Possible mirror shadow help - imaging with DSLR and 8" f/4 newtonian with coma corrector

[strmy]

Hello,

I have recently bought new telescope, Skywatcher Quattro 8" f/4. When i use it with my Canon 800D (APS-C) and coma corrector i get some normal vignetting, but also "bottom edge" of image is darker. I have come to conclusion, that this must be DSLR mirror shadow.

Now the question for you all is - have you come across this and how to get around it if it is possible at all. I Have previously imaged with f/12 scope, so i did not have such problem.

With flats i can of course mitigate this problem a bit, but because shadowing is so severe, there is much less signal present, and this bottom edge becomes useless for processing.

Attached is one stack without flats (first 2 images) and one with flats. As you can see, even flats don't completely resolve the issue. They are taken with white shirt over tube, lit from distance with lamp to achieve even light density across diameter.

 

Thanks for any thoughts and suggestions!

[ONIKKINEN]

First, welcome to SGL!

With my 550D and an f/4.4 newtonian coma corrected to f/4.2 i used to see the same thing, a noticeable dark band on one of the flat sides of the sensor. Used to think its the mirror casting a shadow as it rises out of the way, or shutter shadow because of the mechanical shutter. But if you look closely at your sensor you see that the sensor is very boxed in deep in to the sensor recess. The camera body/mirror mechanism at one side of the sensor is close enough that it could cast a shadow (like the part circled below). In that case there is nothing to do about it, except of course keep taking flats and accept the lower SNR on one side. Usually you can anticipate this and compose your shot so that there is nothing of interest on that side.

Other than that, you probably have a normal sky gradient in the image which could be confused to something originating from the camera. It could be a coincidence that the sky had a gradient that matches the sensor orientation in this case leading you on a wild goose chase.

[strmy]

Thanks for reply! Then i will just have to live with this apparently.

Of course i suspected that, because of above mentioned position of mirror, but hoped that i'm wrong.

 

I am used to all sorts of light pollution associated trouble, but here i just think my flats weren't the best.

Clear skies!