One thing I failed to mention is that many of us use only scientific filters (UBVRI) in our cameras yet these same filters can still produce very nice images. Again it is how you expose the images through the filters and how you combine them into the final image. Patrick you can have your cake and eat it to. Also the reason for median combining is to eliminate any cosmic ray hits that occur in the flats. As for the number of flats or darks that will be combined to form a "master" flat or dark, remember the noise reduction that occurs with using multiple images decreases as the square root of the number of images used. Thus using 15 images reduces the noise by 3.87 (16 images would reduce it by 4). Jerry Foote ScopeCraft, Inc. 4175 E. Red Cliffs Dr. Kanab, UT 84741 435-216-5450 jfoote@scopecraft.com