What is least powerful (magnification) telescope is needed to see an astronaut doing spacewalk?
An astronaut close enough to be seen with a telescope would be moving much to fast to be held in the field of view.I have tracked the ISS with my 6″ dob.but again it is moving very fast,your best chance to see this is with binoculars. Your friend is making a good choice with an 8″ dob.and should be able to see all 110 Messier objects from a dark site. You can go to this site and put in your location and time zone for times the ISS will be visible and try to track it.
Most people would need 150X magnification with a good 12-inch collector, and clear atmosphere. Even then, the astronaut would appear as just a large dot. Since anything in near-Earth orbit is moving very fast, you will have a lot of trouble keeping the telescope aimed steady at the astronaut. An 8-inch collector should have no problem seeing the shape of the ISS (73 m x 108 m).
