Above is a before and after shot showing the effects of using a flat-field filter on DISR images. The image shows the MRI (middle image in triplet) from Triplet 535. An HRI example, using the bottom framelet in triplet 404, can be found here. In both examples, original image can be found on the left and the flat-fielded and noise-filtered image is on the right. The TIF-format flats I used along with a short explanation of how I created them can be found in the zip file here. Flat-fielding can be accomplished in many astrophotography software packages, like Giotto. Flat-fielding is important for removing CCD artifacts like variations in pixel sensitivity which can cause gradients to appear in digital images. In the MRI flat, the brighter region along the tops of each images is removed, for example.
Rene Pascal, who created the wonderful mosaic highlighted below, got me thinking about DISR flat fielding and has posted a detailed explanation of how he processes these images along with the MRI flats he uses on his website. He doesn't have an HRI flatfield, but his work on the MRI flatfield was much more analytical than mine.