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.
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 when the final image is obtained from Registax or Deep Sky Stacker.
The image can be scaled down or scaled up to the required 4:3 aspect ratio
Steve Wainwright