One of the cool iphone apps I use for astronomy is Starmap Pro which includes a great “find comets” feature. Ironically, the brightest one coming up was also the very same one that was mentioned recently in Astronomy magazine as “passing near M15″ and was pretty bright – magnitude 8!

M15 and Comet Garradd

M15 and Comet Garradd

Processing comet images can be a challenge. In particular, comets move fast! Well, they move fast with respect to the background stars. This creates challenges for “stacking” images because it will cause either the comet to streak or the stars to “streak”. There are a few choices:

  • No stacking / single frame. Usually this will limit exposures to about 2 minutes. It will make it hard to get much of a tail and hard to put anything other than stars as background, but can still produce pretty nice images. For best results, you’re going to want to have your camera tracking via polar alignment. If you’re not able to track, short focal lengths or alt-az tracking will also let you take images in the 30-60 sec range which might be enough to catch some detail.
  • Autoguiding on a star/background object. If you have long exposures (again around 2 minutes for most comets), you’re going to have the comet itself turn into a big streak. This will look neat, but probably isn’t what you want. The background of course will be easily stackable and look nice. In fact, this is how many comets and/or asteroids are discovered: they show up as short streaks in long exposure images.
  • Autoguiding on the comet. This produces fabulous comet images if it is bright enough for your autoguider to see. Sometimes I find that binning 2×2 and using longer guiding exposures here helps (5 sec!) for the guide cam to show the comet. Even though this will streak the background, you can take very long exposures of the comet.
  • Single point stacking on the comet. Take multiple frames, but when you go to stack them in your imaging software, chose the comet as the stacking point. This is has the same streaking effects as autoguiding on the comet, but for comets that you can’t pick up in the autoguider, it’s a good alternative.
  • DeepSkyStacker. This software has a great “comet stacking” which will take into account the movement of the comet against the background stars.

Comet C/2009 Garradd

Comet C/2009 Garradd

For some comparison, the above image is a single 2 minute exposure, while the one to the left is a stack of images with the center point on the comet. Another interesting thing to note is that the direction of the comet (marked on the image!) is not the same way that you’d think it were by the tail. If you recall, the comet tail is created by the solar wind, so it will always point away from the sun. However, this comet is actually moving closer to the sun in it’s elliptical orbit. This creates a bit of an illusion when looking at the image — it almost seems like the comet would be rising to the upper left corner from the tail, but in fact it is moving to the upper right!

What do you think? Which framing do you prefer? What other comet stacking methods do you use?

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