Creating a flat field from light exposures

On the 2003-03-06 imaging session, I used my newly obtained FR/0.33 focal reducer for the first time. The images obtained using it show severe vignetting. For example, the following image is of Seyfert's Sextet (50% of original size):

At the lower left, the average ADU is 24% less than in the center of the image. Also, the vignetting is off-center.

By combining 49 images from different fields that night, all with the same camera orientation, and applying a dust and scratches filter with radius 30 and blurring with radius 30, the result is:

Applying this flat field to the above image, the image now looks like this:



Here the black and white points remained the same. There is still some residu of the vignetting, but it is not very severe.
The difference of the average ADU is now much smaller. There is now only a difference of 1.4% between the center and the border. I don't know why this central area is overcorrected by this amount. It is not a replacement for a real flat field of course, as the noise is much higher (not half full-well limit).

I returned this focal reducer, because the 24% vignetting is unacceptable. In addition it also created severe coma. Not only in the corners of the main CCD, but especially in the tracking CCD all stars had a V shape. The replacement only exhibits a 7% vignetting and much less coma.