ISO Surveys of a Dusty Universe

Many of the ISO observers who assembled for this workshop at Ringberg c- tle met for the third time in the Bavarian Alps. At two previous meetings in 1989 and 1990 surveys were only a minor topic. At that time we were excited by the discoveries of the IRAS survey mission and wanted to follow it up with pointed observations using an observatory telescope equipped with versatile instruments. With the rapid development of detector arrays and stimulated by ISOs Observing Time Allocation Committee, however, surveys eventually became an issue for the upcoming mission. In a review paper on Infrared S- veys - the Golden Age of Exploration given at an IAU meeting in 1996, Chas Beichman already mentioned that there are ISO surveys. They were at the bottom of his hit list, while the winners were future space missions (Planck, SIRTF, etc. ) and ground-based surveys in preparation (Sloan, 2MASS, DE- NIS, etc. ). He organized his table according to the relative explorable volume, calculated from the solid angle covered on the sky and the maximum distance derived from the detection sensitivity. Clearly, with this ?gure of merit, ISO, as a pointed observatory, is rated low. Applying the classical de?nition of a survey, i. e. to search in as large a volume as possible for new or rare objects and/or study large numbers of objects of various classes in order to obtain statistical properties, ISO was indeed limited.