Re: Guiding AP mounts with PHD


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