A goal of many compositional measurements of comets is to determine the total abundance of a species in the entire coma. However, most comet observations are of only a small portion of the whole coma, and we must extrapolate the observed abundance to determine the total coma abundance.

Haser, vectoral, and Monte Carlo models all can provide this, but to minimize the number of assumed parameters and because the Haser model can usually successfully reproduce measured spatial profiles, we usually employ the Haser model to perform this extrapolation to a total coma abundance (see Haser 1957, Bull. Cl. Sci. Acad. R. Belgium 43, 740-750).

Parent and daughter Haser scalelengths are used to create the Haser radial profile for each observed daughter species. Often many combinations of parent/daughter values will yield similar profiles. We have deliberately chosen a parent/daughter pair (along with appropriate heliocentric distance dependencies) for each species which works well for comets at a variety of heliocentric distances. Note that while the Haser model is non-physical (assuming that all particles move radial outward from the nucleus), using the appropriate (non-physical) scalelengths which reproduce the radial fall-off yields a true, physical total abundance.

We provide here each parent/daughter pair of scalelengths, both at 1 AU, and again scaled by r² to the heliocentric distance requested. This service also computes the Haser fraction for the given aperture size, i.e. the fraction of the total coma contained within the requested circular aperture. [If a square aperture was used in the observations, the equivalent area circular aperture should be input.] The scalelengths used here for OH are from Cochran and Schleicher 1993 (Icarus 105, 235-253) and for other species from Randall et al. (BAAS 24, 1002). See A'Hearn et al. (1995, Icarus 118, 223-270) for further details.