Thomas,

Yes, I agree that there is almost certainly a fairly complex log(d) term
in the fit which is not currently in these curves but the small aperture
magnitudes show changing biases even over quite small changes in d so it
would be useful to understand what is going on.

The amount of scatter in the small aperture photometry is very small
compared to the total magnitude estimates on COBS so it is easier to see
short-term changes in trends. I don't think that the coma outside the 9
arcsec radius can be the full story here since the lightcurve is clearly
very asymmetrical. There is also something odd going on in mid 2025
November which is not obvious in the COBS total mags or coma diameter
estimates. That is why I think it is useful to try to model these small
aperture measurements.

I'd be very interested if you could point me to the model that you used
for the phase effect so that I can incorporate this into my analysis. In
particular, over what period do you think that the phase enhancement was
significant (say > 0.1 mag).

Thanks,

Nick.

On 31/10/2025 19:35, Thomas Lehmann - t.lehmann at mailbox.org (via
baa-comet list) wrote:
>
> Hi Nick,
>
> in my opinion, it is somewhat difficult to use those 9" measurement for an
> overall analysis of lightcurves if the distance of the comet from earth is
> changing a lot within the time frame considered.
> E.g. in case of C/2023 A3 this distance decreased by more than a factor of 3
> between 2023 April and 2024 June. The early mesurements are therefore tracing
> a much larger volume of the coma and appear brighter by large amount (1-2 mag).
> The lightcurve will be skewed a lot and deriving model parameters similar to
> total coma magnitude lightcurves should be considered with caution.
>
> The peak in April is mostly affected by the phase effect as you mentioned, but
> also by the projection of the dust tail within the mesurement aperture during
> that time.
>
> Best regards
> Thomas
>
>
>
>> Am Fri, 31 Oct 2025 18:38:49 +0000
>> schrieb Nick James <ndj@nickdjames.com>:
>>
>> With all the current excitement about C/2025 A6 (Lemmon) we shouldn't
>> forget the much brighter comet that we had this time last year. C/2023
>> A3 was a really impressive naked-eye object in mid-October 2024 but it
>> has now faded to around 18th magnitude although it is still available in
>> the evening sky.
>>
>> In response to a query from Jonathan Shanklin I have updated the 9
>> arcsec lightcurve. The lightcurve of this comet is very unusual. The
>> blue curve shows a simple fit to the data points from 2024 July 1
>> onwards. It is a passable, although not very good, fit. The period up to
>> 2024 April has a rather poor fit to a different curve. The peak around
>> 2024 April 15 is supposed to be mainly due to the phase angle effect on
>> the very dusty tail which, at that time, was projected behind the comet,
>> but that doesn't really explain why the comet was a lot brighter
>> throughout the period up to 2024 April.
>>
>> Jonathan specifically requests if there are any images that can provide
>> data in 2025 January. The comet was at a small elongation then and was
>> moving from the evening to the morning but if you do have any images
>> which you haven't measured and submitted then please contact me.
>>
>> It is worth keeping this comet under observation and submitting your
>> photometry results. The recent large scatter in data is probably due to
>> the very crowded field and the high probability that faint stars are in
>> the 9 arcsec radius aperture.
>>
>> Nick.
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