Indeed - I neglected to take flats at the time (not sure of an easy way to do this at night during a star party without upsetting the astro neighbours) and of course I forgot to take note of the orientation of the camera with the optical tube for later.
Good news is that most of the tilt I was suffering with this OTA/camera combination is now gone by using an extension tube so I don't have to rack out the focuser very far. Still seems like there's a small amount when pixel-peeping at full-res.[/QUOTE]
Couple of things.
You can do a false flat in Photoshop later if you have to.
Also when taking flats unless you have something sticking in the scope that creates an uneven illumination rotating the camera around makes no difference. The unevenness of illumination is vignetting and is usually symmetrical and turns with it. The scope itself is round.
I take flats at dusk with a white cloth over the lens or sometimes in the morning in a roll off roof with the roof closed, a white cloth over the lens and pointing at a dull wall slightly illuminated. Both work, dusk flats seem more reliable. At a star party that would simply leave dusk flats.
I find flats a bit of an art and now I never assume they are ok and test them on actual images to see how well they correct and retake them as needed.
I am finding the can be too bright and overcorrect but if too dark they don't correct enough.
If all you want to correct on this image is a tad of vignetting simply use the vignetting tool in lens correction in Photoshop or the vignetting slider in Lightroom or any other image processing program. They usually have a vignetting correction slider.
Greg.
