In video astronomy, video is often saved to DVD from analogue video cameras such as the LN300, Mintron, Samsung SDV-435 or SCB-2000 frame-integrating video cameras and other analogue security type cameras, the data are stored in VOB (Video Object) files on the DVD. When the BMP files are extracted from the VOB files using software such as VOB file extractor, The aspect ratio of BMP images extracted from PAL VOB files on DVD have the dimensions 720 x 576 which is NOT the 4:3 VGA ratio required. The result is that circular objects are seen as elliptical in shape with the vertical axis being longer than the horizontal axis. Also, some USB analogue capture cards only offer aspect ratios that are not 4:3.
For a BMP with the dimensions 720 x 576 all that is required is to rescale (not maintaining aspect ratio) to 640 x 480, or to the larger PAL 4:3 image size of 768 x 576. Rescaling should be done at the end of the stacking process when the final image is obtained from software such as Registax or Deep Sky Stacker. Aspect Ratio Converter is a Windows program that installs and runs just as well in Wine under Linux.