Comets are interesting and diverse objects. At mm-cm wavelengths, however, comets are very weak, and one needs a lot of sensitivity to image molecular lines and detect continuum radio emission. Comet Hale-Bopp was an exception, observed widely at many frequencies. The continuum radio emission was easily detected at submm wavelengths, it was mapped at 3 mm wavelength, and just barely seen at 3.6 cm with the VLA. Continuum observations give information on the dust coma around the coma, such as the particle size distribution and halo mass.
The first OH map was obtained by the VLA on comet Halley; only 2 more comets were mapped since comet Halley. Unfortunately, the VLA array configuration during the Hale-Bopp apparition was too spread out to image OH. This gets precisely at the point why it is so hard to image molecular species in comets: one needs a high sensitivity, as well as a sufficient number of short spacings to image the structure, since a cometary coma is many arcminutes on the sky. So to image a comet's OH, one needs the VLA D configuration, and even then only a fraction of the emission is usually seen. Observing the coma simultaneously in autocorrelation mode, one actually knows how much flux is missed. Images of OH can be used to derive information on the dissociation process of water, the excitation mechanisms of OH (maser emission), which can be tied in with the physics in a comet's immediate environment (e.g., interaction with the solar wind).
Molecules such as water, ammonia and formaldehyde, which all have line transitions at short cm wavelengths, have only been detected for a few comets, and have never been imaged. Again, one needs a high sensitivity, and the right antenna spacings. We believe the SKA array might actually be the first array to detect and image such species.