Occultations of stars and radio sources by comets: Predictions and observing prospects

The results that might stem from observations of occultations of stars and radio sources by cometary comae and ion tails are outlined. Taking account of the current orbital accuracy of P/Halley, specific predictions of stellar appulses with that comet, including preliminary topocentric circumstances, are presented. A crude model of the absorption of starlight by grains in a dusty comet supports the notion that stellar extinction may be observable in practice. Some thoughts on observing techniques are set down, discussing the merits of a two-dimensional detector (a CCD camera) and a conventional aperture photometer. The need for accurate photometric observations with time resolution approx. 1 sec is stressed because the interval of greatest extinction will usually be brief. Finally, the efficacy of undertaking accurate relative astrometry of comets during predicted approaches to stars is explored.