A comet for Liege
One such case was that of the Franco-Belgian Schmidt (Grand Schmidt) telescope at the observatory of Haute-Provence, where an important part of the observations were dedicated to the study of known comets, but where new comets could sometimes be searched for. This opportunity yielded results as it led to the discovery, forty years ago, of a “Liege” comet and the only one to date, the C1973A1/Heck-Sause. This is also the only comet ever to have been found at the observatory of Haute-Provence.
In the interest of initiating studentse into astronomical observation, series of fields were chosen which combined astronomical interest and possibility in order to find a comet. The aesthetic aspect was not neglected and fields showing galaxies, nebulae or clusters were prioritized as a motivating factor. The Virgo cluster of galaxies was one of the targets and the presence of many diffuse stars tested the shrewdness of the competing observers. The act of finding a comet cannot be improvised. If chance has an important role to play it is better to lend it a helping hand. Thus, it is not very useful to search for brilliant new comets in the middle of the sky at night; it is highly unlikely that they would be allowed to arrive there unnoticed. On the other hand, weaker objects might well have been neglected. Brilliant objects can appear near the horizon at dawn or dusk which until then were hidden by the light of the sun. However, relatively bright comets could be more easily found by the visual observation of amateurs who rapidly studied the sky with open binoculars or telescopes. It would have been pointless to try to compete with them using an ill-adapted photographic technique. The researchers from Liege therefore applied a comprehensive strategy in order to maximize their chances of success with the Schmidt telescope by concentrating on faint comets. The exposures, which were several tens of minutes long only made it possible to comb a very small part of the sky each night, but this could be done in depth.
These long exposures required careful and continuous guidance of the eyepiece in the absolute darkness of a dome that was often freezing accompanied by the incessant ticking of sidereal and universal time clocks. One the exposure was finished, the photo was developed then examined while still wet in case something appeared. An in-depth study followed the next day when the film was completely dry.
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Comparison of the images with existing atlases couldn’t be done by computer as is the case nowadays. It was a laborious process which required starting with maps that were not very detailed such as those of the Star Atlas of David Phillip Norton or the Coeli Atlas of Antonin Bečvář in order to identify the general field, then to progress with more detailed atlases (for example Bečvář’s Borealis, Eclipticalis and Australis Atlases) to finally produce the large photographs of the Palomar Sky Survey. Another technique consisted in repeating the same fields at intervals of one hour and comparing them with an apparatus that made it possible to rapidly sequence the images. A mobile object then seemed to jump between two positions. This effect is currently reproduced much more conveniently with the “/blink/” function of astronomical instruments. Calculations of scale, interpolation of coordinates and precessions all had to be done manually, with all the risk of error that this entails while working in haste. The positions were traced onto tracing paper but sometimes directly onto negatives and maps. Thus the hesitations and hopes of our predecessors can sometimes be detected in the archives.
The research strategy with the Grand Schmidt telescope proved worthwhile when, on the night of the tenth to the eleventh of January 1973, André Heck and Gérard Sause noticed a diffuse-looking intruder among the Virgo cluster of galaxies. Two extra negatives taken the same night showed that the object had moved. Therefore it was indeed an object of the solar system, it was a comet. It was still necessary to verify that this was not a comet that was already known and which had been discovered very recently and not yet officially catalogued. These verifications required some phone calls and the precious help of François Dossin (1927®C1998), it was established that it was indeed a new comet. A telegraph was then sent to the CBAT. It was still nighttime in the US and François Dossin and Jean-Pierre Swings were able to warn the observatory at Mount Palomar where the astronomer Wallace L.W. Sargent (1935®C2012) succeeded in capturing an image with the famous 1m22 Grand Schmidt telescope, barely 12 hours after it was discovered.
