A Snowball Lobbed in our Direction!

Being that Winter is near, seems appropriate that a snowball is flying past the earth! It was scientist Fred Whipple back in the early 50s that theorized the "dirty snowball" model of comets which stands today with few modifications. The comet 46P/Wirtanen is a small, short period (5.4 years) comet discovered in 1948 by American astronomer Carl Wirtanen. It is normally very faint and rarely observed, but last weekend happened to be the closest object to the Earth besides the Moon!


I've been on the road, but am now located at "Ketelsen East" and this time, brought up a couple of telescopes so some observing can be done locally near St Charles. In fact, this last Saturday (15 December), I was signed on at a Christmas party in a local county park to show families the moon and other bright objects. Well, I arrived, but the event was so crowded there was no place to park, so went on home, photographed the nearly perfect quarter moon, and tried imaging the comet. That is the moon at left, photographed with an 8" RC reflector of 1600mm focal length. I've got no tracking mount, but the 250th second exposure froze the earth's rotation...

On this particular night the comet's orbit brought it almost opposite the sun, and almost at the closest point of its orbit, only 7.2 million miles from us. From a dark site it was said to be readily visible to the naked eye, but from my semi-urban area it was barely spotted in binoculars!

The comet provided a new set of challenges! I chose a 200mm lens which perfectly fit both the comet and Pleiades star cluster in the frame. Shooting from between St Charles and Elgin (my yard), I suffered a lot of light pollution, not only from artificial lights, but also from the moon! I also knew that the comet was moving at a pretty good clip, and it might take some effort to get sharp images. In my first attempt, I took 30 second exposures every minute (with in-camera noise reduction on), and took 30 of them to stack. The initial results are at left - over the 30 minute time frame, the comet moved quickly enough to show up as a streak, showing its motion over that time!

Another thing you can do is to align the stacked images to the moving comet, as shown at right with the same data files. While the comet is nice and sharp, the stars and cluster are now trailed! I know there is a way to render both comet and stars sharp, but I've not yet learned that trick.

What I ultimately did was to take short exposures (10 seconds!) at a higher ISO (1600) and F/number (F/3.2). stacking 20 of them together (about 3.5 minutes total exposure).  Shown at left, the stars are nice and sharp, and the comet is pleasingly rendered also without showing much trailing...  It was tough dealing with colored gradients, likely from the bright sky - a combination of moonlight and city lights.  But it will have to do for now...

No other "snowballs" expected in the near future, we were just lucky to have this one!