600 divided by focal length = max shutter speed on stationary mount to avoid apparent celestial movement. So in your case 600/85 gives you about a seven second exposure before things start to smear. Sharpness is, of course, subjective, but this is a good rule of thumb that yields satisfactory results by most persons standards, the aforementioned 250 divided by focal length being about twice as conservative. IMO the wider you are the sharper things look. Print size and viewing distance are of course factors as well. The 600/fl formula was taught to me by Wally Pacholka (one helluva nice guy), and he knows his shinola. You will notice he tends to shoot wide to give himself more exposure time, and also time to "paint" terrestrial objects in his compositions with a light. You can see his stuff here:http://astropics.com/ What will be satisfactory to you will depend mostly your own subjectivity.
Moon and stars in the same exposure would be very difficult. Compared to stars the moon is very very bright.
The mount to follow celestial movement is called an equatorial mount. It wouldn't help you shoot the moon against moving stars, however, because the moon's apparent movement is in sync with the stars. It is you that is moving, relative to the moon and stars. Of course it, and they, and everything else are moving too, but the mechanics lend the POV movement of the observer as the perceptible factor for rude observation, such as ours.
The only time I have made an observation that yielded a movement of the moon against the stars was when the moon occulted a star, and I was out with my scope looking for it to happen. It took a long time, relatively speaking, but things like that blow my skirt up.
Last edited by JBrunner; 06-16-2011 at 04:29 PM. Click to view previous post history.