Debugging AP130GTX star elongation


Gabe Shaughnessy
 

I've had some star elongation issues with my AP130GTX that I'm hoping to get some advice among the group here to resolve it.  I've been in contact with George from AP, and the consensus there is that it's within spec.  However, I'm having difficulty with that since the R and G channels don't show any elongation.  The pixel scale with my experiments was 1.32"/px, so I don't think I'm over-sampled, yet the differences are still pretty clear among the different channels.  There is some slight elongation in the B channel as well.  I received the scope originally in Dec of last year, noticed triangular shapes in the stars and dropped it off around early March.  
 
I received it not long after and noticed now the star elongation.  Here are the things I've tried to diagnose where the issue lies:
- Multiple exposures to determine if it's tracking.  Same elongation found in 60s and 300s.
- Two cameras and filter sets: ASI2600MM + Astrodon and ASI6200MM + Chroma.  The Chroma and Astrodon have slightly different cutoffs, so I wanted to isolate that effect.  Same elongation on both sets.
- Unfiltered - same elongation, with slightly more bloated stars, but that's expected due to the lack of a UV/IR cut. 

Below are some crops of LRGB frames.  I hope the resolution is okay.  I can share links to the raw files as well.  I’m using the QUAD-TCC with this scope and it’s mounted on a Mach 2.  

Also attached are a few screenshots of an inspector tool I wrote to show the elongation direction of the stars in the frame with an eccentricity color-coded heatmap - color is Eccentricity, the line in each cell indicates the elongation direction.  You can see that over the frames, there's a uniform elongation direction that's not what you'd expect from a reducer/flattener spacing issue.  

Any help on diagnosing or solving the elongation issues in L and B would be very much appreciated!  Thanks!

Gabe





Gabe Shaughnessy
 
Edited

Hmm, I see the elongation crop wasn't the greatest resolution.  Here's a link to the fits files and heatmap images.


Chris White
 

If you hadnt swapped out cameras and seen the same effect I would suggest that you make sure the filter isnt tilted in the carousel. 

This is a strange one (at least to me).  Do you have the field flattener as well?  You could maybe rule out the scope and camera accessories if you cant reproduce the issue. 


Gabe Shaughnessy
 

Good call.  I did take the carousel apart and check that the screws were tightened.  I also flipped the lum filter on the off chance the orientation had anything to do with it.  

I don't have the flattener.  The common elements are the AP130GTX and QUADTCC.  I don't have access to the field flattener, and I'm not too keen on spending that much money on something to debug the issue.  I could image w/o the QUADTCC and inspect the center of the fov to rule out the TCC.  


ROBERT WYNNE
 

For what it's worth, and you may know this already, but a FFC and QUADTCC can't be used together. Been round that tree several times on different A-P scopes. -Best, Robert

On 07/04/2021 6:56 PM Gabe Shaughnessy <gshaughnessy@...> wrote:
 
 
Good call.  I did take the carousel apart and check that the screws were tightened.  I also flipped the lum filter on the off chance the orientation had anything to do with it.  

I don't have the flattener.  The common elements are the AP130GTX and QUADTCC.  I don't have access to the field flattener, and I'm not too keen on spending that much money on something to debug the issue.  I could image w/o the QUADTCC and inspect the center of the fov to rule out the TCC.  


M Hambrick
 

Hi Gabe

The frame analysis suggests the possibility of tilting. How is your camera attached to the scope ? Is there a possibility that you have an adapter that is not quite square.

I noticed that the L and R FITS images are inverted compared to the G and B images. Do the frame analysis for each image correspond to the orientation of the FITS images ?

Mike


Chris White
 

On Mon, Jul 5, 2021 at 09:45 AM, M Hambrick wrote:
Hi Gabe

The frame analysis suggests the possibility of tilting. How is your camera attached to the scope ? Is there a possibility that you have an adapter that is not quite square.

I noticed that the L and R FITS images are inverted compared to the G and B images. Do the frame analysis for each image correspond to the orientation of the FITS images ?

Mike
Mike,

The funny thing is that he is not seeing the problem with Red and Green.  Only Lum and Blue.


Gabe Shaughnessy
 

There is a minor amount of tilt, but I don't think that's the dominant effect here.  As Chris remarked, it's only strong in Lum and more weakly in Blue.  

Robert, thanks for the advice.  I hadn't planned on combining the two.  

Gabe


M Hambrick
 

Very strange.

I was looking at the FITS header information for the four images to see if there may be a clue there.

The red and then the blue images were taken on 04/02/2021. The luminance and then green images were taken a month later on 05/12/2021. On both dates, the later images (blue and green ) are inverted with respect to the earlier images.

Did you have some kind of meridian flip going on ? If so is it possible that there is some kind of guiding or maybe guider calibration issue ? What about flexure ?

It is really strange because the "bad" images were taken on different dates with the blue image being taken after the red image on 04/02/2021 and the luminance image being taken before the green image on 05/12/2021.

These observations probably don't have anything to do with what you have been seeing, but I will be curious to find out what the real cause is.

Mike


Chris White
 

Good observation Mike.  Gabe might already have this, but looking at a fresh comparison where test images were taken back to back to back to back might be useful.  I still can't make sense of it though when he is seeing the same issue with a second camera with a separate set of filters.  That suggests some sort of channel management by something that the photons are passing through.  There is definitely a component of tilt going on, and I wonder if that could be exascerbating this problem or even causing the effect of how different color channels are handled differently.  Whenever I have done my tilt testing I've only ever used Lum, as thats the only filter I can get Voyager to do a focus routine with when "on the fly".  So I've never compared different filters directly.  I wonder if you fix the tilt if you will either fix the issue you are seeing, or at least have a meaningful impact on it. 


Gabe Shaughnessy
 

Thanks guys for the responses!  They're on separate nights because I was trying to find images w/ good focus.  Below is a set where they are all on the same dates - April 2.  Similar story there. 

These are all on one side of the meridian (suffix "_E"), and I use an OAG for guiding.  I calibrate every night before imaging also (also using Voyager Dragscript to automate makes things more repeatable).  This isn't a flexure issue since I'm imaging with the same subs for these channels.  Also, a test run on shorter subs show the same elongation.  If it were flexure, the elongation would be smaller w/ shorter subs. 

I've also attached a scatter graph (FWHM vs. eccentricity) showing how consistent the issue is across the night - these are all subs taken on April 2.  There are some focus issues w/ some Blue subs related to the dual imaging rig I'm working on (130GTX and Stowaway!), but the story is consistent.

Gabe


Chris White
 

Do you have an osc camera?  I wonder if you'd see this effect in the colors there. 

I wish you were in Vermont,  I'd bring my FF over. Lol.  Where are you?  I can't remember from CN.  

How about this for further testing:

Since you've ruled out the camera, have you tried rotating the qtcc to see if that changes things?  Have you tried taking images without the qtcc to compare colors to see if they are all manged tge same way with the OTA only?


Lawrence Lopez
 

Just a red herring:

have you ruled out atmospheric refraction.
It is color dependent so the 2 channels would be different.


Gabe Shaughnessy
 

Great suggestions Chris!  I'll rotate the TCC and see if the orientation tracks the rotation.  I do have a 2600MC, so I will investigate more how I could get that working.  Most likely will replace the front M42 plate with the M68 plate to match with the Precise Parts adapter I have for the TCC.  I'm in Wisconsin, so a good deal away from you, and quite close to AP - no more than 90 min away.  It's great having 1 day shipping when I order something from AP :) 

Lawrence, good suggestion.  I thought about that as well, but the elongation direction is more in the RA, and since my subs were taken reasonably close to the meridian, this translates to an azimuthal elongation, not one in altitude.  For atmospheric refraction, I'd expect an elongation along the altitude direction.


M Hambrick
 

Hi Gabe

One other thing about this keeps bugging me, and that is that the problem is occurring only on the L and B filters. I realize that you have seen the same problem with the same filters with two different cameras, but I wonder if it wouldn't be a good idea to switch the position of the L and B filters with the R and G filters in one of your cameras to see if the elongation stays with the same filters. 

If nothing else, you could rule this out as a possibility. The cameras are both ASI. Who knows ? They may have some common manufacturing flaw.

Mike


Roland Christen
 

Take a snapshot to rule out guiding issues. Find an area of sky with some bright stars that will show up in a 1 second exposure. Take a shot just inside focus, then at exact focus and then just outside focus. Repeat the above with no TCC and no flattener. Shoot a star or stars at or very near the center of the frame.

With those sets of 3 images you will know what's really happening. Just the images of the stars is all you need, the analysis program won't tell you anything.

Rolando

-----Original Message-----
From: Gabe Shaughnessy <gshaughnessy@...>
To: main@ap-ug.groups.io
Sent: Sun, Jul 4, 2021 8:56 pm
Subject: Re: [ap-ug] Debugging AP130GTX star elongation

Good call.  I did take the carousel apart and check that the screws were tightened.  I also flipped the lum filter on the off chance the orientation had anything to do with it.  

I don't have the flattener.  The common elements are the AP130GTX and QUADTCC.  I don't have access to the field flattener, and I'm not too keen on spending that much money on something to debug the issue.  I could image w/o the QUADTCC and inspect the center of the fov to rule out the TCC.  


Roland Christen
 

Shoot the following sequence:

Star at or very near the zenith, centered in the camera frame, 1 sec exposure, L channel
1) slightly inside focus
2) at focus
3) slightly outside focus

Shoot the above sequence with QTCC
Repeat the above sequence with no QTCC, no Flattener

Roland Christen
Astro-Physics Inc.



-----Original Message-----
From: Gabe Shaughnessy <gshaughnessy@...>
To: main@ap-ug.groups.io
Sent: Sun, Jul 4, 2021 2:25 pm
Subject: [ap-ug] Debugging AP130GTX star elongation

I've had some star elongation issues with my AP130GTX that I'm hoping to get some advice among the group here to resolve it.  I've been in contact with George from AP, and the consensus there is that it's within spec.  However, I'm having difficulty with that since the R and G channels don't show any elongation.  The pixel scale with my experiments was 1.32"/px, so I don't think I'm over-sampled, yet the differences are still pretty clear among the different channels.  There is some slight elongation in the B channel as well.  I received the scope originally in Dec of last year, noticed triangular shapes in the stars and dropped it off around early March.  
 
I received it not long after and noticed now the star elongation.  Here are the things I've tried to diagnose where the issue lies:
- Multiple exposures to determine if it's tracking.  Same elongation found in 60s and 300s.
- Two cameras and filter sets: ASI2600MM + Astrodon and ASI6200MM + Chroma.  The Chroma and Astrodon have slightly different cutoffs, so I wanted to isolate that effect.  Same elongation on both sets.
- Unfiltered - same elongation, with slightly more bloated stars, but that's expected due to the lack of a UV/IR cut. 

Below are some crops of LRGB frames.  I hope the resolution is okay.  I can share links to the raw files as well.  I’m using the QUAD-TCC with this scope and it’s mounted on a Mach 2.  

Also attached are a few screenshots of an inspector tool I wrote to show the elongation direction of the stars in the frame with an eccentricity color-coded heatmap - color is Eccentricity, the line in each cell indicates the elongation direction.  You can see that over the frames, there's a uniform elongation direction that's not what you'd expect from a reducer/flattener spacing issue.  

Any help on diagnosing or solving the elongation issues in L and B would be very much appreciated!  Thanks!

Gabe