  We present the results of a Monte Carlo technique to calculate the absolute
magnitudes (H) and slope parameters (G) of about 240000 asteroids observed by
the Pan-STARRS1 telescope during the first 15 months of its 3-year all-sky
survey mission. The system's exquisite photometry with photometric errors <
0.04 mag, and well-defined filter and photometric system, allowed us to derive
accurate H and G even with a limited number of observations and restricted
range in phase angles. Our Monte Carlo method simulates each asteroid's
rotation period, amplitude and color to derive the most-likely H and G, but its
major advantage is in estimating realistic statistical+systematic uncertainties
and errors on each parameter. The method was tested by comparison with the
well-established and accurate results for about 500 asteroids provided by
Pravec et al. (2012) and then applied to determining H and G for the
Pan-STARRS1 asteroids using both the Muinonen et al. (2010) and Bowell et al.
(1989) phase functions. Our results confirm the bias in MPC photometry
discovered by (Juric et al. 2002).
