Be aware that with a standard 50mm f.l. lens on a 35mm SLR, you can take a time exposure of no more than about 60 seconds without the celestial objects "trailing" on the film (or CCD) due to the rotation of the earth. Shorther focal lengths = longer exposures without trailing, at the expense of smaller image size of the object.
Hope that helps.
Thanks, longshadow. I'll use hi-res, wide open (f 2.8), ISO 400 and 30 seconds. That's the best I can do with the camera. I imagine I'll get a lot of noise, but have noise reduction if I need it, if it doesn't wipe out the comet pixels. Later.