You're better off buying a fully manual camera like the FM. Shooting star trail requires very long exposure and is not possible in aperture priority mode. The meter would be wrong because startrail pictures include mostly dark sky. The light level is too low and is below what the meter can read accurately. None camera in aperture priority mode would have long enough shutter speed for star trail. You must do it in manual using B or T. If you use a film that is faster than the max speed of the camera you can compensate but I don't think you can find film that is faster than the max ISO on most cameras. When someone say pushing 400 to 800 they mean that they would set the ISO at 800 and shoot it just like 800 film. And then when they have the film processed the film will have to have be pushed processed generally by leaving the film in the developer longer than normal.