This question came to me from a neighbor during a discussion in July 2020 about an image of Comet NEOWISE (C/2020 F3) that I had recently posted. My recollection at the time was that I had not seen such an image. A few weeks later during a Saturday morning breakfast discussion with some astronomy friends in early August, I learned that two comets would be aligned as such that they could be seen within the same camera frame, so I set about to produce an image of two comets.
The star of the two-comet scene was NEOWISE which had dominated astronomers’ attention during the entire month of July 2020. By the second week of August, however, NEOWISE had faded to magnitude 7.6. At this point it could only be seen with telescopic magnification. The other comet was PANSTARRS (C/2017 T2) at a much dimmer magnitude 10.7.
As luck would have it, as I was looking for these two objects’ placement in Stellarium, I noticed a third comet, Comet Lemmon (C/2019 U6) at magnitude 10.3. It was significantly farther from NEOWISE than was PANSTARRS, but I could still fit the trio into a single camera frame using a 55 millimeter camera lens.
Luck still staying with me, the three comets were located in a star field that included Arcturus and some other stars that would make it easier to identify the star field while I was working at the camera, even if I could not actually see the comets. I was counting on post processing to make them visible in the image.
The same day that the breakfast discussion had occurred, one of Howard Astronomical League’s dark observing sites was opened for its members, and I went out on my first attempt to image the three comets. I set up with my Canon 60Da mounted on a Omegon LX2 Mini (mechanical wind up) tracker. I had not used this device for some time, so I found myself fumbling with it way too long. By the time I was polar aligned, had the target framed, and decided on the exposure, I was exactly one hour later than I had hoped to start imaging. By then the comets were very low in the mucky part of the atmosphere, and ground fog had started to appear. I did not have a dew heater for the camera lens, so it was completely overwhelmed by moisture.
While still in the field, I was able to see NEOWISE in my unprocessed images on my dewy laptop screen, and I was able to verify that I was capturing the star field that contained the other two comets. Even though I could not see the comets in these images, I was hopeful that I would be able to pull them out in post-processing. My luck had run out. In processing the next morning, I found that I did not have a useable image of the three comets.
Amazingly, we had another lucky break in the clouds, and the site was opened again the next night night. I added a battery and dew heater to my pack up, and headed out again. Equally as important, I spent some time reacquainting myself with the tracker in the clear light of day. It was another very moist evening, but not nearly as challenging at the night before. Because I was not fumbling with the tracker, I was able get up and running and was shooting sub-frames as soon as it was dark enough. I could see right away that my data quality was much higher than the night before. I couldn’t see the other two comets, but I saw enough stars that I was pretty sure that I could pull the other two comets out in post processing.
Getting the image that I wanted out of the data that I had captured the night before required that I stretch my processing skills to the limit, but somehow I managed. Not only was I able to see the two dim comets, I cross checked against Stellarium to verify that the two green blobs were indeed Lemmon and PANSTARRS. Here is the Three Comets image that I posted to Flickr.
I must add that I am pretty impressed with the tracker. Of the fifty sub-frames that I shot, I threw away only one for a tracking issue. Not bad for a $200, low-tech wind up device?!!
Technical information for Three Comets: Canon EOS 60Da with 55mm lens at f/2.8, and mounted on a Omegon LX2 Mini Tracker. Twenty minutes total integration time in 47 x 25 second light frames. Calibration frames consisted of 25 dark frames, 25 flat frames, and 25 flat dark frames. Light frames and calibration frames were stacked in Deep Sky Stacker, and post-processing was done in Photoshop.
© 2020 Jim Johnson
