Large circle in my images with a 200PDS which won't calibrate out.

[andyofastro]

Any ideas what is causing the large ring in my images?  It is a 200PDS with a 269C camera, Baader Coma Corrector, L Pro Filter.  I don't believe it is light leaks, the primary is covered up, focuser is covered up with felt and taped up at all joins, massive dew/light shield is used to stop stray light as well and most importantly, it is not present on individual subs. I have retaken all calibration frames, including flats, short flats, long duration flats, Darks, Dark Flats etc. The only other report of this I can find is on astrobin:

https://www.astrobin.com/.../help-please-some-circle-is.../

There was no resolution described.  It wasn't a 200PDS it was a 200P but very similar.

Thank you for any assistance you may be able to offer.

[fwm891]

Could it be that your flats are rotated by 180° ? therefore having the opposite effect on the areas they should be correcting? Could happen especially if lights and correction frames are shot at different times...

Just a thought

[ONIKKINEN]

Did you take the flats at the end of the session? Rings like these come from flatframes not matching light frames because they were taken at different times/different camera orientations/different collimations (100 little things in newtononians = always take flats for each night).

Not saying its definitely caused by flats, but sure looks like it since you mention they are not present in the light frames.

[andyofastro]

1 hour ago, ONIKKINEN said:

Did you take the flats at the end of the session? Rings like these come from flatframes not matching light frames because they were taken at different times/different camera orientations/different collimations (100 little things in newtononians = always take flats for each night).

Not saying its definitely caused by flats, but sure looks like it since you mention they are not present in the light frames.

Thanks for the reply.  I took my flats after imaging.  The scope Parked around 4am and the flats were taken mid morning.  However the camera would have been warmed and powered down between this time.  Also it was two nights imaging, should I be taking flats for each night, straight after imaging?  Thanks

[andyofastro]

1 hour ago, ONIKKINEN said:

Did you take the flats at the end of the session? Rings like these come from flatframes not matching light frames because they were taken at different times/different camera orientations/different collimations (100 little things in newtononians = always take flats for each night).

Not saying its definitely caused by flats, but sure looks like it since you mention they are not present in the light frames.

I took the flats after the second night (scope was left rigged) but would have been de powered between each night.  Should I be taking my flats immediately after each imaging session?  Like darks with a DSLR?  Thanks

[ONIKKINEN]

1 hour ago, andyofastro said:

I took the flats after the second night (scope was left rigged) but would have been de powered between each night.  Should I be taking my flats immediately after each imaging session?  Like darks with a DSLR?  Thanks

If the scope was left assembled during this time the flats should work. I have no such option and take flats each time and they always work, it doesn't take too long to take the flats so its time well spent as far as im concerned. But with a newtonian you have other things to worry about than whether the camera was removed, namely the mirror which can and should be able to move (but not accidentally, and not that much), focuser which might sag and the tube itself which might deform under different orientations/temperatures. All issues that result in any of the parts between the primary mirror and the camera sensor moving even a tiny little bit between taking the lights and flats will result in weird gradients (such as this ring).

Its all wild guessing at this point, but you have one way to see if the flats worked at all and if they only worked for some of the data. Check the calibrated frames and see if this artifact appears in all of the subs or just some of them. If some of the subs have this ring and some dont, it means you have some mechanical problems somewhere in the scope that lead to flats and lights not matching.

[andyofastro]

8 hours ago, ONIKKINEN said:

If the scope was left assembled during this time the flats should work. I have no such option and take flats each time and they always work, it doesn't take too long to take the flats so its time well spent as far as im concerned. But with a newtonian you have other things to worry about than whether the camera was removed, namely the mirror which can and should be able to move (but not accidentally, and not that much), focuser which might sag and the tube itself which might deform under different orientations/temperatures. All issues that result in any of the parts between the primary mirror and the camera sensor moving even a tiny little bit between taking the lights and flats will result in weird gradients (such as this ring).

Its all wild guessing at this point, but you have one way to see if the flats worked at all and if they only worked for some of the data. Check the calibrated frames and see if this artifact appears in all of the subs or just some of them. If some of the subs have this ring and some dont, it means you have some mechanical problems somewhere in the scope that lead to flats and lights not matching.

Thank you for your reply.  I'll blink through the calibrated frames tonight to see what I can see.  As a wider question about imaging with Newtonian's and when to take flats:  I would like to gather imaging time over multiple nights, which may necessitate removing the rig from the garden between imaging sessions.  I am therefore assuming that I should be taking flats before removing the scope between imaging sessions.  I am also assuming that those flats should only be used to calibrate the images from the same night.  Then, only after all frames from multiple nights are calibrated with their corresponding calibration frames (Darks, Flats, Dark Flats), should the calibrated frames be stacked together?

Thanks.

 

[ONIKKINEN]

3 hours ago, andyofastro said:

Thank you for your reply.  I'll blink through the calibrated frames tonight to see what I can see.  As a wider question about imaging with Newtonian's and when to take flats:  I would like to gather imaging time over multiple nights, which may necessitate removing the rig from the garden between imaging sessions.  I am therefore assuming that I should be taking flats before removing the scope between imaging sessions.  I am also assuming that those flats should only be used to calibrate the images from the same night.  Then, only after all frames from multiple nights are calibrated with their corresponding calibration frames (Darks, Flats, Dark Flats), should the calibrated frames be stacked together?

Thanks.

 

Yep, that's the best plan. I keep a folder of calibrated subs for every work-in-progress type target i am shooting so eventually when they are ready for stacking much of the work has already been done.

[andyofastro]

1 hour ago, ONIKKINEN said:

Yep, that's the best plan. I keep a folder of calibrated subs for every work-in-progress type target i am shooting so eventually when they are ready for stacking much of the work has already been done.

Thank you.  I think I now know what has caused the issues.  I only took flats on the second night/morning.  Potentially the scope focuser may have sagged/moved/temp variation/list goes on.  Therefore I think my theory, which I need to test, is that if I had taken Flats at the end of each night/morning/session and calibrated each session accordingly before stacking all data, that this issue will hopefully not re appear.  I will test on the next successive clear nights.  Thank you for your help, it has really assisted with clearing my mind and getting a workflow.

[Padraic M]

5 hours ago, andyofastro said:

Thank you for your reply.  I'll blink through the calibrated frames tonight to see what I can see.  As a wider question about imaging with Newtonian's and when to take flats:  I would like to gather imaging time over multiple nights, which may necessitate removing the rig from the garden between imaging sessions.  I am therefore assuming that I should be taking flats before removing the scope between imaging sessions.  I am also assuming that those flats should only be used to calibrate the images from the same night.  Then, only after all frames from multiple nights are calibrated with their corresponding calibration frames (Darks, Flats, Dark Flats), should the calibrated frames be stacked together?

Yes indeed, that's a good and commonly used plan. Just also fyi, most stacking software allows you to group lights by session, and assign calibration frames to each session so you don't need to manually calibrate each session before stacking the outputs. I use Astro Pixel Processor and it handles this almost seamlessly. DSS has a way to do it too. 

[Dan_Paris]

I had rings artifacts with my Newtonian (same size but not exactly the same look), it was coming from reflections inside the coma corrector (whose entry lens was about 10mm above the tube wall). I solved it by adding a baffle at the telescope end of the corrector: an M48 extension tube lined with flocking material (Protostar).  

 

 

 

 

Edited February 22, 2023 by Dan_Paris