4. THE ISOPHOT FAR IR DEEP SURVEYS

ISOPHOT was used to survey the sky at 90 and 170 µm , mainly (plus a few surveys at 60, 120, 150 and 180 µm, see Juvela et al. 2000; Linden-Voernle et al. 2000; cf review by Dole 2002). At 90 µm, the 46 arcseconds pixels and FWHM of ISOPHOT represented a significant improvement with respect to IRAS, however due to k-correction and sensitivity limitations, only the local universe could be probed at such wavelengths. The 170 µm band was more favorable for the detection of distant ULIRGs as shown in the Fig. 1 where the peak emission of a ULIRG of 4 × 10¹² L and redshifted at z = 1 is shown (normalized SED of Arp 220). A major issue at these wavelengths with a 60 cm telescope is obviously the identification of optical counterparts for the determination of a redshift and the separation of local and distant galaxies.