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Old 12-03-2022, 01:47 PM
Stephane
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FWHM & Eccentricity problems - Sombrero

Hi all,

Recently I finally managed to fix my diffraction spikes. Great news. However, my image of the Sombrero galaxy suffers from bad eccentricity and poor focus in the corners. Could I please pick your brains to analyse my image and give me some possible indication of where I might start?

My images often suffer from bad eccentricity with about a third of my frames above 0.7, but last imaging session was worse than ever where 2/3 were above 0.7. Coma looks particularly bad in 2 corners. Previous images have almost been coma free.

Thanks so much for your help.

Regards,
Stéphane

Notes:
- I am using a Newtonian reflector with a coma corrector at about 56 or 57mm from the sensor.
- Poor polar alignment could be a problem. I might be doing something wrong, but for me, NINA's 3-pt polar alignment is inconsistent.
- Flexure could be another problem. My guidescope is not tight enough and my OTA image may not have been at the centre of the guiding image.
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Old 12-03-2022, 03:10 PM
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Joshua Bunn (Joshua)
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Hello Stephane,


First of all, you have resolved some good detail in those dust lanes.


A few questions...
How long was this exposure?


It looks like you do have some tilt in the image train somewhere, as the elongation of the stars is mostly in one direction. If the star elongation was radiating from the center out equally in all directions (or equaly in a circular direction at the edges all the way around the image), that could suggest a lack of tilt.
I say the elongation is mostly in one direction, because in the bottom left, the direction is a little different. This may suggest the trailing is not tracking (tracking issues would all be in the same direction)


Take a series of short 2, 5, 15, 30 sec exposures on a bright star field, this will tell you if the issue is tracking or optical in nature.



I would say looking at the image, its probably optical...tilt, flexure, corrector spacing, or a combination of them.


Cheers
Joshua
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Old 12-03-2022, 03:35 PM
Stephane
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Quote:
Originally Posted by Joshua Bunn View Post
Hello Stephane,


First of all, you have resolved some good detail in those dust lanes.


A few questions...
How long was this exposure?


It looks like you do have some tilt in the image train somewhere, as the elongation of the stars is mostly in one direction. If the star elongation was radiating from the center out equally in all directions (or equaly in a circular direction at the edges all the way around the image), that could suggest a lack of tilt.
I say the elongation is mostly in one direction, because in the bottom left, the direction is a little different. This may suggest the trailing is not tracking (tracking issues would all be in the same direction)


Take a series of short 2, 5, 15, 30 sec exposures on a bright star field, this will tell you if the issue is tracking or optical in nature.



I would say looking at the image, its probably optical...tilt, flexure, corrector spacing, or a combination of them.


Cheers
Joshua
Thank you Joshua for your response. This is a stacked image of about 40 x 300s exposures. I believe the elongation is worse on longer subs. The stars looked much better at 10s exposures (better, but not perfect).

I wonder if bottom-left and upper-right corners are a “destructive” combination: coma (elongation toward corner) combined with tilt in the other direction. Both combining at right angles to make blocky stars in the bottom left and upper right corners? Whereas in the upper-left and bottom right, both issues combine “constructively” to create very elongated stars.

Overall I am pleased with the detail in the galaxy. It’s just a shame the stars ruin this image. Hopefully I can fix this. Additional comments most welcome!!

P.S. Guiding was excellent at 0.6

Last edited by Stephane; 12-03-2022 at 03:48 PM.
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Old 12-03-2022, 03:49 PM
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Single unprocessed sub exposures are whats needed really to make any kind of evaluation. The less variables the better.
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Old 12-03-2022, 03:54 PM
Stephane
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Single unprocessed sub exposures are whats needed really to make any kind of evaluation. The less variables the better.
Ok, here is one of the unprocessed subs.
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Old 12-03-2022, 03:56 PM
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if there are tracking issues, its definitely not just tracking issues. But, its best to do as I mentioned in my first message and go from there.
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Old 12-03-2022, 04:18 PM
Startrek (Martin)
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As you know I have the same scope
Joshua is right on the money !!
Looks like “tilt” to me but also could be a combination of tilt, tracking and PA
Unfortunately a process of elimination , testing and time will resolve the problem
By the way your diffraction spikes on the larger stars are spot on too !!
Be patient and you will eventually resolve the issue
Good luck !!
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Old 12-03-2022, 05:14 PM
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A few tests you can do to eliminate/isolate things.


take a photo then rotate your camera 90 deg. If the star elongation is in the same position relative to the camera chip, it is likely NOT tracking and the issue could be a tilt problem.


If the star elongation does not follow the chip after rotation, the issue could be collimation or a tracking issue.
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Old 12-03-2022, 05:32 PM
Stephane
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Thank you both so much for your comments. I will experiment a little tonight and see if I can improve things. Sometimes one needs to take one step back to then move two steps forward… hopefully…

Thanks again,
Stéphane
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Old 13-03-2022, 12:07 PM
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Hi all,

I set up my rig last night. Unfortunately, I genuinely forgot to try rotating the camera. I can try that tonight. But I did remember to try different length exposures:

10 seconds: stars are nice and round. No noticeable coma.
20 seconds: same
30 seconds: same
60 seconds: same
120 seconds: same
3 minutes: eccentricity starts becoming noticeable
5 minutes: eccentricity only gets worse and always toward the top-left corner as in the image attached at the start of this post.

Overall, I noticed that my stars are drifting toward the top-left corner throughout the night. I guess this explains why the eccentricity is worse for longer subs.

Tonight I will rotate the camera to see how this affects things. In the mean time, any diagnostics and tips? (other than the obvious of taking 2-minute subs from now on) Before imaging, I centred the guidescope image with the main camera image.

Thank you for your help.

Regards,
Stéphane
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Old 13-03-2022, 12:45 PM
Startrek (Martin)
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Stephane
A couple of things I just thought of over and above previously mentioned tilt etc.... or even coma which it’s not
1/ You mentioned your guiding was 0.60 arc sec error total ?
What was the individual errors on Ra and Dec
If Ra and Dec are significantly different, especially Dec , this will affect your Star shape even though the guiding rms or average is good
Maybe check that out
For example, say Ra 0.52 and is Dec 0.78

2/ Also you rig could be suffering from some flexure in your guiding scope set up
Are you using a central axis mounted guide scope piggybacked on the OTA or just using the offset shoe bracket with a single bracket mounted finder / guide scope ??
Recommend the central axis piggybacked mount using a solid losmandy bar

3/ To prove your PA is ok ( forget relying on the software for the moment ) just find a bright star up in the east / north east ( say magnitude 1.5 to 1.0 ) at 40 to 60 degrees in Alt , centre it , start tracking , don’t use any guiding and take a 30 sec , 45 sec , 60 sec and 80 sec exposure
If your PA is good you should be getting a round Star to at least 60 sec unguided. If your only getting 30 to 45 sec exposures before the Star starts to morph then your PA is not good ( possibly +5 arc minutes ) I normally can take up to 60 to 80 sec exposures unguided on the same rig before Star shape is affected and my PA is usually around an arc minute or less
Even with guiding, poor PA can cause all your smaller stars ( not the big bright ones ) to be eggy or odd shaped usually in one direction , on subs longer than 3 minutes over a long period of tracking time

Just a few other things I thought of which may help !!
Cheers
Martin
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Old 13-03-2022, 01:05 PM
Stephane
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Quote:
Originally Posted by Startrek View Post
Stephane
A couple of things I just thought of over and above previously mentioned tilt etc.... or even coma which it’s not
1/ You mentioned your guiding was 0.60 arc sec error total ?
What was the individual errors on Ra and Dec
If Ra and Dec are significantly different, especially Dec , this will affect your Star shape even though the guiding rms or average is good
Maybe check that out
For example, say Ra 0.52 and is Dec 0.78

2/ Also you rig could be suffering from some flexure in your guiding scope set up
Are you using a central axis mounted guide scope piggybacked on the OTA or just using the offset shoe bracket with a single bracket mounted finder / guide scope ??
Recommend the central axis piggybacked mount using a solid losmandy bar

3/ To prove your PA is ok ( forget relying on the software for the moment ) just find a bright star up in the east / north east ( say magnitude 1.5 to 1.0 ) at 40 to 60 degrees in Alt , centre it , start tracking , don’t use any guiding and take a 30 sec , 45 sec , 60 sec and 80 sec exposure
If your PA is good you should be getting a round Star to at least 60 sec unguided. If your only getting 30 to 45 sec exposures before the Star starts to morph then your PA is not good ( possibly +5 arc minutes ) I normally can take up to 60 to 80 sec exposures unguided on the same rig before Star shape is affected and my PA is usually around an arc minute or less
Even with guiding, poor PA can cause all your smaller stars ( not the big bright ones ) to be eggy or odd shaped usually in one direction , on subs longer than 3 minutes over a long period of tracking time

Just a few other things I thought of which may help !!
Cheers
Martin
Martin, thank you for your detailed response. My guide scope is on a shoe bracket fitted where the finder scope was originally. I have had problems with it moving. And although I can’t pinpoint the exact issue, I believe it is the main cause of my problem (there may be multiple problems, but this one I think is the cause of star drift). I will consider getting a solid Losmandy bar and mounting it on solidly.

As for guiding, I will need to check in detail tonight weather permitting.

I will also try your PA test tonight. I wouldn’t be surprised if this also is an issue, although I wouldn’t expect an overall star drift throughout the night. I’ll take an unguided 60 sec exposure on a bright star and see if there is any drift. Anything to look out for in guiding graph for poor PA?

Thanks again, Martin. Your comments are much appreciated.

Regards,
Stéphane

P.S. Attached is a 2-minute sub from last night. Note that stars are much rounder than in my 5-minute image. (Sorry the compression is terrible but you can see the stars)
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Old 13-03-2022, 02:48 PM
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Joshua Bunn (Joshua)
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To add to Martins comments, Have you got periodic error? have you trained your mounts periodic error?
On Polar alignment, You can still have great polar alignment but still see drift, this is because of atmospheric refraction (more at lower altitudes) and because of tube flexure. If you're not using a mount model like Tpoint and Protrack to correct for these things, you are likely to still see drift in an unguided exposure. Of course, it will vary depending on altitude of the subject and the stiffness of your setup.


Cheers, Joshua.
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Old 13-03-2022, 03:32 PM
Stephane
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Quote:
Originally Posted by Joshua Bunn View Post
To add to Martins comments, Have you got periodic error? have you trained your mounts periodic error?
On Polar alignment, You can still have great polar alignment but still see drift, this is because of atmospheric refraction (more at lower altitudes) and because of tube flexure. If you're not using a mount model like Tpoint and Protrack to correct for these things, you are likely to still see drift in an unguided exposure. Of course, it will vary depending on altitude of the subject and the stiffness of your setup.


Cheers, Joshua.
Thank you, Joshua, for these additional comments. I have not trained the mount’s periodic error. I will need to do some research on this.

I always image above 30 degrees altitude, never less. Would atmospheric refraction cause the image to continuously drift throughout the night though? I believe tube flexure is the issue, but I need to continue experimenting. This drift occurred throughout the night even whilst guiding. If I were to make a video of my subs, you would see the image drifting toward the top-left corner despite good guiding (not much, but enough to elongate my stars at 3 to 5 minute exposures.

Again, thank you for all these comments. They are very helpful.

Regards,
Stéphane
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Old 13-03-2022, 03:36 PM
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atmospheric refraction will cause image drift throughout the night, yes, If you are not guiding or if you don't have a software solution to it such as ProTrack.


What you're describing sounds like differential flexure between the guide camera and imaging camera. What happens if you go unguided, try a series of subs like that...
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Old 13-03-2022, 06:56 PM
Startrek (Martin)
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Quote:
Originally Posted by Stephane View Post
Thank you, Joshua, for these additional comments. I have not trained the mount’s periodic error. I will need to do some research on this.

I always image above 30 degrees altitude, never less. Would atmospheric refraction cause the image to continuously drift throughout the night though? I believe tube flexure is the issue, but I need to continue experimenting. This drift occurred throughout the night even whilst guiding. If I were to make a video of my subs, you would see the image drifting toward the top-left corner despite good guiding (not much, but enough to elongate my stars at 3 to 5 minute exposures.

Again, thank you for all these comments. They are very helpful.

Regards,
Stéphane
As you know I have 2 x EQ6-R mounts at different locations and have never needed to PEC train my mounts as both do not suffer from bad PE ( I may be lucky but both mounts were purchased 1 year apart )
Any residual PE is usually taken care of by PHD2 using the Predictive Periodic Error Correction algorithm which only requires 2 worm cycles to activate ( 16 minutes )
Furthermore, if the EQ6-R mount is PEC trained, the Auto Restore Calibration function of PHD2 will not work, also PEC restricts the use of Dithering your subframes which I use for all my imaging.
PEC training does work and provides smooth guiding but it’s not an absolute necessity for all mounts. There are many good PEC software programs to use but it’s a matter of choice, most EQ6-R mounts are reasonably well behaved.
To tighten my guiding performance I tuned both my mounts ( worm gear backlash and drive belt backlash), re orientated and rebalanced which has improved guiding performance by a considerable margin

Cheers
Martin
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Old 14-03-2022, 12:26 PM
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Thanks Martin, that’s very helpful information, especially in regards to PEC. I didn’t get to experiment last night. Too cloudy. Hopefully I get to the bottom of it soon.
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Old 29-03-2022, 08:19 PM
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Hi all,

I would like to provide an update on my eccentricity issues. Hopefully you will be in a better position to help. I re-centered the image in the guide scope, and re-positioned the guide scope in its bracket. Stars look a little better (maybe).
  • Shorter subs have rounder stars. At 1 minute, stars look great.
  • Elongation rotates with camera.
  • Polar alignment seems good.
  • Guiding overall is good, although RA is worse than dec. Eg: RA 0.6, dec 0.4.
  • RA pulses are up and down. Dec pulses are always down for some reason.

Any additional suggestions are welcome.

Many thanks,
Stéphane
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Old 29-03-2022, 08:57 PM
Startrek (Martin)
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How’s your collimation ? Maybe re collimate the OTA as best you can get
Secondly did you check or Re balance the mount / scope first to ensure you are properly balanced in Ra and Dec ( and slightly east heavy in Ra )
Are you balancing Dec correctly with the mounts Ra axis horizontally level and locked so the Ra axis and OTA form a T shape, then rotating Dec 180 degrees both ways to find good balance.

Then you need to check the mounts tracking and polar alignment first , guiding can come later ( as per my previous recommendation )
When you say “PA seems good” did you perform progressive unguided subs on a test Star ( say mag 1.5 to 2.0 ) in the east ?
Starting with 30 sec , then 45 sec , then 60 sec , then 80 sec and review all for signs of poor PA ( Star shape )
Don’t just rely on your guiding software to tell you “PA seems good”
I always run an unguided sub of up to 60 sec or longer to check PA even though my PA says under an arc min
Then proceed on to other checks like tilt and guiding .....

I’d still try and get that guide scope out of that damn offset shoe bracket and bolt it solidly onto a Losmandy bar attached to the tube rings so it’s centrally mounted along the centre axis of that big OTA. A solid mounting and tightened guide scope and camera will minimise flexure !!! The bigger bar and bolts the better !!! Those shoe brackets are prone to micron movement as it’s just relying on the thin metal wall of the OTA with this 0.5 to 0.75 kg guide scope and camera and cable hanging out at 90 degrees up to 200mm with various torque applied when the OTA slews to different angles

That’s enough for now .........
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Old 29-03-2022, 09:31 PM
Stephane
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Originally Posted by Startrek View Post
How’s your collimation ? Maybe re collimate the OTA as best you can get
Secondly did you check or Re balance the mount / scope first to ensure you are properly balanced in Ra and Dec ( and slightly east heavy in Ra )
Are you balancing Dec correctly with the mounts Ra axis horizontally level and locked so the Ra axis and OTA form a T shape, then rotating Dec 180 degrees both ways to find good balance.

Then you need to check the mounts tracking and polar alignment first , guiding can come later ( as per my previous recommendation )
When you say “PA seems good” did you perform progressive unguided subs on a test Star ( say mag 1.5 to 2.0 ) in the east ?
Starting with 30 sec , then 45 sec , then 60 sec , then 80 sec and review all for signs of poor PA ( Star shape )
Don’t just rely on your guiding software to tell you “PA seems good”
I always run an unguided sub of up to 60 sec or longer to check PA even though my PA says under an arc min
Then proceed on to other checks like tilt and guiding .....

I’d still try and get that guide scope out of that damn offset shoe bracket and bolt it solidly onto a Losmandy bar attached to the tube rings so it’s centrally mounted along the centre axis of that big OTA. A solid mounting and tightened guide scope and camera will minimise flexure !!! The bigger bar and bolts the better !!! Those shoe brackets are prone to micron movement as it’s just relying on the thin metal wall of the OTA with this 0.5 to 0.75 kg guide scope and camera and cable hanging out at 90 degrees up to 200mm with various torque applied when the OTA slews to different angles

That’s enough for now .........
Martin, some great comments. Thank you for taking the time to write that all up. Collimation is good, I’ve tried collimating using different techniques. All work fine, but don’t fix the elongation, so I don’t think collimation is the issue. Next you mentioned something that interests me that I’ve heard before but never applied: RA slightly East heavy. How much is slightly East heavy? Half a counterweight away from perfect balance? I do the T shape when balancing Dec, but must admit I don’t lock RA. I might try that too.

I tried your previous experiment up to 60 seconds. The star looked good. But I will try 80 seconds next time too.

I will purchase a Losmandy bar. Everyone seems to recommend this. I’m just not that good with all the DIY. I’ll give it my best shot. I’m convinced it will help.

Thanks so much, Martin. I have some more experiments to do. Please let me know how East heavy I need to go.

Regards,
Stéphane
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