The largest planet, Jupiter is around 45 arc seconds - 0.013 degree! So to get a decent number of pixels in the image you must use very long focal lengths - 10,0000 mm is not too long. First off planets have an incredibly small angular size (measured in arc seconds). So why stack planetary images? Bear with me. So you'll have to use something like Bootcamp or Parallels to run a virtual Windows session. Unfortunately both of these programs are Windows only. Registax is free, and I believe Avistack is also. For that type of imaging, one of the best programs is Registax, but others use Avistack.
I will just answer your question in the area I'm most knowledgeable - lunar/planetary imaging.
There are a number of different kinds of astrophotography that benefit from stacking. If I may ask a rudimentary question: what is the purpose/goal of stacking astrophotography photos? I understand merging exposures for hdr, but what is the deal with stars, the moon, whichever? I would like to find out as my girlfriend loves everything astronomy.
