2000A&A...359...51L

We present here the results of a deep survey in a 0.4deg² blank field in Selected Area 57 conducted with the ISOPHOT instrument aboard ESAs Infrared Space Observatory (ISO) at both 60µm and 90µm. The resulting sky maps have a spatial resolution of 15x23arcsec² per pixel which is much higher than the 90x90arcsec² pixels of the IRAS All Sky Survey. We describe the main instrumental effects encountered 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 60µm and 50 mJy at 90µm. To these limits we find cumulated number densities of 5±3.5deg^–2 at 60µm and 14.8±5.0deg^–2 at 90µm. 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.