Last Saturday it was almost midnight when I changed my mind about photographing 46P/Wirtanen. I had talked myself out of it, but Saturday the 16th was the closest approach of this comet that was getting a lot of press (“Green Christmas Comet”) considering how dim it was. Also the skies here were pretty clear, and by midnight the moon had set below the Sierra. Barely visible unaided, near the Pleiades, it looked rather nice through binocs (though unspectacular). The moon was going to get worse in the coming nights, so it was now or never.
I made the decision to go for it, taking some shortcuts with the setup and setting a goal to be done within two hours. It went fine.
Later, when I pulled the ten 1.5 minute exposures off the camera, I was surprised how much the comet moved over the course of 15 minutes. Of course comets move, but so do a lot of things in space; just not many move noticeably in such a short span. This repeat visitor was indeed close to us.
It’s kind of neat to see how quickly it cooks along. Here is a GIF of the 10 frames and 15 minutes of motion:
Now to stack the images rather than animate them. The software I use to combine multiple exposures is called Deep Sky Stacker, and it has some interesting comet options. In the first example, here’s how much the stars moved in relation to the comet as a fixed point:
If you stack the 10 exposures and fix to the comet, you get the above star trails. If you fix to the stars, you get one very blurry comet:
Finally, deep Sky Stacker does have an option called “Star Freeze,” allowing the 10 stacked images to keep both the stars and the comet fixed:
Clearly some extra image processing voodoo going on here, but I like it anyway. It also makes me realize that if I were to go to the trouble of tracking a comet with the mount, I’d be stuck with star trails and it would be a challenge to get a similar picture. The 1.5 minute exposures would yield a crisper comet if tracked, but hey, they’re fuzzy anyway, right? Lots of trade-offs when you’re trying to shoot comets, I guess.
Images 400mm refractor (AT-66), ISO 1600
Scott's Sky Shots 
Very cool, Scott. Your astrophotography skills are impressive!
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Thank you. You’re a great photography cheerleader as well as a great photographer.
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I am always impressed with your astrophotography, but you managed to to make star watching dynamic when it is usually pretty static. (aside from meteors, eclipses, and occultations.
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I almost went back to correct the line “comets move, but so do a lot of things in space.” _Everything_ moves, of course, but it’s all relative. Sort of like the comments on my posts. (Relatives, get it?)
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The Green Christmas Comet seems to be moving a lot swifter than Halley’s Comet did! Nice work Scott, looking forward to checking out your setup.
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You’ll check it out real up close and personal if I don’t move all of my gear out of the guest room -heh-.
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I like the 3 different stacking modes that you created; great demo.
It is really nice that you have clear skies in December. I won’t see a star, or any comet, until about July 4 😦
Deep Sky Tracker is a fantastic program that I have used. I found the “star freeze” option to be very uncommon in these types of programs, and a real neat feature. It worked well with a comet that I photographed about 10 years ago. I almost burned up a kitchen (for a second time) while photo-ing the comet, but that is another story…
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DSS is amazing, but though I’ve been using it for years I still feel really clumsy with it. I want to better understand how each setting will effect the output but there’s just too many combinations to test and I don’t understand the principles of many of them well enough to really know what I’m doing. Fun to try though. Looking forward to that kitchen story some day.
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Neat-o! So happy we are here. Jim’s comment makes me realize how fortunate our sky clarity in winter is!
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