How Good Can Guiding be With Non-AE Mount


M Hambrick
 

I  have a 1100 GTO mount without absolute encoders, and I have been wondering how good I can expect the guiding to be. The guiding graph below is pretty typical of what I get. Is this pretty typical, or should I be able to do better than this ?

Suggestions are welcome.

Mike


Mike Dodd
 

On 2/7/2021 7:23 PM, M Hambrick wrote:
I have a 1100 GTO mount without absolute encoders, and I have been
wondering how good I can expect the guiding to be. The guiding graph
below is pretty typical of what I get. Is this pretty typical, or should
I be able to do better than this ?
Your X-axis RMS is 0.168 (pixels, I assume). At 3.21 arcsec/pixel, that's0.539 arcsec RMS error.

I typically get that or slighter better with my AP-1200 and a separate guide scope/camera. With this guiding performance, I get good images with an image scale of 0.86"/px on my main camera.

Does your guiding software let you specify guider scope and camera parameters, so it will display the guide error graph in arcseconds? I've found that very helpful in MaxIm.

--- Mike
http://astronomy.mdodd.com


M Hambrick
 

Hi Mike Thanks for the comments.

Yes, I use MaxIm DL Pro for imaging and guiding. I always use a separate guide scope piggybacked on top of the main imaging scope (in spite of Roland's recommendations against doing so to avoid flexure). My guide scope is a Tele-Vue Pronto (70 mm, f 6.86). I had been using a SBIG ST-i as my guide camera, but more recently I started guiding with my old ST2000-XM. It is much easier to find a guide star with this 2 megapixel camera.

On my main imaging camera (SBIG STXL16200) I have the SBIG FW8G-STXL filter wheel which has an off-axis guide chip located in front of the filters, but I have never been able to get the off-axis chip to image anything. I think I am doing something wrong, but I have decided to switch over to a standard filter wheel without the guider to get some extra backfocus.

I recently set up configurations for all of my scope / camera combinations in MaxIm so that I can set up the guiding graph to display the error in arc-seconds, but until now I have not done this. I think I saw a recent comment from Roland where he stated that he normally displays his guiding graph in pixels because that is what the camera sees.

I have owned the 1100 GTO mount since 2017, but I never updated the PEC that was set up with the mount at the factory. I am not sure if this is necessary, and I am not sure how to do this if it is required.

I have also been experimenting with the guide exposure time and exposure delay to see if I can do better.

Mike


Mike Dodd
 

On 2/7/2021 10:13 PM, M Hambrick wrote:

Yes, I use MaxIm DL Pro for imaging and guiding. [...]

I recently set up configurations for all of my scope / camera
combinations in MaxIm so that I can set up the guiding graph to display
the error in arc-seconds, but until now I have not done this.
I recommend doing that; see below.

I think I
saw a recent comment from Roland where he stated that he normally
displays his guiding graph in pixels because that is what the camera sees.
I displayed pixels, but then it occurred to me that pixels and plate scale on the guide camera have no relation to pixels and plate scale on the imaging camera, so why display guide camera pixels?

I use arc seconds because an arc second is an an arc second on both cameras. No mental gymnastics to go from one to the other.

I know what the plate scale is on my ASI-1600 imaging camera (0.8 arcsec/px) so if I see a guiding error of 0.6 arcsec RMS, I know that's less than one pixel on the imaging camera, and that's what I care about.

I have also been experimenting with the guide exposure time and exposure
delay to see if I can do better.
I use ACP automation software, and it sets the exposure based on guide star brightness. It also doesn't begin an exposure until the error is within a specified limit. Then MaxIm applies a settling delay before opening the shutter.

With my setup, I get good guiding with a 1-second exposure and a guide star SNR above 50.

--- Mike


Worsel
 

Mike

From the factory, my 1100 (2016 - non-AE) PE was 7 a-s peak to peak, which is within spec.  After using Ray Gralak's PemPro to analyze and load a PEC curve, the PE is 1.1 a-s P-P.  The upshot is that it is much easier for PHD2 (or any guide software) to guide out that error than the 7 a-s p-p value.

Also, I get RMS values ranging from .4 to .8 a-s using PHD2 (OAG - ZWO 174), which, given the seeing, are equivalent as far as round stars.  Your value for that one example is excellent.

Bryan


Roland Christen
 

Guiding is going to depend on your seeing as well as the parameters that you set in the guide software. If you have a good PE curve it can help a lot in RA.

Rolando

-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Sun, Feb 7, 2021 6:23 pm
Subject: [ap-gto] How Good Can Guiding be With Non-AE Mount

I  have a 1100 GTO mount without absolute encoders, and I have been wondering how good I can expect the guiding to be. The guiding graph below is pretty typical of what I get. Is this pretty typical, or should I be able to do better than this ?

Suggestions are welcome.

Mike


--
Roland Christen
Astro-Physics


Roland Christen
 


I use arc seconds because an arc second is an an arc second on both
cameras. No mental gymnastics to go from one to the other.
The problem with MaximDL is that in RA the calculated arc seconds are divided by CosineDec. So as you go higher in the sky towards the pole, the calculated value of 1 pixel will be a large number of arc seconds compared to what it would be at 0 Dec. I consider this a bug in the program which affects the guide graph but not the guiding. You can turn off the Auto Dec feature, but most people don't and then get a wild RA graph when imaging objects toward the north.
When I suggested using pixels, I was of course referring to use of an off-axis guider which looks at star motion from the same scope as the imaging camera. Of course it isn't the same if you use a separate guide scope with short focal length.

Rolando


-----Original Message-----
From: Mike Dodd <mike@...>
To: main@ap-gto.groups.io
Sent: Sun, Feb 7, 2021 10:38 pm
Subject: Re: [ap-gto] How Good Can Guiding be With Non-AE Mount

On 2/7/2021 10:13 PM, M Hambrick wrote:

> Yes, I use MaxIm DL Pro for imaging and guiding. [...]
>
> I recently set up configurations for all of my scope / camera
> combinations in MaxIm so that I can set up the guiding graph to display
> the error in arc-seconds, but until now I have not done this.

I recommend doing that; see below.

> I think I
> saw a recent comment from Roland where he stated that he normally
> displays his guiding graph in pixels because that is what the camera sees.

I displayed pixels, but then it occurred to me that pixels and plate
scale on the guide camera have no relation to pixels and plate scale on
the imaging camera, so why display guide camera pixels?

I use arc seconds because an arc second is an an arc second on both
cameras. No mental gymnastics to go from one to the other.

I know what the plate scale is on my ASI-1600 imaging camera (0.8
arcsec/px) so if I see a guiding error of 0.6 arcsec RMS, I know that's
less than one pixel on the imaging camera, and that's what I care about.

> I have also been experimenting with the guide exposure time and exposure
> delay to see if I can do better.

I use ACP automation software, and it sets the exposure based on guide
star brightness. It also doesn't begin an exposure until the error is
within a specified limit. Then MaxIm applies a settling delay before
opening the shutter.

With my setup, I get good guiding with a 1-second exposure and a guide
star SNR above 50.

--- Mike







--
Roland Christen
Astro-Physics


Mike Dodd
 

On 2/8/2021 12:14 PM, Roland Christen via groups.io wrote:

The problem with MaximDL is that in RA the calculated arc seconds are
divided by CosineDec. So as you go higher in the sky towards the pole,
the calculated value of 1 pixel will be a large number of arc seconds
compared to what it would be at 0 Dec. I consider this a bug in the
program which affects the guide graph but not the guiding.
Thanks, Roland; I didn't know this.

--- Mike


M Hambrick
 

Thanks Roland

Is there a recommended time limit for how long a PE curve can be used before it should be updated ? I am still using the original PE curve that was programmed into the mount when I bought it in March, 2017.

Mike


Roland Christen
 

Depends how much the mount is used. Over time the periodic error changes shape, so what was in the mount when it was manufactures is not going to be accurate now. You have a super easy way to determine what is the PE and whether turning the PE correction on will make it better. All you need is a guide program like PHD2 or MaximDL.

Simply run the mount with guide corrections turned off for 15 - 20 minutes and record the cyclical motion of the guide star (ignore drift, just look at the cycles). You will see every 6.4 minutes the start of a new cycle, with each new cycle resembling the last one. Do this with PE OFF and then repeat with PE ON. Examine the results and make your judgement of what to do.

Rolando



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Mon, Feb 8, 2021 6:30 pm
Subject: Re: [ap-gto] How Good Can Guiding be With Non-AE Mount

Thanks Roland

Is there a recommended time limit for how long a PE curve can be used before it should be updated ? I am still using the original PE curve that was programmed into the mount when I bought it in March, 2017.

Mike

--
Roland Christen
Astro-Physics


Nick Iversen
 

I did exactly what Rolando said. My mount is 3 years old. This is what I got. There is a definite periodic error. I used PemProV3 to fix the error. PemPro was surprisingly easy to use (and free for 60 days). And it worked. Not only did it remove the error but it reduced my PHD2 RMS from about 0.5" to 0.4". Nice!