Comet Pojmanski

Discovered in early January, this comet swung by the sun in late February, appearing first for southern hemisphere observers and then in early March emerging into the northern hemisphere morning sky. Foul weather was a problem, but finally on the morning of March 8 there was a break between storms and I drove up to the Upper Meadow on UCSC property to photograph it. The sky looked clear the evening before, for Astro 8A at the observatory, and also at 3am when I awoke and drove off. But arriving at the site I could see cirrus clouds rapidly coming in from the north. The comet rose and was about 17 degrees up when I began my first 5 minute exposure. Most of the exposure happened with the comet behind thin cirrus. The second exposure was clear for about 2/3 of the 5 minute exposure. Three more 5 minute shots were all contaminated mostly by cirrus, and the last by thick cirrus and dawn. The comet was moving quickly and 5 minutes was too long to avoide smearing detail in the tail. However, the cloudiness demanded that a long exposure be taken in order to record the tail, so it was a compromise.

I tried flat fielding these, but it raised the noise significantly (underexposed flats??) and so the shots below do not include flats. They were shot through the 5" f/6.5 Megrez flourite triplet APO refractor, with the SBIG ST2000XCM camera at -30C, dark subtracted, and single-shot color processed. In Photoshop I did a very mild curve, small adjustment in brightness and contrast, and raised the saturation just a bit. I also cropped out the outer 30% of the area of the photo, which showed significant vignetting what with the Santa Cruz illuminated cloudy bright sky. The stack of the two best photos, below right, shows less Poisson noise (color grain), but the sky is brighter and the tail is more streaked; I can't call it a better photo. The left photo shows both the long ion tail and the very short faint dust tail angling below it. Using the stars of Delphinus, I estimated the head had a magnitude of 5.5.

The second exposure only: 12:52UT. 5min.

Stack of 1st and 2nd exposures, total of 10 minutes (Registax3). Note the coma is horizontally widened by comet motion during this time.