I have posted my recent analysis of the brightness behavior of 103P/Hartley 2 at
the current apparition as well as at its 1991 and 1997 returns. This analysis is
based on a subset of the visual and CCD observations from 1991 and 1997, a
subset of CCD data from the current return, and a all of the visual data from
the current return.

Most visual and pretty much all recent CCD magnitude estimates show the comet
fainter (up to 2 magnitudes) than it was in 1997. As alluded to by Ignacio
Ferrin in an earlier post, many observers may be missing a large fraction of the
coma when making their measurements. This comet is very large and very diffuse.
My recent images taken with the LightBuckets 8" astrograph show a coma up to
~25' across. Even that may be an underestimate. Of course, the dense Milky Way
isn't helping matter either.

Still after an August when visual estimates were falling ~2 magnitudes behind
what was expected based on the '97 return, this month more observers (and my CCD
data) has the comet just a tad fainter than the average from the '97 visual
data.

Based on the '97 visual data, 103P should reach magnitude +4.5 at its peak.
Based on what we have seen so far, that might still be possible though a peak of
+5.0 might be more realistic.

I agree with those posters who have called for very wide-field images of the
comet for photometry. I would add that filter photometry would be better than
clear photometry.

Anyway, you can read more of my analysis and see some actual lightcurve plots
and images on my blog.
http://transientsky.wordpress.com/2010/09/22/introducing-comet-hartley-2/

BTW, let me know what you think. Getting accurate photometry for large comets,
whether bright ones like Hale-Bopp or faint ones like 103P, is a complicated
problem but one we should be able to solve. I do fear that more and more visual
observers will not be able to get close to the true total magnitude of large
diffuse comets due to ever increasing light pollution.

As an additional note, please check out the new 103P page at the Cometary
Science Center/International Comet Quarterly. We are slowly expanding this
concept though finding the time and money has been slow. The goal is to archive
not only all photometric observations but also all image data. Let us know what
you think.

http://www.csc.eps.harvard.edu/index.html

- Carl