Weird image produced of M42

[maksovnik]

Hi there, I tried to take an image of the M42 Orion Nebula using 20mm Wide Angle eyepiece projection with a T ring adapter on my Nikon D3200 and my SkyWatcher 130/900 telescope. I managed to get the core of the nebula mostly in focus however upon observing the images I noticed that the stars nearer the edges of the image are completely blurred and out of focus. This meant that when I tried to put my images into Deep Sky Stacker the result was not what I wanted.

As a sidenote I have tried prime focus and I can not move the focuser in enough to get into focus via this method. I can use a barlow but again the same issue as mentioned in the previous paragraph occurs where things near the outside of the image because blurred.

One of my exposures (20 second for each)

Deep Sky Stacker Result

How can I fix this??

Thanks
Maks
 

Edited January 21, 2020 by maksovnik

[JamesF]

I'm not sure there's much you can do about this.  I think it's an artefact of the optical design using a spherical rather than parabolic mirror and is probably impractical to correct.  I don't think this particular telescope is really designed for wide field imaging so you may have to live with what you have as the best you can get.

James