Tracking and guiding with and without encoders - Part2


Roland Christen
 

Hello again,
Thanks for being patient in reading my previous post. You have earned a chocolate bar.
Below I have posted two graphs (I will post them also to our files section and soon to our web page).

The first one shows the difference when the Mach2 is running with encoders and without. The graph starts with encoders on, then I switch them off and run thru one complete worm cycle (about 6.4 minutes) and then turn the encoders back on. Although the tracking error with encoders off is only around 4 arc sec, you can see instantly which one you would rather have running in the background while guiding and imaging. Yes, you can guide out the majority of that error and yes you can even compensate most of it with PE correction (PEM), but do you really want to and will it be as accurate? My tests show that if the seeing is 0.1 arc sec rms, the mount will guide at 0.1 rms. It will not add little sniggles and snorts to the tracking that the guide software can't fully compensate for. If the seeing is 0.8 rms and your pixel scale is 3 arc sec per pixel, then you may not see any of these errors and your stars will be round regardless.

The second graph shows the response to outside commands which I sent at various intervals. In this case I sent 0.5 arc sec move commands similar to what a guide program would do. The mount will respond accurately to any size command from .05 arc sec on up. It can also follow a custom guide rate smoothly without steps.

Graph1:


Below is the tracking response to outside commands. In this test I sent 0.3 arc sec move commands at various intervals to the mount and measured the response, the actual shaft position after each command.Graph2:


Dean Jacobsen
 

For some reason I am not able to see anything after "Graph 1:" when I look at this post in my browser... and I would like to see the graphs and the rest of the text.  ;-)

Dean Jacobsen


Rick Socarras
 

Roland,

I can see by the graphs that these motors are constantly correcting and therefore working hard.  I'm located in BOILING HOT south Florida and expect to receive a Mach 2 in December if all goes well.  My question is simple...is there any concern about the motors overheating?  I also have solar telescopes so the daytime temperatures can get really hot.  Thoughts?  Sorry, I just worry about things getting too hot - experience!

Rick Socarras


Chad A
 

Roland, 

Without encoders your mounts already guide up to the limit imposed by seeing.  How much will encoders improve the results of guided imaging?  No one is questioning whether encoder mounts track better or are superior for unguided imaging.

A test like this would be helpful:

- Have encoders on, calibrate PHD2, use default PHD2 settings, set a 3 second guide interval, run 200-400 samples .... check the total RMS error

- Turn encoders off, have PEM disabled, recalibrate PHD2, default PHD2 settings, 3 second guide interval, run 200-400 samples .... check the total RMS error

Repeat the test a few times and compare.  It would be best to use optimal PHD2 settings with both encoders on and off.  The above test would be simple and give at least some idea of the difference though.

Chad





Ray Gralak
 

Hi Chad,

Without encoders your mounts already guide up to the limit imposed by seeing. How much will encoders improve
the results of guided imaging?
I did lots of tests without encoders many years ago by turning PEC on and off. I could easily measure the difference in average FWHM of non-saturated stars. The reduction in FWHM was about 10% with 5-second autoguider exposures with a mount with 5-arc-second peak-peak periodic error. It is more dramatic with mounts with higher periodic error.

For example here is a comparison of actual star images taken with PEC enabled versus disabled and auto-guided with a mount with 40 arc-secs periodic error:

https://www.siriusimaging.com/PEMProV2/PECComparison.png

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: https://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver

-----Original Message-----
From: ap-gto@yahoogroups.com [mailto:ap-gto@yahoogroups.com]
Sent: Sunday, September 15, 2019 1:11 PM
To: ap-gto@yahoogroups.com
Subject: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Roland,

Without encoders your mounts already guide up to the limit imposed by seeing. How much will encoders improve
the results of guided imaging? No one is questioning whether encoder mounts track better or are superior for
unguided imaging.

A test like this would be helpful:

- Have encoders on, calibrate PHD2, use default PHD2 settings, set a 3 second guide interval, run 200-400
samples .... check the total RMS error

- Turn encoders off, have PEM disabled, recalibrate PHD2, default PHD2 settings, 3 second guide interval, run
200-400 samples .... check the total RMS error

Repeat the test a few times and compare. It would be best to use optimal PHD2 settings with both encoders on
and off. The above test would be simple and give at least some idea of the difference though.

Chad

< /div>





Chad A
 

Ray,

The stars in those images are horribly shaped as if tracking is turned off.  It's apparent even in the 1 second exposure.  Something is obviously going on there besides PEC being turned on or off.  Regardless, my question isn't about PEC.

Roland recently said on here, "Of course encoders improve guiding accuracy."  If that's true there should be a measurable and consistent improvement in Total RMS in PHD2 with encoders on vs. off.  I don't have encoders so I can't test how much improvement there might be - I suspect it's minimal and imperceptible in images.  It would be helpful for potential buyers like myself if this were tested and the results shared.  Valid results should be repeatable by non-biased users.

Chad


Bill Long
 

Encoders improve guiding accuracy for sure. My typical guiding was about 0.5" before the encoders, and is about 0.3" after them. 

I also shared 15 minute frames taken with no guiding, and no pointing/tracking model either. That would have been impossible to do prior to adding the encoders (trust me, I tried). 

Those frames can be found here:


I also tried 20 and 30 minute exposures and those were no good. Considering the environment I image on (wooden deck, 2 second story) I would call those results rather impressive. If you added a tracking correction model and guiding, the results would only get better. That I think is obvious.


From: ap-gto@... on behalf of badgerz49@... [ap-gto]
Sent: Sunday, September 15, 2019 7:45 PM
To: ap-gto@...
Subject: RE: [ap-gto] Re: Tracking and guiding with and without encoders - Part2
 
 

Ray,


The stars in those images are horribly shaped as if tracking is turned off.  It's apparent even in the 1 second exposure.  Something is obviously going on there besides PEC being turned on or off.  Regardless, my question isn't about PEC.

Roland recently said on here, "Of course encoders improve guiding accuracy."  If that's true there should be a measurable and consistent improvement in Total RMS in PHD2 with encoders on vs. off.  I don't have encoders so I can't test how much improvement there might be - I suspect it's minimal and imperceptible in images.  It would be helpful for potential buyers like myself if this were tested and the results shared.  Valid results should be repeatable by non-biased users.

Chad


Ray Gralak
 

The stars in those images are horribly shaped as if tracking is turned off. > It's apparent even in the 1 second exposure.
It's not a 1-second exposure. All the images are 5-minute images. The 1-5 second images are the autoguide interval. That's what you get if you have a 40 arc-second periodic error. That's why you want to correct your periodic error.

And, with a mount with 5 arc-secs peak to peak, the FWHM I measured improved by 10% in 5 minute images. That was measurable and repeatable.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: https://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver


-----Original Message-----
From: ap-gto@yahoogroups.com [mailto:ap-gto@yahoogroups.com]
Sent: Sunday, September 15, 2019 7:45 PM
To: ap-gto@yahoogroups.com
Subject: RE: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Ray,

The stars in those images are horribly shaped as if tracking is turned off. It's apparent even in the 1 second
exposure. Something is obviously going on there besides PEC being turned on or off. Regardless, my question
isn't about PEC.

Roland recently said on here, "Of course encoders improve guiding accuracy." If that's true there should be a
measurable and consistent improvement in Total RMS in PHD2 with encoders on vs. off. I don't have encoders so
I can't test how much improvement there might be - I suspect it's minimal and imperceptible in images. It would be
helpful for potential buyers like myself if this were tested and the results shared. Valid results should be repeatable
by non-biased users.

Chad


Chad A
 

Ok, that explains it.  Yes, PEMPro works great.


Roland Christen
 

The Mach2 mount needs the encoders to be on in order to produce smooth tracking. Without the encoders the error of the microstepper will dominate and you will have elongation of the stars in RA even though the rms guiding might be sub-arc seconds. The rms value does not take into account the peak to peak excursions of the guide star, which can be 5 times the rms value. It's a lot like sticking your head in the oven and your feet in the freezer. Your average rms body temp might indeed be very comfortable.

You all may not realize this but most of the inexpensive mounts being sold today with microsteppers and belt drives have these fast moving p-p excursions. That's because they don't have any kind of encoder feedback, either on the motor shafts or on the mount axes. It's a well known effect of microstepping where the individual step sizes are not constant but vary by 5% in a typical stepper motor. These are fast moving errors that occur between 1 to 3 times per second depending on pulley ratio. Without encoder feedback you simply won't get smooth tracking. These show up as fast oscillations in the RA direction when used with longer focal length scopes such as SCTs. They may not show up when using short wide field instruments and they certainly won't show up when guiding at 3 second frame rates.

That's why DC and AC servo motor drives with spur gear reduction have smooth tracking rates because they must use encoders on the motor shafts simply because these motors won't work without them. Microsteppers do work without encoders and have been the cheap solution for low end mount manufacturers, but the downside is tracking wiggles.

Rolando



-----Original Message-----
From: badgerz49@... [ap-gto]
To: ap-gto
Sent: Sun, Sep 15, 2019 3:11 pm
Subject: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Roland, 

Without encoders your mounts already guide up to the limit imposed by seeing.  How much will encoders improve the results of guided imaging?  No one is questioning whether encoder mounts track better or are superior for unguided imaging.

A test like this would be helpful:

- Have encoders on, calibrate PHD2, use default PHD2 settings, set a 3 second guide interval, run 200-400 samples .... check the total RMS error

- Turn encoders off, have PEM disabled, recalibrate PHD2, default PHD2 settings, 3 second guide interval, run 200-400 samples .... check the total RMS error

Repeat the test a few times and compare.  It would be best to use optimal PHD2 settings with both encoders on and off.  The above test would be simple and give at least some idea of the difference though.

Chad







Roland Christen
 


I can see by the graphs that these motors are constantly correcting and therefore working hard. 
No, they are not working hard while constantly correcting. This is a fallacy. Microsteppers get fed a constant current which varies not one iota whether they are moving or not. That is completely different from DC servo motors where the motor current is dependent on the load. The current that is sent to a microstepper is always the same whether they are at rest (not moving) or turning at various rates while tracking.

Rolando


-----Original Message-----
From: tgrick@... [ap-gto]
To: ap-gto
Sent: Sun, Sep 15, 2019 4:41 pm
Subject: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Roland,

I can see by the graphs that these motors are constantly correcting and therefore working hard.  I'm located in BOILING HOT south Florida and expect to receive a Mach 2 in December if all goes well.  My question is simple...is there any concern about the motors overheating?  I also have solar telescopes so the daytime temperatures can get really hot.  Thoughts?  Sorry, I just worry about things getting too hot - experience!

Rick Socarras



miguelmjr14@...
 

Hey Bill,

Just interested in what camera was used to take those images.


Miguel   8-)

.


Miguel Morales <miguelmjr14@...>
 

 

Hey Bill,

 

Just interested in what camera was used to take those images.

 

 

Miguel   8-)

 

.

 


Bill Long
 

That was a FLI ML16200 camera. 


From: ap-gto@... <ap-gto@...> on behalf of Miguel Morales miguelmjr14@... [ap-gto]
Sent: Monday, September 16, 2019 9:23 AM
To: ap-gto@...
Subject: Re: [ap-gto] Re: Tracking and guiding with and without encoders - Part2
 
 

 

Hey Bill,

 

Just interested in what camera was used to take those images.

 

 

Miguel   8-)

 

.

 


Rick Socarras
 

Thankyou!

Rick


Chad A
 

Roland,

P-P excursions are rare even with my belt-driven Atlas Pro.  Here's an image of guiding with two AP1100s and an Atlas Pro (top left) all at the same time with PEC disabled.  Excursions above 1" are infrequent (approx. 1/100 samples) and they're quickly guided out by PHD2.  Virtually none go above 1.5".  Total RMS is usually around .3" but seeing was mediocre this night.  


A PHD2 test of encoders vs. non-encoders would show excursions in the Target window.   Encoders should instantly remove excursions but when guiding are they really necessary and do they make a difference in the final result?  Maybe the belt-driven Mach 2 is a different story as you've noted.

Chad



Roland Christen
 

You are mixing apples and oranges. No wonder there is so much confusion. When you do 2 to 4 second guide exposures you are certainly not going to see very fast RA oscillations in the guide data. These oscillations happen quickly and can only be seen when you actually look at a star at rates faster than a guider can record. In my graphs I used sample rates of 100msec. You can do this with video on a small guide camera, but not at normal guide sample rates. Therefore all mounts can bury small fluctuation in RA tracking rates simply because they do not show up at long integration times.
If you don't know what these oscillations look like, here is one of many examples:https://www.cloudynights.com/topic/676345-cem-60ec-tracking-oscillations/

In this particular case it shows up at long focal lengths and is completely buried in the tracking graph when using 3  - 4 second integration times. Even though the guide chart shows maybe sub-arc sec guiding, the stars with that focal length are always a bit oval due to these p-p variations. And again, a belt drive isn't what does this, it's a microstepper motor without shaft encoder that can produce these fast moving oscillations. No amount of guiding can eliminate these.

Also, your seeing in both your examples is not good, average maybe for your skies, but we have to build mounts that can perform their best at sites that have excellent seeing without introducing mount tracking errors that would spoil the best results that could be obtained. This guiding graph (below) is what I would want the mount to achieve under good seeing, which in our case happens maybe once per month, but in places like Tuscon may happen every clear night. And that's taken at fairly long focal length with a long refractor:

-----Original Message-----
From: badgerz49@yahoo.com [ap-gto] <ap-gto@yahoogroups.com>
To: ap-gto <ap-gto@yahoogroups.com>
Sent: Mon, Sep 16, 2019 3:13 pm
Subject: Re: [ap-gto] Re: Tracking and guiding with and without encoders - Part2

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Roland,
P-P excursions are rare even with my belt-driven Atlas Pro.  Here's an image of guiding with two AP1100s and an Atlas Pro (top left) all at the same time with PEC disabled.  Excursions above 1" are infrequent (approx. 1/100 samples) and they're quickly guided out by PHD2.  Virtually none go above 1.5".  Total RMS is usually around .3" but seeing was mediocre this night.  
https://imgur.com/a/RDhtTaI

A PHD2 test of encoders vs. non-encoders would show excursions in the Target window.   Encoders should instantly remove excursions but when guiding are they really necessary and do they make a difference in the final result?  Maybe the belt-driven Mach 2 is a different story as you've noted.
Chad


Chad A
 

Roland,

It seems you're not claiming guiding results, particularly Total RMS, will be improved with encoders.  That's what I wanted to know.

It's never been in doubt whether encoders remove fast oscillations.  The oscillations in your graph are very small over a typical 2-3 second guide interval.  Whether they're detectable in guided images is a different topic.  I don't know what the seeing threshold is to notice them but I image it's beyond what most/all of us will ever have.

Chad


Ray Gralak
 

Hi Chad,

RMS guide error is the wrong measurement because it depends on the sampling of autoguider moves.

What you want to compare is average stellar FWHM and eccentricity of non-saturated stars in images. The image integrates light over the entire duration of an image and is the way to best determine guide quality. You want to compare the guided image's FWHM and eccentricity to those values in a very short duration image (e.g. 1 second duration).

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: https://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: https://www.siriusimaging.com/apdriver

-----Original Message-----
From: ap-gto@yahoogroups.com [mailto:ap-gto@yahoogroups.com]
Sent: Tuesday, September 17, 2019 7:00 AM
To: ap-gto@yahoogroups.com
Subject: Re: [ap-gto] Re: Tracking and guiding with and without encoders - Part2



Roland,


It seems you're not claiming guiding results, particularly Total RMS, will be improved with encoders. That's what I
wanted to know.

It's never been in doubt whether encoders remove fast oscillations. The oscillations in your graph are very small
over a typical 2-3 second guide interval. Whether they're detectable in guided images is a different topic. I don't
know what the seeing threshold is to notice them but I image it's beyond what most/all of us will ever have.


Chad


Chad A
 

Ray,

Yes, comparing FWHM seems like a good way to quantify the difference.  Rapid changes in seeing might make it difficult to test properly.  Any difference in results should be statistically confirmed over many samples and repeatable.  The test would be:

Guided encoder off FWHM vs. 1 second FWHM

against

Guided encoder on for same duration vs. 1 second FWHM

Chad