Strange circles in my image Generic equipment discussions · Anthony (Tony) Johnson · ... · 6 · 119 · 2

Posted is a screen grab from Pixinsight. This image is very curious to me because I'm not sure whats going on here. So some info

51 images stacked in Siril, with no flats, darks, biases or dark flats. I wanted to see the raw image with no calibration frames.

Equipment used:

12" Older Meade SCT
Celestron reducer
a TPO focuser on the back of the celestron reducer
Camera is a modified Canon 60D
ISO was set at 1000
Filter used was the Antlia Triband filter
Camera was connected to the focuser via a t ring adapter to a 2" barrel slid into the focuser
The backfocus for the combination of the focuser on the reducer was around 140mm to the focal plane of the camera. I realize the backfocus requirement for this reudcer is around 105mm. So I'm outside of that by about 35mm, not sure if that would account for what we are seeing here.

Also I realize there may be some collimation issues going on as well, but thats not what I'm seeking help with, that can be easily fixed.
The reported focal length of the scope, reducer, and focuser combo by the ASIAir plus was 1451mm, the native focal lenght of the Meade 12" SCT is 3048mm.
The 2 questions I have is why do I see a center shadow as though I'm getting a secondary mirror shadow due to low magnification, which I can't see this be an issue in astrophotography, but I do know it can be an issue with eyepieces, and why am I seeing an outer ring. Is this indicative of a backfocus issue or something else. And if this is not a collimation issue then why is the outer ring and the center shadow not centered in the frame, like just simple vignetting would be.


One other thing before its mentioned, I do not have any way to adjust tilt. I don't believe its a tilt issue but I'm at a loss.  Also the conner stars don't seem to exhibt the characteristics of bad collimation or backfocus issues. If its vignetting, thats a lot of vignetting. Again that may come back to a backfocus issue, Any help will be greatly appreciated.  I do plan on not using the existing focuser and using one I have on hand that is more in line with the 105mm backfocus requirement of the Celestron reducer. The reason I didn't use it before, 1 I had to adapt it to the SCT read cell threads. and 2 its a single speed focuseer and the TPO is a dual speed.

Never had any issues with light leaks. Took 3min darks and there was no evidence of a light leaks before, those were taken in the day time with the end cap on.

That is an internal reflection, the RASA 8 suffered with something similar in the early days and looked exactly like that

Tony Gondola:
Whatever it is, I'm sure it will calibrate out. I would do that and see if it's still an issue. You might be chasing a problem that isn't.

In doing some more exploration I’ve figured, the camera was too far back, and the scope is out of culmination. I didn’t some adjusting with a Cheshire eyepiece I have and taken new flats they are not perfect but much better, using the flat contour plot in pixinsight I could see exactly what was going on with the flats, they were whacked out very badly. When I stacked the image at first the problem calibrated out somewhat but I was left with what looked like a swirl in the image, that ended up being part of the circles that didn’t calibrate out. I’ve move the camera in and will adjust culmination better when I can get it out under the stars and use a star to adjust it with. Thanks for your feedback.