Comet Catalina’s “flyby”

Comet C/2013 US10 was discovered on 31 October 2013 by the Catalina Sky Survey, an automated system aimed at searching for asteroids and comets. As perihelion was expected on 15th November 2015 and the closest approach to Earth on 17th January 2016, the best “observing window” would be between late 2015 and early 2016.

In the fall of 2015, the comet was a southern sky object, but its swift northward motion would rapidly increase its declination, so that it would emerge in the morning sky in constellation Virgo in late November 2015. Later, it would zip past Arcturus and it would become circumpolar in the Big Dipper later in January. This would ensure optimal observing conditions from the entire northern hemisphere.

What’s so special about this comet? The orbit. Calculations have shown that it most likely originated in the Oort cloud, and with an eccentricity greater than 1, it has a hyperbolic orbit. Therefore, comet Catalina is very soon bound to plunge back into the depths of outer space, thus leaving the solar system for good.

Unfortunately, the peak brightness turned out to be lower than expected, just shy of magnitude 6 compared to early estimates of about 4.5. Nevertheless, it offered some nice imaging opportunities between December 2015 and January 2016.

Of course, I did my best not to miss this comet. However, due to several factors (weather, often with cloudy or hazy skies in spite of the long high pressure spell, busy times and some mistakes I made in choosing the right equipment), in the end I didn’t get much for the effort spent. I made a total of seven imaging attempts:

1. 12th December 2015 (pictures already published on my website)
2. 31st December 2015
3. 7th January 2016
4. 12th January 2016
5. 18th January 2016
6. 21st January 2016 (from home)

Below are my pictures, taken with a variety of OTAs (70-300 mm Canon zoom lens, Pentax 75 SHDF, Pentax 105 SDHF, 8” GSO RC) and detectors (Canon EOS 500D and 6D DSLRs, Moravian Instruments G2-8300 CCD). More info is available in the caption of each image.

First off, here’s my image from December 2015:

Then comes first image of 2016, taken on 13th January with a 70-300 Canon zoom lens and a Canon 6D camera. Tracking was far from perfect. The oscillation in the star trails is probably due to some periodic error, or the tripod got bumped (any hypotheses are more than welcome  ).

On 13th January I also took a pic with a narrower field rig (Pentax 75 SDHF + Canon 500D). Short exposure, but decent result:

This photo clearly shows that the comet’s appearance changed significantly from the December picture. In particular, the ion tail was much fainter, probably due to the comet lying farther away from the Sun and different geometric conditions.

Here’s another shot from 18th January, taken with a Pentax 75 a Canon 6D in harsh weather (temperature ranging from -8 to -10 C with wind gusting up to 40 km/h):

Finally, here we come to 21st January, where I managed to take a longer FL exposure through a GSO 8” RC from my backyard, just in time before the sky was engulfed in full Moon’s light:

On the whole, the overall quality of my exposures was affected by some mistakes I made in choosing the right equipment and the exposure times, a bit too short to detect the elusive wisps in the comet’s ion tail in January. In particular, I should have taken advantage of the 6D’s very low electronic noise by using it at 6400 ISO. Finally, worth mentioning is the comet’s fast proper motion, which even exceeded 400 arcsecs/hour in the days of the closest approach, which caused blurring even in short exposures (1-2 mins).

Anyway, I enjoyed my time spent hunting down the comet. Dear Catalina, so long… It was fun meeting you!