|Comet C/2013 R1 Lovejoy and a Geminid Meteor, December 2013|
I see that my exposure was 8 seconds at ISO 5000 on a 50mm lens. It was an f/1.4 lens, so I was able to shoot at f/1.6. No star tracking mount was needed for up to a 10 second exposure with that lens. With that long of an exposure, shouldn't the stars appear to be moving across the sky? Using the "500 Rule", as long as the focal length times the exposure in seconds doesn't exceed 500, you should be fine. So 50 x 8 = 400, and you get no visible star movement.
If you try a 400 mm lens however your maximum exposure goes to 1.25 seconds, so you'd need a star tracking mount. Most people don't have star tracking mounts, so a good compromise might be an 85 mm lens offering f/1.8 or wider, keep the exposure around 5-6 seconds or less, and boost the ISO a bit is necessary. The comet is approaching Mars now, so try it tonight, Saturday, for practice.
Seen from mid northern latitudes this weekend, Mars will be visible to the southwest from the end of evening twilight until it sets around 9/9:30 pm or so. Good luck!
I performed lens tests with Comet C/2-013 R1 Lovejoy in 2013. I've put together a time-lapse video today to show you how they did. I've uploaded the video to YouTube here:
+NASA Goddard has produced a cool visualization showing how Comet Siding Spring will pass Mars, in this video: