Wide field imaging and the velocity structure in the coma of Hale-Bopp

The comae of very active comets have a substantially more complex coma than their weaker cousins. The primary cause of this is photolytic heating and collisions that occur over an everlarger volume of the coma as Q_H2O increases. Collisions with the photochemical daughters of water in this region modify the radial distributions and outflow velocity of each species, excite and quench metastable emissions, and introduce velocity gradients from photolytic heating. Comet Hale-Bopp was the first comet for which the collisional coma was both spatially resolvable and comparable in extent to the scale lengths of major coma species. In the case of this object, the classical assumptions that make it possible to invert radial emission line profiles, brightnesses, and lineshapes to production rate and velocity either do not hold or require adjustment to work. Here we describe how a large collision zone modifies the coma, how it affects the classical methods for obtaining production rate and velocity, and discuss how wide field imaging may be combined with modified versions of simple models to address the complications and extract some structural information.