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Hi John
I agree with Eric. You need to use good flats, especially if trying to image from light polluted areas with a focal reducer. I would also recommend separate flats for each colour filter. The focal reducer worsens the vignetting situation since it steepens the light cone.
The off centred nature of the light cone suggests an optical alignment issue. Most likely either in the focuser or the secondary. Looking at the image, the star shapes look reasonable with minor distortion but it is not clear from their shapes how far off the collimation is. I am unsure if this is a cropped image or if the chip is sufficiently small enough to not show up the problems further out. The focal reducer should not be causing the off centred effect.
Ted
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