Deep far infrared ISOPHOT survey in "Selected Area 57" I. Observations and source counts

We present here the results of a deep survey in a 0.4 deg 2 blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observa- tory (ISO 1 ) at both 60 m and 90m. The resulting sky maps have a spatial resolution of 15 23 arcsrc 2 per pixel which is much higher than the 90 90 arcsec 2 pixels of the IRAS All Sky Survey. We describe the main instrumental effects en- countered in our data, outline our data reduction and analysis scheme and present astrometry and photometry of the detected point sources. With a formal signal to noise ratio of 6.75 we have source detection limits of 90 mJy at 60 m and 50 mJy at 90 m. To these limits we find cumulated number densities of 53:5 deg 2 at 60 m and 14.85:0 deg 2 at 90 m. These number densities of sources are found to be lower than previ- ously reported results from ISO but the data do not allow us to discriminate between no-evolution scenarios and various evo- lutionary models.