We present here a composite phase function for the scattering of sunlight by dust grains in cometary comae. Our primary purpose in creating such a phase curve was to adjust A(θ)fρ values for phase angle effects we compute from narrowband photometry measured for comets with time and/or distance from the Sun. In practice these effects can reach up to a factor of 3 over a range in phase angles of between 0° and 110°, and much more at higher phase angles. Background information, the curves used in the composite, and how these were combined are described in detail here.
To distinguish this curve from others, we designate this as the HM phase function, for the sources of the two components: Halley and Marcus, where the Halley curve for smaller phase angles comes from our previous work (Schleicher et al. 1998) while Joe Marcus has fit a Henyey-Greenstein function to a variety of mid- and large-phase angle data sets (Marcus 2007); see here for details. Note that we do not consider our composite curve to be a definitive result, but rather appropriate for performing first-order adjustments to dust measurements for changing phase angle.