AN OPTIMIZED DETECTION TECHNIQUE FOR FAINT MOVING-OBJECTS ON A STAR-RICH BACKGROUND - A SEARCH FOR THE NUCLEUS OF COMET 46P WIRTANEN/

A novel method is proposed for the efficient detection of faint solar system objects moving in front of a star-rich field. After the removal of the background objects by means of seeing template frames and accu rate image alignment on the moving object, subsets of frames selected by different criteria (e.g. seeing) are co-added to improve the signal -to-noise ratio for the image of the object. Requirements for the succ essful application of this procedure are discussed and verification ch ecks for identified candidates are outlined. The procedure provides ac curate astrometric positions and approximate magnitudes for identified objects. The method was applied to a search for the similar to 24-mag nucleus of comet 46P/Wirtanen, the prime target of ESA's ROSETTA miss ion, while at heliocentric distance 4.6 AU and located in a field near the direction to the Galactic Centre. A 24.2 mag candidate image was marginally detected in CCD frames totalling 176 min integration time a nd taken on June 26, 1995, with the 2.2-m telescope at ESO La Silla. H owever, because of insufficient, corroborating data from other nights (caused by adverseadverse atmospheric conditions). an unambiguous conf irmation of this detection is not possible. The corresponding upper li mit of the radius (assuming albedo of 0.04) is only 0.8 Ecm (the likel y radius is 0.69 km), indicating that comet 46P/Wirtanen possesses one of the smallest cometary nuclei known so far.