Re: Unusual brightening of comet C/2023 Q1
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Jakub and all,
This discussion has inspired me to observe C/2023 Q1 in my 38-cm reflector at the next opportunity. I also have a very dark sky especially in the north direction — when the aurora isn't active! It would be nice if we had some additional visual observers so there would be a pool of estimates to draw upon. J.J. Gonzalez Suarez's observation is the only visual sighting of the comet I'm aware of.
Bob
Hello all,
If I have read the posts correctly, we have two observations:
- CCD magnitude of 14.1 mag with a 5' aperture.
- Visual magnitude of 11.6 mag with a 6' aperture.
How can we explain such a large difference? The CCD method
measures flux in a chosen aperture, and we know that a comet’s
surface brightness decreases with distance from the central
condensation. However, even if the surface brightness is
decreasing, the increase in the area of the coma can still
significantly change the total measured magnitude.
Regarding the well-known “CCD vs. visual” issue, it often arose
in the past from insufficient exposure times, which led observers
to measure only the central condensation in a smaller aperture,
whereas visual observers saw more of the coma. Even among visual
observers alone, an “aperture effect” has been described—smaller
telescopes can show a larger apparent coma and thus yield brighter
magnitude estimates. Hence, the 2–3 mag discrepancy was typically
caused by different coma sizes captured in visual vs. CCD
observations. However, this problem has largely faded in recent
years as CCD observers have become more experienced. With proper
techniques, CCD observations now usually match visual data
closely, differing by only a few tenths of a magnitude due to
variations in spectral sensitivities (especially for unfiltered or
narrowband photometry vs. visual V or g' bands).
It is unlikely that a difference of 2.5 mag could arise from
nearly the same apertures (5' vs. 6'), because an increase of just
1' in aperture should not produce such a large magnitude gap under
normal circumstances.
Although it is possible to measure an object as 2.5 mag brighter
by including a much larger coma with very low surface brightness
(for example, if the visual observation indicated a coma diameter
of 15'–20'), this is not the situation described here. Therefore,
the reported visual observation may be in error—either in the
estimated magnitude or the coma diameter. The observer should
re-examine their observation to identify where the discrepancy
arose.
No one is flawless—myself included. It has been demonstrated many
times that observers can report objects or details they did not
actually see. There have even been cases where a comet was
reported at the wrong position. We must learn from such instances
in order to provide scientifically valuable data.
Best regards,
Jakub ?ern?
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