I am not sure if this is a beginner or expert question. I love to take long exposure photos for star trails. The longest I've done so far is 35min. I use a 35mm traditional SLR and I typically have my negatives scanned(just bought a Epson V770 scanner). So the question is, I heard about combining multiple shorter exposures, such as 3x10mins instead of one 30min exposure, but how would I combine the photos digitally? I use photoshop all the time for work, but it's more for illustrations. I may not be as informed about photo editing tools in PS. Any tip will be very helpful. Thanks in advance!