The comet passed the perihelion on the 5th of October 1972 in the southern hemisphere. Drowned out by the brightness of the Sun, and at two AU behind it, it was unobservable.  Climbing rapidly into the sky in December and January 1973, it became a target for comet hunters. In summer, observations were prevented due to a weak solar elongation, but from September the comet was photographed with Grand Schmidt telescope by François Dossin and it was seen to be astonishingly bright despite its distance away.

The last images were taken on the 25th and 26th of January 1974 at more than 5 AU from the Earth and the Sun (François Dossin with the Grand Schmidt of Haute-Provence, and Elisabeth Roemer with the 2m29 telescope of the Steward Observatory).

The condensed object was given a magnitude of 19.6 by Roemer, but the image taken by the Liege Schmidt a few hours earlier seem to indicate a much brighter object than this estimate. The absence of reference stars is the main reason for these differences.

The comet is now an insignificant point in Pisces at 63 AU, two times further than Pluto and Neptune (at 30 and 32 UA respectively) and Halley’s comet (33 AU) which passed the perihelion in 1986. By comparison, the voyager probes launched in 1977 have been much faster and are now at 123 and 100 AU. As for the contemporary of Heck-Sause, the comet Kohoutek, it took an entirely different direction and is now moving through the Gemini constellation at around the same distance, 64 AU.

Though the scientific contribution of the Liege comet was not so important, we can be certain that it was educationally very beneficial to the students who participated in these observations many of whom have become teachers. This episode constituted the astronomical experience of their lives and undoubtedly they still speak to their own students about it: real astronomy in a real observatory, the routine of observations, the excitement of the discovery, the doubts and the verifications not to forget the media aspect.

The Schmidt telescope of Haute-Provence.

The Schmidt telescope of the observatory of Haute-Provence (the « Grand Schmidt » as opposed to another more modest telescope from the same observatory) is a large field telescope made up of a primary spherical mirror of 87 cm in diameter and a focal length of 209 cm (cf A. Heck, L’astronomie, 1973, p 241). A corrector plate 62 cm in diameter is placed at the center of the curve of the apparatus which gives it an opening of 3.36. The scale of the images is around 100 arcseconds per millimeter. Their diameter of 16.5 cm corresponds to a field of 4.6 degrees. Such a field is still out of reach of CCD sensors due to the fact that the focal surface is spherical. Photographic films existed in such dimensions and could easily follow the focal surface without difficulty. In order to benefit from a better stability of the emulsion, they were replaced by photographic plates after the perfection, ®C following a lot of damage, ®C of a system making it possible to cause them to swell in order to follow the spherical shape of the focal surface of the Schmidt telescope.

Modern instruments approaching the same field characteristics have to use more complex optics, mosaics of sensors, complicated observation strategies, and all of this backed up by very heavy computer equipment.

The idea of this Franco-Belgian telescope dates from 1959, and the definitive installation took place in 1970. The essential element of the Liege observation program consisted in tracking comets with a view to studying their shape: structure of the tail, the comas, and their development. Monitoring heavenly objects was done with an eyepiece by means of a guiding glass fixed to the telescope. This solution which works well for fixed stars is imprecise where comets are concerned as their core is often diffuse or too faint. An ingenious system was perfected making it possible to program a movement of the eyepiece which compensates for the movement of the object. The guidance could then be done, not on comets but on a neighboring star.

The death of photographic emulsions signified the end of wide-field imaging on this telescope and lead to a reconversion of sorts with a small-dimension to a sensor.