Thanks everyone for writing back! Haven't logged on in a couple days so I'll try to respond to everyone. Sorry if this is bad form, but I'm new to the forum.
Hello Mark, the EF USM and IS Canon lenses that I shoot with have settings slightly past infinity to compensate for changes in temperature. For infinite focus at normal temperatures, whatever normal may be, +70 I guess, manual focus is dead on the vertical line, but temps here on clear winter nights can be -35 F. Setting the infinity focus on the Minolta Maxxum with AF lens works fine by just twisting it to the end regardless of temperature. Just guessing, but I think temperature may be the issue here.
Hi Frank, I've tried autofocus just before dark and taping the lens at that point, but possibly the falling temperatures cause slight distortion within the lens elements that alter focus. Also have tried leaving the camera out for a while before shooting to equalize its temp with the outdoors, the same as one does with telescopes for best imagery. For whatever it's worth, there is nothing light around home to autofocus on at infinity at night. The stars are extremely bright and look in focus to me at a range of settings. Sometimes the Auroras are dim and-or almost motionless so a 10-20 second shot builds up image. Other times they are extremely bright and move so rapidly any shot longer than a second can produce blurred images. That variation produces the learning curve for me.
Hi to you too, Bolantej. Side-by-side shooting with the Minolta at ISO 400 has produced very nice photos with no graininess and sharp star images. I was concerned about grain at higher ISO, but will try faster shutter speed and higher ISO. Maybe I have a bad 50 mm f/1.4 USM. I may stop it down a little or just try another lens next time for a point of comparison. Thanks again to everyone for all the help and suggestions. Ed