1.06 AU from Earth, 1.5 AU from Sun (Wikipedia; probably not too far off but I can’t vouch for distances on 3 Apr 2020)

Clear nights have been rare of late, and there was a lot of Moon on this night. But as has been said, astrophotography is the perfect physical distancing activity during the pandemic! Reasoning that the comet is fairly bright and not too close to the Moon in the sky, I decided to try it. This is my first try to do a comet with a monochrome camera and filters. This posed a stacking problem because of course the comet is moving with respect to the background stars. I’m way out of practice. Deep Sky Stacker did a reasonably good job. Stacking the background stars was easy; I just did that the way I always do, in this case registering to the highest-scoring B subframe. On combining the color layers, I got nicely separated red, green, and blue spots for the comet with nicely registered colors for the stars in the resulting composite image. Although I think there is a way to automatically register the comet, I didn’t use it; I manually selected the comet in all 60 subs (which was a pain and wasn’t done particularly well). That allowed me to stack the R, G, and B subs on the comet, producing some blur in the stars. These then combined fine in StarTools to make a nice color-registered comet with offset red, green, and blue spots for each star. I overlaid this comet-only stack on the stacked star background and aligned the comet only stack with the green comet spot and the green star spots with the composite stars. Finally applied a hide-all layer mask to the comet layer with a hole in the mask to let the composite comet show up. The green color is interesting. Wikipedia indicates it is due to diatomic carbon.

Date: 3 Apr 2020

Subject: C/2019 Y4 (ATLAS)

Scope: AT8IN+High Point Scientific Coma Corrector

Filters: ZWO 31 mm diameter unmounted R, G, B

Mount: EQ-6 (EQMOD 2.000j)+PEC

Guiding: Orion Thin Off-axis Guider + ASI120MM-mini +PHD 2.6.7 (Win 10 ASCOM)

Camera: ASI1600MM-Cool, -20 °C, Gain 139 Offset 21

Acquisition: Sequence Generator Pro 3.1.0.457

Exposure: 20x60 R, 20x60 G, 20x60 B

Stacking: Deep Sky Stacker 4.2.3 (64-bit) dark+flat+bias, κ-σ stacking with κ = 1.5 (see above!).

Processing: StarTools 1.6.392 beta: Combined R, G, & B in StarTools. Software binned 2x2, cropped, wiped, developed, color with a lot of saturation boost, untrack denoised (grain elimination). Layered as described above in Photoshop. Several rounds of levels and curves, and deep space noise reduction. AstroFrame.