  A photometric method is described for accurately quantifying the brightness
of short-period comets far from perihelion. The method utilizes the Sloan
Digital Sky Survey Catalog (Data Release 7) as a homogeneous source of
reference star magnitudes. Results are based on SDSS-r' filtered images taken
using 2.0-m aperture telescopes for which the exposure time was adjusted to
achieve a constant motion-blur of 2.0 pixels (0.56 arcsec) on the CCD chip.
Aperture photometry using circular and tilted elliptical apertures was
performed on images, which were stacked to increase signal to noise. Magnitude
dependence on 'seeing' was determined, and this calibration was used to
normalize photometry to constant seeing thereby maximizing photometric
accuracy. From observations of comet 17P/Holmes between 2008 October and 2009
March, a very significant outburst of 17P was found to have occurred on 2009
Jan 4.7 (\pm0.5 day). Night-to-night measurements of the brightness of the
inner coma (3000-km radius) exhibited a scatter of only 0.015-0.019 mag. No
short time-scale (<36 hr) periodicity was found in the fading lightcurve. From
literature data, it was estimated that reflected light from the nucleus
contributed 7-11% of the signal within the inner coma and it is concluded that
either the nucleus of 17P must be relatively spherical (projected axial ratio
of <1.25), or, if its shape is more typical of other comet nuclei, it has a
rotational period in excess of 10 days (assuming the observations were not made
with the nucleus 'pole-on' to the Earth). Evidence from intermittent activity
displayed by the nucleus is indicative of a possible 44-day rotation period.
