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Lately, I have been imaging comets; here's why:
- not a lot of coverage by imagers when they are fainter than 8th mag
- brighter than 12th mag I can capture them from the backyard
- hones my technique for brighter comets or dark-sky imaging
- never sure if a comet is undergoing an eruption
- interesting to see evolution and tail changes, compare to predictions
My basic set up is a 70-200mm telephoto (67mm aperture), sitting on a Star Adventurer tracker.
I use a wraparound heater to keep the frost off the lens, and a Bahtinov mask for focusing.
To overcome the skyglow from the city, I typically expose for a total of 60 minutes, in 30 second subs. The shorter exposure is to mitigate the loss due to periodic error on the drive. That way, the stacking program automatically rejects the 1/5 of images that trail due to the error and I do not have to go over it manually.
I've been shooting a lot from the backyard, in part because it is easier on family life, or it's too cold, but it also gives me the experience I need to collect to beat back the gremlins and develop a good routine. Because the signal is so low compared to the foreground light pollution, it takes a lot of exposures to bring the object out from the backyard. So when the opportunity came to travel with Mike Noble for some potential aurora, it was a no lose situation: I could do long exposures on the comets if the aurora did not materialize.
C/2016 R2 (PanSTARRS):
R2 has been pretty consistent 9th mag because it has been on a slow arc through the solar system, never getting closer than Mars, maintaining a relatively constant distance from the Sun and Earth. I could catch it from the backyard quite easily, but even with 48 minutes exposure, there is still a fair bit of noise from the light-polluted site. I wanted to get a shot of it under darker skies because of the lovely blue tail seen on APOD. You can see the blue sheen here, but the 67mm aperture can only do so much.
C/2017 T1 (Heinze): this traveller from the Oort cloud was a bit of a surprise, being discovered only in June, brightening quickly, but never up to 10th mag. Other imagers estimated 10.5-11th mag, but I failed to pick it up. I pulled out the stops to catch this from the backyard, giving it 1h46min total exposure! Still I could not extract it out of the background. I knew I had to guide on the comet, but the software I used (ImagesPlus) can only do it when you click on the comet on each frame. That only works if you can see it. So I developed a program to shift the images with the predicted movement, then put them back into ImagesPlus for stacking:
Sadly T1 Heinze was too low by the time we were set up out in the country.
62P/Tsuchinsan:
I also knew 62P was too faint for the backyard,One of the fun things is finding NGC galaxies in the field, which give me a reasonable yardstick for estimating magnitudes, since the NGCs are well calibrated. Comets and galaxies do have different morphologies, surface brightness and sizes, and my backgrounds are not photometrically great, but it's close enough to say that a comet is fainter than 13th as opposed to being between 10-11.
Another fun one happened to be floating up near "the guards" the pair of bright-ish stars at the end of Ursa Minor's Little Dipper bowl. It's not often a comet hangs up by the pole for a couple of weeks. The sign of a slow moving comet. Slow means distant from the Sun. Distant generally means faint.
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