The visual appearance of many comets, particularly in light polluted areas, is somewhat different to the often spectacular views obtained by imagers.

Although it is suggested that visual observers should ignore the contribution of the tail when making magnitude estimates, I don't recall seeing any practical guidance on how to do this. I don't make any correction myself and simply compare the comet to a defocussed star.

There is an argument that if we are trying to monitor the total activity of the comet, then the tail is part of this and should be counted, or at least the part within a roughly circular coma.

In most circumstances this isn't going to make much difference, but there are some geometries where it could - for example where the tail is pointing exactly away from our line of sight, so is behind the nucleus and coma. In such circumstances there isn't much you can do to correct for the tail in any case.

I'm not sure how much that helps in answering the question!

Regards,

Jonathan Shanklin
BAA Comet Section visual observations co-ordinator
https://www.ast.cam.ac.uk/~jds/

-----Original Message-----
From: baa-comet@simplelists.com <baa-comet@simplelists.com> On Behalf Of Nick James
Sent: Friday, October 25, 2024 7:46 AM
To: baa-comet@simplelists.com
Subject: [BAA Comets] Comphot magnitude of C/2023 A3 (Tsuchinchan-ATLAS)

It was clear here in Chelmsford last night almost up to astronomical darkness when clouds arrived so I managed to get wide and narrow field images of the comet. The widefield image in colour is here:

https://britastro.org/observations/observation.php?id=20241025_063719_c5dad64fafe03944

I've used the green channel of this image to get a magnitude using comphot but it is at the low end of what visual observers are getting.
Comphot gives 5.5 with a coma diameter of 15.5 arcmin. The visual observations on COBS for last night are 4.7 - 5.2.

My question for visual observers is how do you eliminate the tail from your magnitude estimate? This is a very dusty comet and it is not obvious where the coma ends and where the bright inner tail starts.
Comphot does this automatically, and possibly wrongly in this case, by assuming that the coma is symmetric around the photocentre. The image shows the photometric aperture that comphot used in this case.

Nick.
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