[BAA Comets] Photometry of C/2012 S1 (ISON)

[Nick James]

This comet is very much in the news at the moment with various media 
outlets predicting that it will be brighter than the full moon at 
perihelion. This sounds very impressive to the general public since the 
full moon is really bright in a dark sky. Of course, even if ISON is 
that bright, it will be very close to the Sun and could be quite 
difficult to spot.

We like to think that comets follow a magnitude law of the form:

m = M + 5 * log10(D) + K * log10(r)

where H and K are constants, D and r are the comet's distance to the 
Earth and Sun (in au) and m is the total observed magnitude. To predict 
a comet's magnitude at any point in the future we need to estimate H and 
K from our observations. We do this by plotting the comet's absolute 
magnitude:

m - 5 * log10(D)

against the log of its distance from the Sun

log10(r)

We then fit a straight line to the points and estimate the intercept M, 
and the slope, K.

Denis Buczynski, Roger Dymock and myself have been observing this comet 
and calculating its total magnitude using Astrometrica and Focas. The 
results we have so far are shown as points on the plots (the both show 
the same thing but on different scales). The dark blue line is the best 
fit to this data. With this very limited dataset we get:

M = 2.31 +/- 1.36
K = 14.24 +/- 1.86

This would imply that the comet would be extremely bright at perihelion 
however it is clear from looking at the first plot that any attempt to 
extrapolate that far is very unwise. Effectively we are using a straight 
line obtained from a very small range of log10(r) to extrapolate down to 
perihelion when log10(r) = -1.9.

Over the coming months we certainly need more photometric observations 
of this comet obtained using the Astrometrica/Focas technique so that we 
can see how the behaviour develops. At present the comet is around mag 
15.5 in a 10 arcsec box. Roger Dymock has been doing a lot of work in 
this area and both he and I can provide advice if needed.

Whatever happens to this comet it will be an interesting object to 
observe throughout 2013 and we will have a lot to talk about at the 
Comet Section meeting in May.

Nick.