Comet C/2021 S3 (Panstarrs)

OTA: Takahashi FSQ106 f/5
Camera: ZWO AS16200MC Pro
Observatory: Insight Observatory
Date of Capture: Apr 24
Date of Processing: May 24

Exposures Used:
Total Exposure time: 58 minutes
Image Width: 3d 54'

Processing Tools:
1.    Commercial: PixInsight, Topaz, Radiant Photo
2.    Pixinsight Addons: NoiseXTerminator, BlurXTerminator, StarXTerminator, GraXpert
3.    My Scripts: NB_Assistant, AC_Restar, Subframe Weighting Tool (Excel w/ J. Hunt), ColorTweaker, StarTweaker

Target Description:
Comet C/2021 S3 (Panstarrs) was discovered in the Panstarrs sky survey. It appears to have originated from the Oort Cloud and probably took millions of years to reach the inner solar system. Its orbit is steeply inclined to the Earth's orbit and crosses the ecliptic plane outside Earth's orbit, disqualifying it and its kind as potential Earth impactors. (Only comets with orbits that interest the narrow orbital path of the Earth can be plausible Earth impactors.)

This comet passed its perihelion on Feb 24, 2024; therefore, when this image was captured, it was already returning to the Oort Cloud. It is scheduled to return in the year 4214! Where to find it and more information and graphics can be found at https://theskylive.com/c2021s3-info.

Processing Description:
I don't do many comet images, so each one relearning a forgotten workflow. This image was complicated because it has three components that would best be processed separately: the stars, the comet, and the background. We have the tools for separating the first two, but the background…that's a struggle. Obviously, a star removal program can separate the stars from the background and the comet, thereby making it possible not to blur or trail the stars when as they are processed separately and added back later. But that leaves the comet subs with the background. If the comet is aligned, the background blurs.

My solution was not perfect. I used an AI inpaint routine to remove the comet, now blurred, from a star-aligned image. That gives me a stars + background image. Next, I made a comet-aligned image. Again, I inpainted the comet and then subtracted that image from its parent image. This yields (almost) a comet-only image that can be combined with the star-aligned image. If you try this, your result will probably be OK but not great. The inpainted areas are not a perfect match to the comet+background image, and it takes extra manipulation to get a reasonable result.

Does anyone have a better workflow—I'd appreciate hearing about it!

Alex Woronow