  All available observations of photometric standard stars obtained with the
Gemini Multi-Object Spectrograph at Gemini North in the period from August 2001
to December 2003 have been used to establish the calibrations for photometry
obtained with the instrument. The calibrations presented in this paper are
based on significantly more photometric standard star observations than usually
used by the individual users. Nightly photometric zero points as well as color
terms are determined. The color terms are expected to be valid for all
observations taken prior to UT 2004 November 21 at which time the Gemini North
primary mirror was coated with silver instead of aluminum. While the nightly
zero points are accurate to 0.02 mag or better (random errors), the accuracy of
the calibrations is limited by systematic errors from so-called "sky
concentration", an effect seen in all focal reducer instruments. We conclude
that an accuracy of 0.035 to 0.05 mag can be achieved by using calibrations
derived in this paper. The color terms are strongest for very red objects, e.g.
for objects with (r'-z')=3.0 the resulting z' magnitudes will be ~0.35 mag too
bright if the color term is ignored. The calibrations are of importance to the
large Gemini user community with data obtained prior to UT 2004 November 21, as
well as future users of achive data from this period in time.
