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 Infrare d Space Observatory (ISO1) at both 60 mu m and 90 mu m. The resulting sky m aps have a spatial resolution of 15 x 23 arcsrc(2) per pixel which is much higher than the 90 x 90 arcsec(2) pixels of the IRAS All Sky Survey. We des cribe the main instrumental effects encountered in our data, outline our da ta reduction and analysis scheme and present astrometry and photometry of t he detected point sources. With a formal signal to noise ratio of 6.75 we h ave source detection limits of 90 mJy at 60 mu m and 50 mJy at 90 mu m To t hese limits we find cumulated number densities of 5+/-3.5 deg(-2) at 60 mu m and 14.8+/-5.0 deg(-2) at 90 mu m. 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 ev olutionary models.