Problem(s) Encountered:

In many images or imaging flat field, there is a dark region, which I don''t know the reason.

Solutions:

I’ve attached two images below.  That dark region on the left of the displayed frames is the guide probe vignetting the image.  This is easy to do when using OSMOS for direct imaging.  See the following link:

http://www.astronomy.ohio-state.edu/MDM/OSMOS/#guiding