bobble in pixels, and set MinMo in pixels.

Normal guiding is better displayed in ", but that is easy to change.

I think Roland's technique is Declination only.

Bryan


---In ap-gto@..., <bill@...> wrote :

Great info. A few questions:


- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would have been 0.8 pixels. Did you guys just do the math to figure out the correct pixel value? 

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can be configured per axis. 


The rest of this looks very interesting and as soon as the weather gets better I plan to test this and see how my guiding performs. 




W Hilmo
 

I am curious about this result, especially the part at the end where you say
that 1 second exposures worked well, but longer exposures "became more and
more sluggish and less accurate".



It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
flexure, etc. If my understanding is correct, it would seem that longer
exposures (within reason) would be just as effective as shorter exposures -
perhaps more effective, since longer exposures would tend to average out
seeing distortion in the individual guider subs. If you were getting better
results at 1 second, that would suggest that you were chasing seeing with
some success.



Do you understand why it worked the way that it did for you? Was it
correcting for slower seeing effects? Is there some other reason?



Thanks,

-Wade



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 9:36 AM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD





Great info. A few questions:



- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the
PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
have been 0.8 pixels. Did you guys just do the math to figure out the
correct pixel value?

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can
be configured per axis.



The rest of this looks very interesting and as soon as the weather gets
better I plan to test this and see how my guiding performs.



_____

From: ap-gto@... <ap-gto@...> on behalf of
chris1011@... [ap-gto] <ap-gto@...>
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD





Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night
to help him with some guiding issues using PHD. He had sent his calibration
graph and it looked quite good, but he was having some problems guiding.
After doing some mechanical tests we set up parameters in PHD to get the
mount to track and respond accurately. Since PHD is somewhat different from
MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second
guide exposures, and look at the guider graph to see what the maximum
excursions are in Declination. The guide star will bounce around a certain
amount and this peak error will be the seeing that you cannot guide out with
normal guide software. This P-V value, which in our case was between +-0.5
and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
to correct for that with guide moves to the mount is impossible (only a fast
acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
want to set the initial Min Move setting in PHD to be approximately this
value as a starting value. We set the Min Move to 0.8 arc sec, the guide
rate at 1x and the aggressiveness to 100%. This means that no correction
pulses are sent to the mount while the guide star is bobbling within that
envelope, but once it exceeds even slightly, the mount gets a full
correction command (-0.8 arc sec) to bring it back toward the zero position.
We turned guiding on and the result was that the mount responded quickly
whenever the error exceeded the bobble limits and overall guiding was tight,
accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to
a lower value. We tried longer guide star exposures, but found that the
response to tracking errors became more and more sluggish and less accurate.
So for these seeing conditions a faster guide rate of 1 per second resulted
in the lowest RMS error on both axes. In pristine seeing it might allow
longer guide exposures, at least that is what we found using PHD Guiding.

Rolando


Ray Gralak
 

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

-----Original Message-----
From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 10:39 AM
To: ap-gto@...
Subject: RE: [ap-gto] Guiding AP mounts with PHD



I am curious about this result, especially the part at the end where you
say
that 1 second exposures worked well, but longer exposures "became more and
more sluggish and less accurate".

It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
flexure, etc. If my understanding is correct, it would seem that longer
exposures (within reason) would be just as effective as shorter exposures
-
perhaps more effective, since longer exposures would tend to average out
seeing distortion in the individual guider subs. If you were getting
better
results at 1 second, that would suggest that you were chasing seeing with
some success.

Do you understand why it worked the way that it did for you? Was it
correcting for slower seeing effects? Is there some other reason?

Thanks,

-Wade

From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 9:36 AM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD

Great info. A few questions:

- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
have been 0.8 pixels. Did you guys just do the math to figure out the
correct pixel value?

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness
can
be configured per axis.

The rest of this looks very interesting and as soon as the weather gets
better I plan to test this and see how my guiding performs.

_____

From: ap-gto@... <ap-gto@...> on behalf of
chris1011@... [ap-gto] <ap-gto@...>
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD

Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night
to help him with some guiding issues using PHD. He had sent his
calibration
graph and it looked quite good, but he was having some problems guiding.
After doing some mechanical tests we set up parameters in PHD to get the
mount to track and respond accurately. Since PHD is somewhat different
from
MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second
guide exposures, and look at the guider graph to see what the maximum
excursions are in Declination. The guide star will bounce around a certain
amount and this peak error will be the seeing that you cannot guide out
with
normal guide software. This P-V value, which in our case was between +-0.5
and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
to correct for that with guide moves to the mount is impossible (only a
fast
acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
want to set the initial Min Move setting in PHD to be approximately this
value as a starting value. We set the Min Move to 0.8 arc sec, the guide
rate at 1x and the aggressiveness to 100%. This means that no correction
pulses are sent to the mount while the guide star is bobbling within that
envelope, but once it exceeds even slightly, the mount gets a full
correction command (-0.8 arc sec) to bring it back toward the zero
position.
We turned guiding on and the result was that the mount responded quickly
whenever the error exceeded the bobble limits and overall guiding was
tight,
accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS
to
a lower value. We tried longer guide star exposures, but found that the
response to tracking errors became more and more sluggish and less
accurate.
So for these seeing conditions a faster guide rate of 1 per second
resulted
in the lowest RMS error on both axes. In pristine seeing it might allow
longer guide exposures, at least that is what we found using PHD Guiding.

Rolando






Roland Christen
 


- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would have been 0.8 pixels. Did you guys just do the math to figure out the correct pixel value? 
- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can be configured per axis. 
If the graph shows arc seconds then you have to change the value to pixels, which we did. Since both RA and Dec experience the same atmospheric seeing, then it makes sense to do the same Min-Mo for both. Remember, these are starting values, and you can experiment by changing any variable up or down and watch the resultant RMS value.

Rolando


-----Original Message-----
From: Bill Long bill@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 11:46 am
Subject: Re: [ap-gto] Guiding AP mounts with PHD



Great info. A few questions:

- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would have been 0.8 pixels. Did you guys just do the math to figure out the correct pixel value? 
- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can be configured per axis. 

The rest of this looks very interesting and as soon as the weather gets better I plan to test this and see how my guiding performs. 



From: ap-gto@... gto@...> on behalf of chris1011@... [ap-gto] gto@...>
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD
 
 
Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night to help him with some guiding issues using PHD. He had sent his calibration graph and it looked quite good, but he was having some problems guiding. After doing some mechanical tests we set up parameters in PHD to get the mount to track and respond accurately. Since PHD is somewhat different from MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second guide exposures, and look at the guider graph to see what the maximum excursions are in Declination. The guide star will bounce around a certain amount and this peak error will be the seeing that you cannot guide out with normal guide software. This P-V value, which in our case was between +-0.5 and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying to correct for that with guide moves to the mount is impossible (only a fast acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will want to set the initial Min Move setting in PHD to be approximately this value as a starting value. We set the Min Move to 0.8 arc sec, the guide rate at 1x and the aggressiveness to 100%.  This means that no correction pulses are sent to the mount while the guide star is bobbling within that envelope, but once it exceeds even slightly, the mount gets a full correction command (-0.8 arc sec) to bring it back toward the zero position. We turned guiding on and the result was that the mount responded quickly whenever the error exceeded the bobble limits and overall guiding was tight, accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to a lower value. We tried longer guide star exposures, but found that the response to tracking errors became more and more sluggish and less accurate. So for these seeing conditions a faster guide rate of 1 per second resulted in the lowest RMS error on both axes. In pristine seeing it might allow longer guide exposures, at least that is what we found using PHD Guiding.

Rolando



Roland Christen
 


I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.
In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando


-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 10:39 AM
> To: ap-gto@...
> Subject: RE: [ap-gto] Guiding AP mounts with PHD
>
>
>
> I am curious about this result, especially the part at the end where you
say
> that 1 second exposures worked well, but longer exposures "became more and
> more sluggish and less accurate".
>
> It's my understanding that the mount is going to track well, assuming that
> PEM is programmed correctly and enabled, and that guiding should basically
> be correcting any drift due to slight polar misalignment, refraction,
> flexure, etc. If my understanding is correct, it would seem that longer
> exposures (within reason) would be just as effective as shorter exposures
-
> perhaps more effective, since longer exposures would tend to average out
> seeing distortion in the individual guider subs. If you were getting
better
> results at 1 second, that would suggest that you were chasing seeing with
> some success.
>
> Do you understand why it worked the way that it did for you? Was it
> correcting for slower seeing effects? Is there some other reason?
>
> Thanks,
>
> -Wade
>
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 9:36 AM
> To: ap-gto@...
> Subject: Re: [ap-gto] Guiding AP mounts with PHD
>
> Great info. A few questions:
>
> - You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
> PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
> have been 0.8 pixels. Did you guys just do the math to figure out the
> correct pixel value?
>
> - Were these changes made on both axis in PHD2? MinMo and Aggressiveness
> can
> be configured per axis.
>
> The rest of this looks very interesting and as soon as the weather gets
> better I plan to test this and see how my guiding performs.
>
> _____
>
> From: ap-gto@... gto@...> on behalf of
> chris1011@... [ap-gto] gto@...>
> Sent: Wednesday, January 31, 2018 4:51 PM
> To: ap-gto@...; ap-ug@...
> Subject: [ap-gto] Guiding AP mounts with PHD
>
> Howdy,
>
> Howard and I had a chance to log in to a customer's Mach1 mount last night
> to help him with some guiding issues using PHD. He had sent his
calibration
> graph and it looked quite good, but he was having some problems guiding.
> After doing some mechanical tests we set up parameters in PHD to get the
> mount to track and respond accurately. Since PHD is somewhat different
from
> MaximDL, which I use, we had to approach the settings a bit differently.
>
> The first step is to do a quick 2 - 3 minute Unguided run using 1 second
> guide exposures, and look at the guider graph to see what the maximum
> excursions are in Declination. The guide star will bounce around a certain
> amount and this peak error will be the seeing that you cannot guide out
with
> normal guide software. This P-V value, which in our case was between +-0.5
> and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
> to correct for that with guide moves to the mount is impossible (only a
fast
> acting AO system can chase that seeing error).
>
> So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
> want to set the initial Min Move setting in PHD to be approximately this
> value as a starting value. We set the Min Move to 0.8 arc sec, the guide
> rate at 1x and the aggressiveness to 100%. This means that no correction
> pulses are sent to the mount while the guide star is bobbling within that
> envelope, but once it exceeds even slightly, the mount gets a full
> correction command (-0.8 arc sec) to bring it back toward the zero
position.
> We turned guiding on and the result was that the mount responded quickly
> whenever the error exceeded the bobble limits and overall guiding was
tight,
> accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
> exposure with the main camera showed tight round stars.
>
> So, as a starting point:
> Determine the amount of guide star bobble above and below the axis
> Set the Min Move to that value
> Set Guide rate to 1x sidereal
> Set the guide star exposure rate to 1 second
> Set Aggressiveness to 100%
> Begin guiding and note the RMS value for both axes.
>
> You can then change any of the parameters to see if you can tune the RMS
to
> a lower value. We tried longer guide star exposures, but found that the
> response to tracking errors became more and more sluggish and less
accurate.
> So for these seeing conditions a faster guide rate of 1 per second
resulted
> in the lowest RMS error on both axes. In pristine seeing it might allow
> longer guide exposures, at least that is what we found using PHD Guiding.
>
> Rolando
>
>
>
>
>
>



------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@...>
------------------------------------

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W Hilmo
 

If you are taking 10 second guide exposures, doesn’t that mean that the star will have bounced around randomly for the whole 10 seconds? As I understand it, the conventional wisdom is that longer guide exposures will have larger stars due to the atmospheric effects, but that the correct star position is likely to be near the center of the blob. Then the centroid calculation by the guide software shouldn’t be seeing huge swings in the centroid position.



I suppose that this gets down to the effectiveness of the centroid calculation. If it’s just looking for the brightest single pixel, then I can see where longer guide exposures might not be helpful. But I thought that the calculations were more sophisticated than that, and would look at the stellar profile to determine the centroid. Perhaps not, I guess? And perhaps it varies with the guide software.



-Wade



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 1:47 PM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD







I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando





-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

-----Original Message-----
From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
Sent: Thursday, February 1, 2018 10:39 AM
To: ap-gto@... <mailto:gto@...>
Subject: RE: [ap-gto] Guiding AP mounts with PHD



I am curious about this result, especially the part at the end where you
say
that 1 second exposures worked well, but longer exposures "became more and
more sluggish and less accurate".

It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
flexure, etc. If my understanding is correct, it would seem that longer
exposures (within reason) would be just as effective as shorter exposures
-
perhaps more effective, since longer exposures would tend to average out
seeing distortion in the individual guider subs. If you were getting
better
results at 1 second, that would suggest that you were chasing seeing with
some success.

Do you understand why it worked the way that it did for you? Was it
correcting for slower seeing effects? Is there some other reason?

Thanks,

-Wade

From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
Sent: Thursday, February 1, 2018 9:36 AM
To: ap-gto@... <mailto:gto@...>
Subject: Re: [ap-gto] Guiding AP mounts with PHD

Great info. A few questions:

- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
have been 0.8 pixels. Did you guys just do the math to figure out the
correct pixel value?

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness
can
be configured per axis.

The rest of this looks very interesting and as soon as the weather gets
better I plan to test this and see how my guiding performs.

_____

From: ap-gto@... <mailto:gto@...> <ap-gto@... <mailto:gto@...> > on behalf of
chris1011@... <mailto:chris1011@...> [ap-gto] <ap-gto@... <mailto:gto@...> >
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@... <mailto:gto@...> ; ap-ug@... <mailto:ug@...>
Subject: [ap-gto] Guiding AP mounts with PHD

Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night
to help him with some guiding issues using PHD. He had sent his
calibration
graph and it looked quite good, but he was having some problems guiding.
After doing some mechanical tests we set up parameters in PHD to get the
mount to track and respond accurately. Since PHD is somewhat different
from
MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second
guide exposures, and look at the guider graph to see what the maximum
excursions are in Declination. The guide star will bounce around a certain
amount and this peak error will be the seeing that you cannot guide out
with
normal guide software. This P-V value, which in our case was between +-0.5
and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
to correct for that with guide moves to the mount is impossible (only a
fast
acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
want to set the initial Min Move setting in PHD to be approximately this
value as a starting value. We set the Min Move to 0.8 arc sec, the guide
rate at 1x and the aggressiveness to 100%. This means that no correction
pulses are sent to the mount while the guide star is bobbling within that
envelope, but once it exceeds even slightly, the mount gets a full
correction command (-0.8 arc sec) to bring it back toward the zero
position.
We turned guiding on and the result was that the mount responded quickly
whenever the error exceeded the bobble limits and overall guiding was
tight,
accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS
to
a lower value. We tried longer guide star exposures, but found that the
response to tracking errors became more and more sluggish and less
accurate.
So for these seeing conditions a faster guide rate of 1 per second
resulted
in the lowest RMS error on both axes. In pristine seeing it might allow
longer guide exposures, at least that is what we found using PHD Guiding.

Rolando

[Non-text portions of this message have been removed]





------------------------------------
Posted by: "Ray Gralak &#92;(Groups&#92;)" <groups3@... <mailto:groups3@...> >
------------------------------------

To UNSUBSCRIBE, or for general information on the ap-gto list
see http://groups.yahoo.com/group/ap-gto
------------------------------------

Yahoo Groups Links


Matthew Hughes
 

Hi Rolando,

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

Very interesting.
Would you recommend a different technique then with a 1100 AE mount with no dec backlash? Very little in the way of jet streams here downunder and seeing and transparency has been quite good. Looking forward to experimenting whilst this moon is out. For starters I’ll try the one second exposures and pixel setting etc.

Regards,

Matt

On 2 Feb 2018, at 08:49, chris1011@... [ap-gto] <ap-gto@...> wrote:

 


I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.
In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando


-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 10:39 AM
> To: ap-gto@...
> Subject: RE: [ap-gto] Guiding AP mounts with PHD
>
>
>
> I am curious about this result, especially the part at the end where you
say
> that 1 second exposures worked well, but longer exposures "became more and
> more sluggish and less accurate".
>
> It's my understanding that the mount is going to track well, assuming that
> PEM is programmed correctly and enabled, and that guiding should basically
> be correcting any drift due to slight polar misalignment, refraction,
> flexure, etc. If my understanding is correct, it would seem that longer
> exposures (within reason) would be just as effective as shorter exposures
-
> perhaps more effective, since longer exposures would tend to average out
> seeing distortion in the individual guider subs. If you were getting
better
> results at 1 second, that would suggest that you were chasing seeing with
> some success.
>
> Do you understand why it worked the way that it did for you? Was it
> correcting for slower seeing effects? Is there some other reason?
>
> Thanks,
>
> -Wade
>
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 9:36 AM
> To: ap-gto@...
> Subject: Re: [ap-gto] Guiding AP mounts with PHD
>
> Great info. A few questions:
>
> - You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
> PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
> have been 0.8 pixels. Did you guys just do the math to figure out the
> correct pixel value?
>
> - Were these changes made on both axis in PHD2? MinMo and Aggressiveness
> can
> be configured per axis.
>
> The rest of this looks very interesting and as soon as the weather gets
> better I plan to test this and see how my guiding performs.
>
> _____
>
> From: ap-gto@... <ap-gto@...> on behalf of
> chris1011@... [ap-gto] <ap-gto@...>
> Sent: Wednesday, January 31, 2018 4:51 PM
> To: ap-gto@...; ap-ug@...
> Subject: [ap-gto] Guiding AP mounts with PHD
>
> Howdy,
>
> Howard and I had a chance to log in to a customer's Mach1 mount last night
> to help him with some guiding issues using PHD. He had sent his
calibration
> graph and it looked quite good, but he was having some problems guiding.
> After doing some mechanical tests we set up parameters in PHD to get the
> mount to track and respond accurately. Since PHD is somewhat different
from
> MaximDL, which I use, we had to approach the settings a bit differently.
>
> The first step is to do a quick 2 - 3 minute Unguided run using 1 second
> guide exposures, and look at the guider graph to see what the maximum
> excursions are in Declination. The guide star will bounce around a certain
> amount and this peak error will be the seeing that you cannot guide out
with
> normal guide software. This P-V value, which in our case was between +-0.5
> and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
> to correct for that with guide moves to the mount is impossible (only a
fast
> acting AO system can chase that seeing error).
>
> So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
> want to set the initial Min Move setting in PHD to be approximately this
> value as a starting value. We set the Min Move to 0.8 arc sec, the guide
> rate at 1x and the aggressiveness to 100%. This means that no correction
> pulses are sent to the mount while the guide star is bobbling within that
> envelope, but once it exceeds even slightly, the mount gets a full
> correction command (-0.8 arc sec) to bring it back toward the zero
position.
> We turned guiding on and the result was that the mount responded quickly
> whenever the error exceeded the bobble limits and overall guiding was
tight,
> accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
> exposure with the main camera showed tight round stars.
>
> So, as a starting point:
> Determine the amount of guide star bobble above and below the axis
> Set the Min Move to that value
> Set Guide rate to 1x sidereal
> Set the guide star exposure rate to 1 second
> Set Aggressiveness to 100%
> Begin guiding and note the RMS value for both axes.
>
> You can then change any of the parameters to see if you can tune the RMS
to
> a lower value. We tried longer guide star exposures, but found that the
> response to tracking errors became more and more sluggish and less
accurate.
> So for these seeing conditions a faster guide rate of 1 per second
resulted
> in the lowest RMS error on both axes. In pristine seeing it might allow
> longer guide exposures, at least that is what we found using PHD Guiding.
>
> Rolando
>
> [Non-text portions of this message have been removed]
>
>
>
>



------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@...>
------------------------------------

To UNSUBSCRIBE, or for general information on the ap-gto list
see http://groups.yahoo.com/group/ap-gto
------------------------------------

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Roland Christen
 


Would you recommend a different technique then with a 1100 AE mount with no dec backlash?
I have successfully run my 1600 AE mount at 20 second guide exposures and smaller MinMove, but that's with MaximDL and in very good seeing. I think that the bobble-limit envelope will work with the encoder mounts also. Cannot chase seeing because the guide pulses are always after the fact.

Rolando


-----Original Message-----
From: Matthew Hughes matthughes77@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 5:21 pm
Subject: Re: [ap-gto] Guiding AP mounts with PHD



Hi Rolando,

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

Very interesting.
Would you recommend a different technique then with a 1100 AE mount with no dec backlash? Very little in the way of jet streams here downunder and seeing and transparency has been quite good. Looking forward to experimenting whilst this moon is out. For starters I’ll try the one second exposures and pixel setting etc.

Regards,

Matt

On 2 Feb 2018, at 08:49, chris1011@... [ap-gto] <ap-gto@...> wrote:

 

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.
In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando


-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 10:39 AM
> To: ap-gto@...
> Subject: RE: [ap-gto] Guiding AP mounts with PHD
>
>
>
> I am curious about this result, especially the part at the end where you
say
> that 1 second exposures worked well, but longer exposures "became more and
> more sluggish and less accurate".
>
> It's my understanding that the mount is going to track well, assuming that
> PEM is programmed correctly and enabled, and that guiding should basically
> be correcting any drift due to slight polar misalignment, refraction,
> flexure, etc. If my understanding is correct, it would seem that longer
> exposures (within reason) would be just as effective as shorter exposures
-
> perhaps more effective, since longer exposures would tend to average out
> seeing distortion in the individual guider subs. If you were getting
better
> results at 1 second, that would suggest that you were chasing seeing with
> some success.
>
> Do you understand why it worked the way that it did for you? Was it
> correcting for slower seeing effects? Is there some other reason?
>
> Thanks,
>
> -Wade
>
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Thursday, February 1, 2018 9:36 AM
> To: ap-gto@...
> Subject: Re: [ap-gto] Guiding AP mounts with PHD
>
> Great info. A few questions:
>
> - You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
> PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
> have been 0.8 pixels. Did you guys just do the math to figure out the
> correct pixel value?
>
> - Were these changes made on both axis in PHD2? MinMo and Aggressiveness
> can
> be configured per axis.
>
> The rest of this looks very interesting and as soon as the weather gets
> better I plan to test this and see how my guiding performs.
>
> _____
>
> From: ap-gto@... gto@...> on behalf of
> chris1011@... [ap-gto] gto@...>
> Sent: Wednesday, January 31, 2018 4:51 PM
> To: ap-gto@...; ap-ug@...
> Subject: [ap-gto] Guiding AP mounts with PHD
>
> Howdy,
>
> Howard and I had a chance to log in to a customer's Mach1 mount last night
> to help him with some guiding issues using PHD. He had sent his
calibration
> graph and it looked quite good, but he was having some problems guiding.
> After doing some mechanical tests we set up parameters in PHD to get the
> mount to track and respond accurately. Since PHD is somewhat different
from
> MaximDL, which I use, we had to approach the settings a bit differently.
>
> The first step is to do a quick 2 - 3 minute Unguided run using 1 second
> guide exposures, and look at the guider graph to see what the maximum
> excursions are in Declination. The guide star will bounce around a certain
> amount and this peak error will be the seeing that you cannot guide out
with
> normal guide software. This P-V value, which in our case was between +-0.5
> and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
> to correct for that with guide moves to the mount is impossible (only a
fast
> acting AO system can chase that seeing error).
>
> So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
> want to set the initial Min Move setting in PHD to be approximately this
> value as a starting value. We set the Min Move to 0.8 arc sec, the guide
> rate at 1x and the aggressiveness to 100%. This means that no correction
> pulses are sent to the mount while the guide star is bobbling within that
> envelope, but once it exceeds even slightly, the mount gets a full
> correction command (-0.8 arc sec) to bring it back toward the zero
position.
> We turned guiding on and the result was that the mount responded quickly
> whenever the error exceeded the bobble limits and overall guiding was
tight,
> accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
> exposure with the main camera showed tight round stars.
>
> So, as a starting point:
> Determine the amount of guide star bobble above and below the axis
> Set the Min Move to that value
> Set Guide rate to 1x sidereal
> Set the guide star exposure rate to 1 second
> Set Aggressiveness to 100%
> Begin guiding and note the RMS value for both axes.
>
> You can then change any of the parameters to see if you can tune the RMS
to
> a lower value. We tried longer guide star exposures, but found that the
> response to tracking errors became more and more sluggish and less
accurate.
> So for these seeing conditions a faster guide rate of 1 per second
resulted
> in the lowest RMS error on both axes. In pristine seeing it might allow
> longer guide exposures, at least that is what we found using PHD Guiding.
>
> Rolando
>
> [Non-text portions of this message have been removed]
>
>
>
>



------------------------------------
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Joe Zeglinski
 

    One other thing to beware of is that some CCD cameras associated guider chips,  as in the SBIG STL-11000, are turned off and “go blind” for the entire duration of the main image download time. Even the (somewhat independent), external RGH camera takes a rest, because the main camera is too busy downloading the image at that moment. Part of the design, unfortunately – should be live, all the time. Don’t know if it is still that way in newer models, but I suspect it is.
So, there will be a significant guider bump, due to guide star drift or bobble,  compared to real time guiding performance.
 
    In such case, it is best to introduce a delay between frames, somewhat longer than the main camera download time, to let the mount catch up with the guider,  to settle down after download completion.
 
Joe


Roland Christen
 


It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
The problem is not in RA where the axis does not have to reverse direction.

Rolando


-----Original Message-----
From: 'Wade Hilmo' y.groups@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 5:44 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

I am curious about this result, especially the part at the end where you say
that 1 second exposures worked well, but longer exposures "became more and
more sluggish and less accurate".



It's my understanding that the mount is going to track well, assuming that
PEM is programmed correctly and enabled, and that guiding should basically
be correcting any drift due to slight polar misalignment, refraction,
flexure, etc. If my understanding is correct, it would seem that longer
exposures (within reason) would be just as effective as shorter exposures -
perhaps more effective, since longer exposures would tend to average out
seeing distortion in the individual guider subs. If you were getting better
results at 1 second, that would suggest that you were chasing seeing with
some success.



Do you understand why it worked the way that it did for you? Was it
correcting for slower seeing effects? Is there some other reason?



Thanks,

-Wade



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 9:36 AM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD





Great info. A few questions:



- You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in the
PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
have been 0.8 pixels. Did you guys just do the math to figure out the
correct pixel value?

- Were these changes made on both axis in PHD2? MinMo and Aggressiveness can
be configured per axis.



The rest of this looks very interesting and as soon as the weather gets
better I plan to test this and see how my guiding performs.



_____

From: ap-gto@... gto@...> on behalf of
chris1011@... [ap-gto] gto@...>
Sent: Wednesday, January 31, 2018 4:51 PM
To: ap-gto@...; ap-ug@...
Subject: [ap-gto] Guiding AP mounts with PHD





Howdy,

Howard and I had a chance to log in to a customer's Mach1 mount last night
to help him with some guiding issues using PHD. He had sent his calibration
graph and it looked quite good, but he was having some problems guiding.
After doing some mechanical tests we set up parameters in PHD to get the
mount to track and respond accurately. Since PHD is somewhat different from
MaximDL, which I use, we had to approach the settings a bit differently.

The first step is to do a quick 2 - 3 minute Unguided run using 1 second
guide exposures, and look at the guider graph to see what the maximum
excursions are in Declination. The guide star will bounce around a certain
amount and this peak error will be the seeing that you cannot guide out with
normal guide software. This P-V value, which in our case was between +-0.5
and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
to correct for that with guide moves to the mount is impossible (only a fast
acting AO system can chase that seeing error).

So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
want to set the initial Min Move setting in PHD to be approximately this
value as a starting value. We set the Min Move to 0.8 arc sec, the guide
rate at 1x and the aggressiveness to 100%. This means that no correction
pulses are sent to the mount while the guide star is bobbling within that
envelope, but once it exceeds even slightly, the mount gets a full
correction command (-0.8 arc sec) to bring it back toward the zero position.
We turned guiding on and the result was that the mount responded quickly
whenever the error exceeded the bobble limits and overall guiding was tight,
accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
exposure with the main camera showed tight round stars.

So, as a starting point:
Determine the amount of guide star bobble above and below the axis
Set the Min Move to that value
Set Guide rate to 1x sidereal
Set the guide star exposure rate to 1 second
Set Aggressiveness to 100%
Begin guiding and note the RMS value for both axes.

You can then change any of the parameters to see if you can tune the RMS to
a lower value. We tried longer guide star exposures, but found that the
response to tracking errors became more and more sluggish and less accurate.
So for these seeing conditions a faster guide rate of 1 per second resulted
in the lowest RMS error on both axes. In pristine seeing it might allow
longer guide exposures, at least that is what we found using PHD Guiding.

Rolando









------------------------------------
Posted by: "Wade Hilmo" <wade@...>
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Roland Christen
 


Then the centroid calculation by the guide software shouldn’t be seeing huge swings in the centroid position.
First, long exposures in PHD result in saturated stars which the program apparently can't handle, or can't calculate a proper centroid.

Second, it may be stable for a period of time until there is just one single excursion outside the envelope. The software will then send a move command which may not fully reverse the axis, so the guide star stays outside the envelope for another guide cycle until finally it gets pushed back down. This may then start an oscillation in the other direction, but even if not, the correcting signals are few and far between time-wise, so the RMS error gets larger. With faster guide exposures, the mount gets faster correction signals and the guide star spends less time in the wrong place.

All I'm saying is to try this method with PHD Guiding and see if it improves your results. It is not a hard and fast rule, just a suggestion of where to start setting the parameters. You are welcome to experiment and post your results here. We all can learn.

I have PHD now and will experiment further. I may try to add a tiny bit of backlash compensation in Dec to see what effect that has and whether that will reduce the back and forth oscillation or make it worse.

Rolando


-----Original Message-----
From: 'Wade Hilmo' y.groups@... [ap-gto]
To: ap-gto
Sent: Thu, Feb 1, 2018 7:50 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

If you are taking 10 second guide exposures, doesn’t that mean that the star will have bounced around randomly for the whole 10 seconds? As I understand it, the conventional wisdom is that longer guide exposures will have larger stars due to the atmospheric effects, but that the correct star position is likely to be near the center of the blob. Then the centroid calculation by the guide software shouldn’t be seeing huge swings in the centroid position.



I suppose that this gets down to the effectiveness of the centroid calculation. If it’s just looking for the brightest single pixel, then I can see where longer guide exposures might not be helpful. But I thought that the calculations were more sophisticated than that, and would look at the stellar profile to determine the centroid. Perhaps not, I guess? And perhaps it varies with the guide software.



-Wade



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 1:47 PM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD







I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando





-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto]
To: ap-gto gto@...>
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
> Sent: Thursday, February 1, 2018 10:39 AM
> To: ap-gto@... <mailto:gto@...>
> Subject: RE: [ap-gto] Guiding AP mounts with PHD
>
>
>
> I am curious about this result, especially the part at the end where you
say
> that 1 second exposures worked well, but longer exposures "became more and
> more sluggish and less accurate".
>
> It's my understanding that the mount is going to track well, assuming that
> PEM is programmed correctly and enabled, and that guiding should basically
> be correcting any drift due to slight polar misalignment, refraction,
> flexure, etc. If my understanding is correct, it would seem that longer
> exposures (within reason) would be just as effective as shorter exposures
-
> perhaps more effective, since longer exposures would tend to average out
> seeing distortion in the individual guider subs. If you were getting
better
> results at 1 second, that would suggest that you were chasing seeing with
> some success.
>
> Do you understand why it worked the way that it did for you? Was it
> correcting for slower seeing effects? Is there some other reason?
>
> Thanks,
>
> -Wade
>
> From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
> Sent: Thursday, February 1, 2018 9:36 AM
> To: ap-gto@... <mailto:gto@...>
> Subject: Re: [ap-gto] Guiding AP mounts with PHD
>
> Great info. A few questions:
>
> - You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
> PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
> have been 0.8 pixels. Did you guys just do the math to figure out the
> correct pixel value?
>
> - Were these changes made on both axis in PHD2? MinMo and Aggressiveness
> can
> be configured per axis.
>
> The rest of this looks very interesting and as soon as the weather gets
> better I plan to test this and see how my guiding performs.
>
> _____
>
> From: ap-gto@... <mailto:gto@...> gto@... <mailto:gto@...> > on behalf of
> chris1011@... <mailto:chris1011@...> [ap-gto] gto@... <mailto:gto@...> >
> Sent: Wednesday, January 31, 2018 4:51 PM
> To: ap-gto@... <mailto:gto@...> ; ap-ug@... <mailto:ug@...>
> Subject: [ap-gto] Guiding AP mounts with PHD
>
> Howdy,
>
> Howard and I had a chance to log in to a customer's Mach1 mount last night
> to help him with some guiding issues using PHD. He had sent his
calibration
> graph and it looked quite good, but he was having some problems guiding.
> After doing some mechanical tests we set up parameters in PHD to get the
> mount to track and respond accurately. Since PHD is somewhat different
from
> MaximDL, which I use, we had to approach the settings a bit differently.
>
> The first step is to do a quick 2 - 3 minute Unguided run using 1 second
> guide exposures, and look at the guider graph to see what the maximum
> excursions are in Declination. The guide star will bounce around a certain
> amount and this peak error will be the seeing that you cannot guide out
with
> normal guide software. This P-V value, which in our case was between +-0.5
> and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
> to correct for that with guide moves to the mount is impossible (only a
fast
> acting AO system can chase that seeing error).
>
> So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
> want to set the initial Min Move setting in PHD to be approximately this
> value as a starting value. We set the Min Move to 0.8 arc sec, the guide
> rate at 1x and the aggressiveness to 100%. This means that no correction
> pulses are sent to the mount while the guide star is bobbling within that
> envelope, but once it exceeds even slightly, the mount gets a full
> correction command (-0.8 arc sec) to bring it back toward the zero
position.
> We turned guiding on and the result was that the mount responded quickly
> whenever the error exceeded the bobble limits and overall guiding was
tight,
> accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
> exposure with the main camera showed tight round stars.
>
> So, as a starting point:
> Determine the amount of guide star bobble above and below the axis
> Set the Min Move to that value
> Set Guide rate to 1x sidereal
> Set the guide star exposure rate to 1 second
> Set Aggressiveness to 100%
> Begin guiding and note the RMS value for both axes.
>
> You can then change any of the parameters to see if you can tune the RMS
to
> a lower value. We tried longer guide star exposures, but found that the
> response to tracking errors became more and more sluggish and less
accurate.
> So for these seeing conditions a faster guide rate of 1 per second
resulted
> in the lowest RMS error on both axes. In pristine seeing it might allow
> longer guide exposures, at least that is what we found using PHD Guiding.
>
> Rolando
>
> [Non-text portions of this message have been removed]
>
>
>
>



------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@... <mailto:groups3@...> >
------------------------------------

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------------------------------------

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[Non-text portions of this message have been removed]



------------------------------------
Posted by: "Wade Hilmo" <wade@...>
------------------------------------

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Joe Zeglinski
 

Rolando,
 
    On your second point ... wouldn’t it be nice if guider programs did a “limit check” and tossed out any centroid which exceeded a user set star position jump size,  and only accepted them “as real” after 2 or more samples that stayed at that new position. This might eliminate effects of sudden bursts of bad seeing, caused by major drift pockets in air temperature, even the occasional passing aircraft’s lights.
 
    Perhaps this is “already being done”,  in PemPro – if not, maybe it would be a good test vehicle for this kind of guide compensation.
 
Joe


Michael Fulbright <mike.fulbright@...>
 

PHD2 implements a hysteresis algorithm where you can specify how much the next correction sent relies on the most recent sample versus the history of samples.

There are also several options which use a low pass filter to achieve a similar end.

Also you can specify a maximum move per step.  You can also tell if if is see a very large excursion (due to a dither move) it will attempt to recover more quickly than the max move you specified.

Most is summarized here:

http://openphdguiding.org/man/Guide_algorithms.htm

I've found using long (4-6 second) exposures extremely useful when dealing with nights of poor seeing.   It is a simple experiment to incrementally increase the exposure time and watch the random scatter of samples of the DEC axis while guiding is turned off to gauge how well this works. 

If seeing conditions are so bad that you are seeing excursions more than say 1 arcsecond regularly it is probably time to shut down as that night is a lost cause.

Something to consider is there are other factors that can cause large jumps which have nothing to do with mechanical or atmospheric behavior.  For example, for a period apparently some QHY cameras with certain QHY drivers would occasionally output frames that were shifted by 1 pixel!  Imagine the consequences on guiding when using a smaller guide scope and a pixels scale of several arcseconds/pixel.

Michael Fulbright


On 2/2/2018 11:39 AM, 'Joseph Zeglinski' J.Zeglinski@... [ap-gto] wrote:
 

Rolando,
 
    On your second point ... wouldn’t it be nice if guider programs did a “limit check” and tossed out any centroid which exceeded a user set star position jump size,  and only accepted them “as real” after 2 or more samples that stayed at that new position. This might eliminate effects of sudden bursts of bad seeing, caused by major drift pockets in air temperature, even the occasional passing aircraft’s lights.
 
    Perhaps this is “already being done”,  in PemPro – if not, maybe it would be a good test vehicle for this kind of guide compensation.
 
Joe


Roland Christen
 

There may be ways of averaging and weighting the guide data so that only drift is compensated and seeing effects are ignored. In Dec especially, drift is only in one direction, so no reason to do lots of Dec axis reversals. During a dither move the software could adjust the guide pulses to bring the guide star back to zero rapidly, and then resume an average slower drift compensation. All this requires a higher level of sophistication in the guide software, but if the software is free, then nobody pays for the labor required to develop it. Dilemma ramalamadingdong

Rolando



-----Original Message-----
From: 'Joseph Zeglinski' J.Zeglinski@... [ap-gto]
To: ap-gto
Sent: Fri, Feb 2, 2018 11:01 am
Subject: Re: [ap-gto] Guiding AP mounts with PHD



Rolando,
 
    On your second point ... wouldn’t it be nice if guider programs did a “limit check” and tossed out any centroid which exceeded a user set star position jump size,  and only accepted them “as real” after 2 or more samples that stayed at that new position. This might eliminate effects of sudden bursts of bad seeing, caused by major drift pockets in air temperature, even the occasional passing aircraft’s lights.
 
    Perhaps this is “already being done”,  in PemPro – if not, maybe it would be a good test vehicle for this kind of guide compensation.
 
Joe



Roland Christen
 


PHD2 implements a hysteresis algorithm where you can specify how much the next correction sent relies on the most recent sample versus the history of samples.

There are also several options which use a low pass filter to achieve a similar end.

Also you can specify a maximum move per step.  You can also tell if if is see a very large excursion (due to a dither move) it will attempt to recover more quickly than the max move you specified.
That's interesting and I'll be anxious to try it.

Rolando


-----Original Message-----
From: Michael Fulbright mike.fulbright@... [ap-gto]
To: ap-gto
Sent: Fri, Feb 2, 2018 11:07 am
Subject: Re: [ap-gto] Guiding AP mounts with PHD



PHD2 implements a hysteresis algorithm where you can specify how much the next correction sent relies on the most recent sample versus the history of samples.

There are also several options which use a low pass filter to achieve a similar end.

Also you can specify a maximum move per step.  You can also tell if if is see a very large excursion (due to a dither move) it will attempt to recover more quickly than the max move you specified.

Most is summarized here:

http://openphdguiding.org/man/Guide_algorithms.htm

I've found using long (4-6 second) exposures extremely useful when dealing with nights of poor seeing.   It is a simple experiment to incrementally increase the exposure time and watch the random scatter of samples of the DEC axis while guiding is turned off to gauge how well this works. 

If seeing conditions are so bad that you are seeing excursions more than say 1 arcsecond regularly it is probably time to shut down as that night is a lost cause.

Something to consider is there are other factors that can cause large jumps which have nothing to do with mechanical or atmospheric behavior.  For example, for a period apparently some QHY cameras with certain QHY drivers would occasionally output frames that were shifted by 1 pixel!  Imagine the consequences on guiding when using a smaller guide scope and a pixels scale of several arcseconds/pixel.

Michael Fulbright


On 2/2/2018 11:39 AM, 'Joseph Zeglinski' J.Zeglinski@... [ap-gto] wrote:
 
Rolando,
 
    On your second point ... wouldn’t it be nice if guider programs did a “limit check” and tossed out any centroid which exceeded a user set star position jump size,  and only accepted them “as real” after 2 or more samples that stayed at that new position. This might eliminate effects of sudden bursts of bad seeing, caused by major drift pockets in air temperature, even the occasional passing aircraft’s lights.
 
    Perhaps this is “already being done”,  in PemPro – if not, maybe it would be a good test vehicle for this kind of guide compensation.
 
Joe




W Hilmo
 

I don’t know of any way to calculate an accurate centroid on a saturated star, since you have no actual data on any saturated pixels.  I was making an assumption that on a longer exposure, you would need to select an appropriately dim star as the guide star.  I also understand the challenges of guiding in declination with any backlash at all.

 

I actually use Maxim for guiding and am happy with the results I’ve been getting.  Almost all of my imaging is automated, so I am rarely present to see what it’s doing and don’t generally have the opportunity to do a test run to determine the seeing.  I just have Maxim’s parameters set to handle my typical conditions.  If we ever get any more clear nights, I am going to kind of relearn the best practices for my own gear.  I just added absolute encoders to my AP1600GTO and am changing my polar alignment routine per your recommendations for minimum drift at the zenith.  That, along with a good APCC tracking model, will likely change the way that I think about guiding.  I use SCTs fairly often, so I don’t think that I can get away from guiding completely, just to deal with mirror shift and non-repeatable flexure.

 

I do a fair amount of helping other folks to image, though, and have worked with PHD and PHD2 quite a few times.  I’m thinking about putting together a second imaging laptop, built around PHD2, and maybe SGP, since that is what all the cool kids seem to be using and I want to understand it well.  I really appreciate your posting all of this information so far, and I will be able to use it.  I just wanted to follow up on the recommendation for short exposures, because it was unexpected and goes a bit against what I’ve always understood and recommended.

 

Thanks,

-Wade

 

From: ap-gto@... [mailto:ap-gto@...]
Sent: Friday, February 2, 2018 8:00 AM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD

 

 

 

Then the centroid calculation by the guide software shouldn’t be seeing huge swings in the centroid position.

First, long exposures in PHD result in saturated stars which the program apparently can't handle, or can't calculate a proper centroid.

Second, it may be stable for a period of time until there is just one single excursion outside the envelope. The software will then send a move command which may not fully reverse the axis, so the guide star stays outside the envelope for another guide cycle until finally it gets pushed back down. This may then start an oscillation in the other direction, but even if not, the correcting signals are few and far between time-wise, so the RMS error gets larger. With faster guide exposures, the mount gets faster correction signals and the guide star spends less time in the wrong place.

All I'm saying is to try this method with PHD Guiding and see if it improves your results. It is not a hard and fast rule, just a suggestion of where to start setting the parameters. You are welcome to experiment and post your results here. We all can learn.

I have PHD now and will experiment further. I may try to add a tiny bit of backlash compensation in Dec to see what effect that has and whether that will reduce the back and forth oscillation or make it worse.

Rolando

 

 

-----Original Message-----
From: 'Wade Hilmo' y.groups@... [ap-gto] <ap-gto@...>
To: ap-gto <ap-gto@...>
Sent: Thu, Feb 1, 2018 7:50 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

If you are taking 10 second guide exposures, doesn’t that mean that the star will have bounced around randomly for the whole 10 seconds? As I understand it, the conventional wisdom is that longer guide exposures will have larger stars due to the atmospheric effects, but that the correct star position is likely to be near the center of the blob. Then the centroid calculation by the guide software shouldn’t be seeing huge swings in the centroid position.



I suppose that this gets down to the effectiveness of the centroid calculation. If it’s just looking for the brightest single pixel, then I can see where longer guide exposures might not be helpful. But I thought that the calculations were more sophisticated than that, and would look at the stellar profile to determine the centroid. Perhaps not, I guess? And perhaps it varies with the guide software.



-Wade



From: ap-gto@... [mailto:ap-gto@...]
Sent: Thursday, February 1, 2018 1:47 PM
To: ap-gto@...
Subject: Re: [ap-gto] Guiding AP mounts with PHD







I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

In this case it was because of poor seeing, not so much drift. When you get instances of poor seeing that move the guide star back and forth erratically, then you end up with many instances of guider corrections being sent to the mount. If you take a 10 second exposure and it shows the guide star at +1.5 arc second location, the mount will receive a command to move -1.5 arc seconds. But chances are that this +1.5 position occurred sometime before the end of the exposure, and at the exact time the move command is sent (after the fact), that star could already be at the opposite extreme, -1.5 arc sec position. The net result is that the guider software moves the guide star to -3 arc sec position for the next exposure. It's quite possible that the next guider pulse sent to the mount will have to be be +3 arc seconds, and you will see oscillations back and forth for a while until the seeing steadies up momentarily.

This is also complicated by any slight delay in Dec reversal that may exist in the mount which will require more than 1 pulse the do a full reversal.

In order to avoid this scenario, I would suggest setting a no-response limit around zero where the guider does not send any guide pulses, and just let the guide star bobble around in there. As soon as it steps outside this limit, the mount will get an instant reverse command. If the mount needs to reverse in Dec, then the necessary pulses will be sent quickly, perhaps 2 or 3 needed for full reversal. If you have long 10 second delays between each guide pulse, it may take 20, 30 or more seconds to get the guide star to move back to zero point.

Rolando





-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto] <ap-gto@yahoogroups..com>
To: ap-gto gto@...>
Sent: Thu, Feb 1, 2018 1:40 pm
Subject: RE: [ap-gto] Guiding AP mounts with PHD

Hi Wade,

I think that the length of the interval between guider exposures depends on
how much drift is present. A 1-second duration worked best for Howard and
Roland in their particular case probably because there was enough drift to
require a moves after 1 second.

If good-quality tracking rate correction can be used then I think that
autoguider exposures could be increased significantly. Otherwise, how could
doing unguided exposures using tracking rate correction work! :-)

Best regards,

-Ray Gralak
Author of APCC (Astro-Physics Command Center):
http://www.astro-physics.com/index.htm?products/accessories/software/apcc/ap
cc
Author of PEMPro: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
> Sent: Thursday, February 1, 2018 10:39 AM
> To: ap-gto@... <mailto:gto@...>
> Subject: RE: [ap-gto] Guiding AP mounts with PHD
>
>
>
> I am curious about this result, especially the part at the end where you
say
> that 1 second exposures worked well, but longer exposures "became more and
> more sluggish and less accurate".
>
> It's my understanding that the mount is going to track well, assuming that
> PEM is programmed correctly and enabled, and that guiding should basically
> be correcting any drift due to slight polar misalignment, refraction,
> flexure, etc. If my understanding is correct, it would seem that longer
> exposures (within reason) would be just as effective as shorter exposures
-
> perhaps more effective, since longer exposures would tend to average out
> seeing distortion in the individual guider subs. If you were getting
better
> results at 1 second, that would suggest that you were chasing seeing with
> some success.
>
> Do you understand why it worked the way that it did for you? Was it
> correcting for slower seeing effects? Is there some other reason?
>
> Thanks,
>
> -Wade
>
> From: ap-gto@... <mailto:gto@...> [mailto:ap-gto@... <mailto:ap-gto@...?> ]
> Sent: Thursday, February 1, 2018 9:36 AM
> To: ap-gto@... <mailto:gto@...>
> Subject: Re: [ap-gto] Guiding AP mounts with PHD
>
> Great info. A few questions:
>
> - You mention that you set the MinMo to 0.8 arc-secs. I believe MinMo in
the
> PHD interface is shown in pixels. So if you set the MinMo to 0.8 it would
> have been 0.8 pixels. Did you guys just do the math to figure out the
> correct pixel value?
>
> - Were these changes made on both axis in PHD2? MinMo and Aggressiveness
> can
> be configured per axis.
>
> The rest of this looks very interesting and as soon as the weather gets
> better I plan to test this and see how my guiding performs.
>
> _____
>
> From: ap-gto@... <mailto:gto@...> gto@... <mailto:gto@...> > on behalf of
> chris1011@... <mailto:chris1011@...> [ap-gto] gto@... <mailto:gto@...> >
> Sent: Wednesday, January 31, 2018 4:51 PM
> To: ap-gto@... <mailto:gto@...> ; ap-ug@... <mailto:ug@...>
> Subject: [ap-gto] Guiding AP mounts with PHD
>
> Howdy,
>
> Howard and I had a chance to log in to a customer's Mach1 mount last night
> to help him with some guiding issues using PHD. He had sent his
calibration
> graph and it looked quite good, but he was having some problems guiding.
> After doing some mechanical tests we set up parameters in PHD to get the
> mount to track and respond accurately. Since PHD is somewhat different
from
> MaximDL, which I use, we had to approach the settings a bit differently.
>
> The first step is to do a quick 2 - 3 minute Unguided run using 1 second
> guide exposures, and look at the guider graph to see what the maximum
> excursions are in Declination. The guide star will bounce around a certain
> amount and this peak error will be the seeing that you cannot guide out
with
> normal guide software. This P-V value, which in our case was between +-0.5
> and +-0.8 arc seconds is what I call the Guide Star "Bobble" limit. Trying
> to correct for that with guide moves to the mount is impossible (only a
fast
> acting AO system can chase that seeing error).
>
> So, knowing that the minimum seeing error is +-0.8 arc seconds, you will
> want to set the initial Min Move setting in PHD to be approximately this
> value as a starting value. We set the Min Move to 0.8 arc sec, the guide
> rate at 1x and the aggressiveness to 100%. This means that no correction
> pulses are sent to the mount while the guide star is bobbling within that
> envelope, but once it exceeds even slightly, the mount gets a full
> correction command (-0.8 arc sec) to bring it back toward the zero
position.
> We turned guiding on and the result was that the mount responded quickly
> whenever the error exceeded the bobble limits and overall guiding was
tight,
> accurate and almost the same RMS value, ~0.35 arc sec, for both axes. An
> exposure with the main camera showed tight round stars.
>
> So, as a starting point:
> Determine the amount of guide star bobble above and below the axis
> Set the Min Move to that value
> Set Guide rate to 1x sidereal
> Set the guide star exposure rate to 1 second
> Set Aggressiveness to 100%
> Begin guiding and note the RMS value for both axes.
>
> You can then change any of the parameters to see if you can tune the RMS
to
> a lower value. We tried longer guide star exposures, but found that the
> response to tracking errors became more and more sluggish and less
accurate.
> So for these seeing conditions a faster guide rate of 1 per second
resulted
> in the lowest RMS error on both axes. In pristine seeing it might allow
> longer guide exposures, at least that is what we found using PHD Guiding.
>
> Rolando
>
> [Non-text portions of this message have been removed]
>
>
>
>



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Posted by: "Wade Hilmo" <wade@...>
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