[BAA Comets] 46P/Wirtanen

[Nick James]

David,

Magnitudes reported to the MPC are usually a byproduct of people doing 
astrometry and so they tend to use a small, but undefined, photometric 
aperture. They are therefore mostly near to "nuclear" mags. Magnitudes 
on COBS should represent total mags. For a large, extended comet like 
46P the total mag (measured in an aperture of several arcmins) will be 
quite a bit brighter than the nuclear mag (measured in an aperture of a 
few tens of arcsec).

At present visual and CCD observers are getting a total mag of around 10 
with a coma diameter of 10 arcmin or so whereas mags from MPC astrometry 
are much more variable (due to the undefined aperture size) but between 
2 - 4 mags fainter.

The lightcurve here:

http://wirtanen.astro.umd.edu/46P/46P_status.shtml

is the one you are referring to. The MPC mags are not really very useful 
in this case.

Nick.


On 16/10/2018 08:02, David Swan wrote:
> I have been looking at the University of Maryland's 46P campaign page. There's quite a big difference between the fitted (and extrapolated) light curves generated from COBS observations versus MPC obs. Is this due to the former relating to total VEM and latter false nucleus mag? Or does this to some extent reflect very significant uncertainty on peak brightness around perihelion/perigee?