[BAA Comets] Photometry of C/2011 L4 (PANSTARRS) five days after perihelion

[Richard Miles]

C/2011 L4 (PANSTARRS) from Stourton Caundle on March 14.80:

Have managed to carry out V photometry of C/2011 L4 some 4.7 days 
post-perihelion when at a solar elongation of 16.2°. Observing conditions 
were (just) satisfactory in that I was able to take a series of 22 images 
during a clear sky interval whilst the comet was at an altitude of 6.0-6.3°. 
It was extraordinary and fortuitous that an A0 comparison star was located 
just 17' from the nucleus and so an absolute precision of +/-0.03 mag was 
possible.

The tracked and stacked image can be seen at:
http://www.britastro.org/~rmiles/Documents/C2011L4_20130314_RMiles_Vfilter.jpg

The photometric growth curve (circular apertures) is available at:
http://www.britastro.org/~rmiles/Documents/C2011L4_20130314_RMiles_Vphotometry.gif

Total (m1) magnitude (2013 March 14.797): +0.8 mag (+/-0.1)
Tail length: 30'

Note that this determination is based on a limited extrapolation of the 
data.
Photometric integration out to a measured 10' tail length gives a magnitude 
of +1.0.

The ephemeris prediction indicates the comet would have faded by 0.5 
magnitudes on March 14.80 compared to when brightest close to perihelion. 
So the conclusion is that this comet reach a minimum m1 of +0.3 and so never 
attained a negative magnitude. Hopefully these observations should help put 
the lightcurve on a sounder footing.

Richard Miles
British Astronomical Association