What is wrong with the background?

[rigellim]

Hi all,
 
I found that when I stretch the histogram of my deep sky images, I found that the background always has a distinct pattern. The pattern is such that centre portion of the background is brighter, while there is a darker circular shape exactly at the centre of the frame.  Is that caused by my secondary mirror? Or something to do with my sensor??
I am using 200PDS with Nikon D90.
This Leo Triplet is an example, 50 subs, 30", ISO 3200

Sorry about the photo, I can't resize it to a smaller frame.

[rigellim]

In this shot as well.

[swag72]

It is an example of vignetting which all optic trains get to some extent. This effect can be combatted by using flat frames.

[rigellim]

It is an example of vignetting which all optic trains get to some extent. This effect can be combatted by using flat frames.

I just prepared some flat frames, and indeed  the vignetting disappear, but not completely though.

Maybe I have to prepare more flats? Also, I don't really know the exact orientation of the camera on the telescope, I fear that the flats that I prepared are not accurate enough...

[swag72]

Flats really do need to be taken at the same focus and camera orientation as the lights. So after you've done your lights, it's best to do your flats before you move anything within the imaging train.

[rigellim]

Then I will have to discard this whole batch of light subs and recapture again with flats next time.