  We describe a method of determining the V magnitude of an asteroid using
differential photometry, with the magnitudes of comparison stars derived from
Carlsberg Meridian Catalogue 14 (CMC14) data. The availability of a large
number of suitable CMC14 stars enables a reasonably accurate magnitude (\pm0.05
mag) to be found without having to resort to more complicated absolute or
all-sky photometry. An improvement in accuracy to \pm0.03 mag is possible if an
ensemble of several CMC14 stars is used. This method is expected to be less
accurate for stars located within \pm10deg of the galactic equator owing to
excessive interstellar reddening and stellar crowding.
