April 13, 2017

A cold night ranging from about 4.4° to about 0° C. It was just me tonight, my primary goal was to capture some images of Comet 41P. I’ve not had much luck observing comets. This one was positioned fairly high in the sky and made a close pass to earth (0.14 AU) on April 1st. Too bad it was another cloudy night.

I got out there on April 2nd and grabbed a 90 second exposure. It came out kind of dim, but not horrible. I might still be able to do something with it someday.

Meanwhile I discovered a program called “Nebulocity” that does image stacking. Stacking is a technique that combines several shorter exposures to compose one better one. As a bonus, it can align the individual shots in both translation and rotation! Correcting the resulting field rotation imposed by the altazimuth mount is a huge win as long as the individual exposures are short enough to avoid star trails.

You might ask why go to this trouble? Why not bite the bullet and get setup for equatorial tracking as surely that’s what a real astrophotographer needs! True. But even so, I’d want to stack the results anyway and learning these techniques will surely payoff.

It’s tough to find good astronomy software for OS X, but Nebulocity is cross-platform and is reasonably priced. I thought about this problem before I found Nebulocity and thought maybe I could program something myself to remove the rotation? But paying the $95 cost of Nebulocity way quicker and easier than diverting my attention to another time consuming project.

So tonight I thought that I’d take a bunch of exposures and then see if I can process them into something decent.

I endured the cold for about two hours. The coyotes howled eerily in the moonlight but I got my pictures, and here is the result of stacking the seven best frames:

Comet 41P 88s ISO1600 no rotation stacked7 cropped
Comet 41P/Tuttle-Giacobini-Kresak.

The streaking star background is noticeable as 41P moves against to the background. Most objects in the night sky appear static relative to one another. But 41P moves significantly over the course of an hour. The irregularity in the position of the background indicates placement of poorer, omitted shots.

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