1 Introduction

With more than 150 hours of observations, FIRBACK (Far-InfraRed BACKground) is one of the largest observational programmes made with the ISOPHOT instrument on board the Infrared Space Observatory (ISO) satellite. This cosmological deep survey at 170 $\mu$m covers more than 4 square degrees located in two Northern and one Southern fields (Lagache 1998; Dole 2000a). There are 106 sources detected above the sensitivity limit (180 mJy, 4$\sigma$). The number of sources detected above 135 mJy ($3 \sigma$) is 196. The first result of this survey is the high number of sources observed when compared to no, or moderate, evolution models for infrared galaxies (e.g. Dole 2000a). Preliminary results in the so-called Marano1 field were published in Puget et al. (1999), and on the whole survey by Dole et al. (2000b). FIRBACK also allows for the first time the detection of the CFIRB fluctuations (Lagache & Puget 2000; Puget & Lagache 2001).

After a presentation of the observational issues of the FIRBACK survey, we present in this paper the final stage of the data reduction and calibration. We use the Phot Interactive Analysis (PIA, Gabriel et al. 1997) for the standard reduction and calibration (Sect. 3.1) with some extra developments: flat-fielding, transient corrections and reprojection (Sects. 3.2-3.7). In Sect. 4, we review our final map calibration and compare it with absolute photometric calibration from other instruments. We then conclude in Sect. 5.

 

 

Table 1: FIRBACK fields main characteristics.

Field	l	b	surface	$t_{\rm int}$	Number of
	deg	deg	deg2	sec	rasters
FSM	270	-52	0.95	256	2$\times$11
FN1	84	+45	1.98	128	2$\times$9
FN2	65	+42	0.96	128	4$\times$4