Quote Originally Posted by modafoto
So the conclusion must be to manual focus at infinity, use a tripod and expect tack sharp slides?
Almost. The only variable is the focussing, whether or not infinity is at the far mechanical stop, or if the infinity mark on the lens barrel is accurate, etc. You should be able to get very close by looking in the viewfinder. If your camera has an "in focus" indicator, it should work or at least get you close enough that DOF will cover any errors (DOF at 250K miles is pretty big!)

Quote Originally Posted by wfwhitaker
While it sounds like most of your sharpness issue may be related to hand-holding the 300mm lens, don't forget that the moon is a moving object. Motion is magnified along with the image when using a long lens.
From "Astrophotography for the Amateur" by Michael Covington, the rule of thumb for fixed-tripod shots of astronomical subjects:
Longest shutter speed = 250 / Focal length in mm.
For a 300mm lens, 1/2 second or faster will be enough to prevent motion blur of the Moon. You should be using mid-apertures (f/8 or f/11), so your shutter speeds should be 1/125 to 1/800 depending on film ISO, much faster than the 1/2" maximum.