Stars showing trails in different directions across field

[RCRyan]

Hi there, I've recently jumped fully into the DSO side of astrophotography and have been having trouble with stars that aren't quite round. My last two imaging sessions have produced stars seemingly pointing in different directions across the field of the image. After the first image we collimated the scope and the problem has continued but we think our collimation may still be off.

I use a SkyWatcher 130PDS with a Canon 6D Mark II and the Baader MPCC Coma Corrector. The problem persists regardless of whether or not the coma corrector is in the imaging train. It also persisted regardless of the exposure time.  I have uploaded processed images which show the problem, but if unprocessed files would be more helpful I can upload them also. The processed images have been cropped slightly. 

If anyone has suggestions as to what the problem could be I'd really appreciate it.

Kind Regards,

Ryan

 

 

 

[symmetal]

This looks like the distance spacing between the coma corrector and the camera sensor is incorrect, usually called back-focus though the term has different meanings depending on what it's referring to so can be confusing.

The MPCC coma corrector has the 'standard' 55mm required spacing between the rear flange of the CC and the camera sensor. With a Canon EF mount the Canon EF to M42 'T2' adaptor which I assume you have to attach the camera to the CC has a thickness of 11mm. The distance from the camera front plate to the sensor is 44mm, (also called back-focus 😉), giving a total of 55mm as required by the CC. Do you have anything else in the image train after the CC apart from the Canon-T2 adapter as this would cause effects like you are seeing. The effects differ quite considerably between the left and right sides of the image implying the sensor is also not square on to the incoming light from the scope, usually called tilt.

Coma Correctors introduce 'opposite' coma to what's incoming to cancel it out and that is designed to work at a specific distance from the CC.

Alan

Edited July 31, 2020 by symmetal

[Blackware]

1 hour ago, RCRyan said:

Hi there, I've recently jumped fully into the DSO side of astrophotography and have been having trouble with stars that aren't quite round. My last two imaging sessions have produced stars seemingly pointing in different directions across the field of the image. After the first image we collimated the scope and the problem has continued but we think our collimation may still be off.

I use a SkyWatcher 130PDS with a Canon 6D Mark II and the Baader MPCC Coma Corrector. The problem persists regardless of whether or not the coma corrector is in the imaging train. It also persisted regardless of the exposure time.  I have uploaded processed images which show the problem, but if unprocessed files would be more helpful I can upload them also. The processed images have been cropped slightly. 

If anyone has suggestions as to what the problem could be I'd really appreciate it.

Kind Regards,

Ryan

 

 

 

Im sorry I can't answer your question but Woah 😳😳😳 Those images look sick. I got a similar telescope and am wondering how you took these pictures, do you mind sharing the exposure time, iso, amount of stacks ?

[RCRyan]

25 minutes ago, symmetal said:

This looks like the distance spacing between the coma corrector and the camera sensor is incorrect, usually called back-focus though the term has different meanings depending on what it's referring to so can be confusing.

The MPCC coma corrector has the 'standard' 55mm required spacing between the rear flange of the CC and the camera sensor. With a Canon EF mount the Canon EF to M42 'T2' adaptor which I assume you have to attach the camera to the CC has a thickness of 11mm. The distance from the camera front plate to the sensor is 44mm, (also called back-focus 😉), giving a total of 55mm as required by the CC. Do you have anything else in the image train after the CC apart from the Canon-T2 adapter as this would cause effects like you are seeing. The effects differ quite considerably between the left and right sides of the image implying the sensor is also not square on to the incoming light from the scope, usually called tilt.

Coma Correctors introduce 'opposite' coma to what's incoming to cancel it out and that is designed to work at a specific distance from the CC.

Alan

No there's nothing else in the imaging train. The tilt idea makes sense, is there any way I can check this or is it something I'll have to adjust and test as I go.

 

[RCRyan]

12 minutes ago, Blackware said:

Im sorry I can't answer your question but Woah 😳😳😳 Those images look sick. I got a similar telescope and am wondering how you took these pictures, do you mind sharing the exposure time, iso, amount of stacks ?

Thank you! M81/M82 was 20 5-minute exposures at 800 iso and Andromeda was 20 3-minute exposures at the same iso. Obviously a good deal of post processing too. 

[Blackware]

11 minutes ago, RCRyan said:

Thank you! M81/M82 was 20 5-minute exposures at 800 iso and Andromeda was 20 3-minute exposures at the same iso. Obviously a good deal of post processing too. 

Thanks for sharing! Good luck on solving your problem

[symmetal]

30 minutes ago, RCRyan said:

No there's nothing else in the imaging train. The tilt idea makes sense, is there any way I can check this or is it something I'll have to adjust and test as I go.

 

The first image shows the best stars towards the top left, while the second has the best stars towards the bottom right. If the camera was mounted with the same orientation wrt the scope then it looks like the focuser is drooping a bit, and you did a meridian flip between the two images. If everything was aligned on axis then the best stars should be in the middle and any spacing error effects should look similar and get worse towards the edges. Your images indicate one corner is too close spacing wise while the opposite corner is too far and only where the stars look reasonably good is the spacing right.

Is there any movement if you hold the scope steady and try and move the camera left, right or up and down. This lets the focuser droop under gravity causing tilt although the problem you have does look excessive.

Alan 

[RCRyan]

37 minutes ago, symmetal said:

The first image shows the best stars towards the top left, while the second has the best stars towards the bottom right. If the camera was mounted with the same orientation wrt the scope then it looks like the focuser is drooping a bit, and you did a meridian flip between the two images. If everything was aligned on axis then the best stars should be in the middle and any spacing error effects should look similar and get worse towards the edges. Your images indicate one corner is too close spacing wise while the opposite corner is too far and only where the stars look reasonably good is the spacing right.

Is there any movement if you hold the scope steady and try and move the camera left, right or up and down. This lets the focuser droop under gravity causing tilt although the problem you have does look excessive.

Alan 

There is a little movement in the camera so that seems like a possible cause. Just have to wait for a clear night now to do some testing and altering. Thank you for your help!

[symmetal]

1 hour ago, RCRyan said:

There is a little movement in the camera so that seems like a possible cause. Just have to wait for a clear night now to do some testing and altering. Thank you for your help!

Best of luck Ryan. 🙂 You could try taking an image with the scope pointing straight up as that should minimize any droop due to gravity and see if the stars look more similar overall, circling the point of best focus which should hopefully be near the centre. Perhaps take some images say 10 degrees off straight up in different directions to introduce slight gravity tilt in that direction which may indicate what direction the tilt is at a minimum. Correcting tilt is one of the more challenging problems. Your 6D being full frame means getting alignment correct is even more critical. You'll probably still end up with some star distortion nearer the corners even at the optimum alignment, as correcting over full frame is probably beyond the ability of a medium cost CC. It should be a lot better than you have at the moment though.

To get the best focus in the centre it may be worth doing the tests I just mentioned without the CC present as the CC is complicating the resultant effects seen. Without the CC the stars towards the edges should at least be a similar shape, circling the best shape stars. You want to adjust the tilt so they are the same size as well as the same shape with the best stars in the centre. Easier said than done. 🙂

Then put the CC back on and see what things look like.

Alan

Edited July 31, 2020 by symmetal

[RCRyan]

Just an update I went out briefly tonight after re-collimating but I'm suffering from the same problem. I've attached some pictures of what I'm experiencing in the hope they might shed some light on the situation. They're each a single shot of a very out of focus Polaris and show the star becoming skewed as it moves towards the edge of the frame. The problem also persists when I rotate to different stars at different angles.

Any more advice would be greatly appreciated!

[symmetal]

Have you checked the scope is well collimated Ryan? I'm not a reflector expert as I've never used one, but I assume collimation needs to be done without the CC present and should return the point of best focus/stars to the centre of the frame. At the moment it's well off towards one corner. You need to get that right before adding the CC which is possibly compounding the problem.

Edit: Just noticed you said the collimation was checked between the two images, though may not have been done right. My previous post mentioning to do checks pointing straight up to avoid gravity tilt, doesn't really apply to reflectors.

Alan

Edited August 5, 2020 by symmetal

[Jamgood]

I have the same scope and had similar problems when I first started with it. (Still not 100% perfect but getting closer.) My star tests looked like yours. Are you sure it is collimated? Are you using a laser collimator and is that collimated? My laser wasn't, it was terribly out and caused me more trouble. I got a cheshire eyepiece and have had more success with that with a little help from @Pixies in this thread >>> 

 

 Check your laser, if you haven't already. 

 

Regarding tilt with the camera adaptor which is due to the 2 grub screws pushing the connection in one direction or another. I found a layer of masking tape around the edge of it makes it fit nice and snug and reduces much of the tilt. Maybe you could try something similar?

Others with more knowledge will be along to help you I'm sure. Good luck! 👍

Edited August 5, 2020 by Jamgood