By Dean Ashton

I have been spending my evenings working on a new approach to comet processing in PixInsight. The process uses the PI Comet Alignment process as well as the PI StarNet2 process. A full full explanation of the process is at the end of this article, which you can read or download in PDF form, but here is a flow chart showing the workflow I have used here.

In early May, Nick asked me what technique I had used to create the image of Comet 2021-S3, that I took on 03-05-2024. Here is the answer. C/2021-S3 was challenging because it was travelling against a dense Milky Way star field. In this ‘non-standard’ approach I combine the use of StarNet2 with the PI Comet Alignment tool.

The approach has been applied to both C/2021-S3 and C/2022-E3 as well as C/2020 F3 Neowise captured with a Nikon Z6 camera. Answering Nick’s question has taken me on a journey of two-months of further exploring and developing the sequence of PI tools that would give me the best results.

Comet C/2023 F3 Neowise

Captured with a Nikon Z6 Camera at 400mm. Shows PI will work with Nikon RAW files (NEF) and extract the necessary date/time metadata from the NEF files just as it would from the astro-camera FITS images.

C/2022 E3 ZTF

C/2022 E3 ZTF capture last year – my luckiest and best comet image to date. Lucky because I only slewed the telescope towards it because Frank had highlighted the comet in his ‘Whats Up’ presentation at our previous club meeting.

Comet Colour

The colours of the comet depend on its composition and proximity to the Sun. If close enough, vapourised compounds in the comet’s coma are energised by the Sun’s radiation and emit light.

Carbon Monoxide – emits a blue-green colour.

Sodium – emits a yellow hue.

Diatomic Carbon C2 – emits a green colour.

Cyanogen CN – emits a reddish colour.

Hydrogen – emits a red colour.

Oxygen emits a blue/green colour.

Nitrogen emits a blue white colour.

The green coma of C/2022 E3 (ZTF) was thought to be due to diatomic carbon produced by the photolysis of organic molecules that have evaporated from the nucleus.
The red colour of the coma bow shock may be caused by ionised cyanogen vapour. However, Magnesium-Iron silicates in the coma may also create the reddish colour. The red colour that can be identified very close to the comet’s bright nucleus may be dust resettling on the nucleus that is scattering light as the comet moves away from us.

C/2021 S3 Panstarrs

Comet Alignment and Processing PDF

By Dean Ashton