[BAA Comets] Imaging and photometry of C/2011 L4 (PANSTARRS)

[Richard Miles]

Here are URLs of two deep images of this bright comet in the same field as 
Messier 31 taken from Dorset, UK and stacked at sidereal rate.

April 1st, V-filter image (1.9 Mb) with a logarithmic stretch applied:
http://britastro.org/~rmiles/Images/C2011L4_M31_20130401_RMb.jpg

April 2nd, V-filter image (2.0 Mb) with a linear stretch applied:
http://britastro.org/~rmiles/Images/C2011L4_M31_20130402_RMiles.jpg


V photometry has now been performed at Golden Hill Observatory on 5 epochs 
between March 14 and April 2. Results to date are shown at the following 
URLs:

Photometric growth curves (0.3 Mb):
http://britastro.org/~rmiles/Documents/C2011L4_Mar14-Apr02_phot_curves_RMiles.jpg

V magnitude vs. time of the 1-arcmin diameter coma (0.16 Mb):
http://britastro.org/~rmiles/Documents/C2011L4_Mar14-Apr02_Vmag_1arcmin_RMiles.jpg


The growth curve at all five epochs are well fitted by a power law which is 
gradually diminishing with time, probably tending towards zero as the coma 
activity declines and its shape takes on a more spherical appearance. For 
the last weblink, a simple 2nd-order polynomial has been fitted to the V 
magnitude plot, which models the observed magnitudes to a precision of about 
0.02 mag.

As can be seen, using CCD photometry it is possible to derive very accurate 
magnitudes. The big question is how can the CCD methodology be employed so 
that it is well correlated with mean visual magnitude data. The BAA Comet 
Section is putting together some plans to quantify this correlation using 
the apparition of Comet 2P/Encke later this year (from about September 01 to 
perihelion on November 21). Since Comet Encke is a fairly 'standard' comet 
and will brighten from about 15th mag to 5th mag during this time interval 
we stand a good chance of nailing the correlation. All participants both 
visual and CCD (equipped with V- or Green filters) are of course welcome to 
participate in this!

>From the trend in brightness over time, Comet PANSTARRS appears to be a very 
well-behaved object exhibiting a gradual fall-off in brightness without any 
fragmentation or minor outbursts as yet. However, with good photometric data 
any slight change in activity can be picked up clearly. If we follow 
PANSTARRS closely over the next few months both visually and with V-filtered 
CCDs then we also stand a chance of correlating CCD and visual studies of 
this relatively bright comet in time for the BIG one, Comet ISON!

Richard Miles
BAA