Comet PanSTARRS, question

Check out the exposures of the successful shots already posted. You will see that they range from about 1 second to 15 seconds, typically at f/8 or so and ISO 100-200. (My "best" shots were shorter exposures at higher ISO to minimize swaying trees in the foreground. Would have done better to go longer exposure, lower noise.)

The answer really is, it depends. The clarity and brightness of your local sky and the altitude of the comet make a huge difference. The last 5 degrees above the horizon add up to 5 f-stops to your exposure. I shot most of my shots using auto exposure with the spot about 50% dark horizon and 50% twilight sky to get me to a starting exposure, and then bracketed widely, not to mention a large measure of intuition.

Use as wide an aperture as you can to keep the exposure short without degrading the image quality. (Hence my cautious f/6.3).

As for focal length, most comet pictures benefit from the context of the foreground in silhouette or interesting lights. I shot at 90 mm, but only because in my haste I didn't bring my 70-300 mm zoom. The best shots from last night seem to be in the 150mm range. You are not going to see a lot of detail in the comet even at 600 mm and you will lose context.

If you keep the exposures under about 10 seconds, you don't need tracking.

--
Bob in Baltimore