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July 25, 2020

The July Comet C/2020 F3 NEOWISE: Quite a Show!

by John A. Blackwell

Comet C/2020 F3 NEOWISE was definitely the comet of the year, one of the best in a couple of decades. Hovering low in the morning sky before dawn, then, later in July making its evening appearance low in the northwest sky after sunset, it provided hauntingly beautiful views of the night sky. Not much impresses more than a comet with a visible tail.

By July 20th, the comet was easily seen without optical aid, about 25 degrees above the northwest horizon just after sunset when the stars started to come out. It sported a long dust tail: to my eyes it appeared some 5 degrees long, but others were calling for longer, even up to 15 degrees. In either case, it was impressive. In binoculars it gave an even more stunning view. Below is a wide field photo I took of the comet on July 20 taken with a tripod-mounted Nikon D-810 and a 28-300mm Nikkor zoom wide open for 10 seconds at iso 1000. No tracking was used. Click on it for larger image.

Comet C/2020 F3 NEOWISE 20 July 2020

This wide field show is a 10s exposure with 28-300mm lens on Nikon D810 iso 1000.

Desiring a closer look at the details in the comet’s tails, I needed to use a tracking system with higher focal length for better image scale. The best bet for such a large object is a wide field astrograph. I grabbed the Takahashi FSQ-106ED4X, a splendid modified Petzval refractor telescope, and placed it on a portable Losmandy G-8 mount, battery operated, for the field. Attached to this was a Nikon D-810a, the modified version of the 810, allowing more H-alpha light to shine through. Nikon made this camera deliberately for astro-imagers, but this comet doesn’t have a whole lot of H-alpha light to speak of.  I set the system to tracking the stars and had the comet imaged for 1-minute exposures throughout the night at iso 1000, f/5, FL-530mm. By the end of the evening, all the gear was coated in dew except the heated objective lens (phew!) and I had some 30GB of images to deal with.

Once home and slept, the arduous task of slewing through all the images began. There were so many! I culled off the ones that had interference from automobile lights, lasers from people nearby shining them at the comet, kids with flashlights, and airplanes passing through the field. I kept the rest, some 30 near-perfect images. An example here, an unedited 1-minute exposure:

Comet C/2020 F3 NEOWISE 20 July 2020

Unedited 1-minute exposure of the comet.

It’s a very nice image!  The issue is that there is still some vignetting to deal with and some image noise from the heat and readout of the camera’s chip. There is always inherent noise to any digital system, and the solution? Stacking those images together in a median and getting a much cleaner and sharper image. The process is time consuming and CPU intensive, but well worth the effort. Stacking also gets rid of any one-time events like satellite passes (keep reading for more) which streak across the frames throughout the night. Surprisingly there are many. The end result of all the processing is this image below. It pretty much presents a fine portrait of Comet NEOWISE. The dust tail certainly spreads widely throughout the frame, and the blue ion tail is more than evident.

Comet C/2020 F3 NEOWISE 20 July 2020

This was taken through a Takahashi FSQ-106ED4X telescope at f/5. iso 1000 with Nikon D-810a. This is a stack of 30, 1 minute exposures.

What about those satellites? Just how many are flying through all those images? Watch this video to see!

So many satellites!  If you go through this video frame by frame you will also see one frame that has two streaks from a nearby green laser pointer. Alas.

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