Deep far infrared ISOPHOT survey in Selected Area 57

We present here the results of a deep survey in a 0.4 deg2 blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory (ISO1) at both 60 μm and 90 μm. We describe our data reduction and analysis scheme with special emphasis on source extraction and flux calibration. With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 μm and 50 mJy at 90 μm. To these limits we find cumulated number densities of 5±3.5 deg-2 at 60 μm and 14.8±5.0 deg-2 at 90 μm. These number densities of sources are found to be lower than previously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evolutionary models.