2 FIRBACK observations

FIRBACK covers about 4 square degrees in 3 high galactic latitude fields (chosen to have very low HI column density, typically $N_{\rm HI} \le 10^{20}$ cm^-2), the so-called N1 (FN1), N2 (FN2) and Marano (FSM) fields. The observations were performed at an effective wavelength of 170 $\,{\rm\mu m}$ with the C200 array ($2\times 2$ pixels) of ISOPHOT. The pixel field of view is 1.5 arcmin. We use the AOT (Astronomical Observation Template) PHT22 in the multi-pointing staring raster mode. Each FIRBACK field (Table 1) consists of several rasters of 17$\times$17 pixels:

• Eleven rasters were done in the so-called FN1 field. These rasters were re-observed with a shift in position that corresponds to a fraction of an ISOPHOT pixel to provide proper sampling where possible. Observations were done from the ISO revolution 753 to 774. This field covers the same area as the ELAIS N1 15 $\mu$m and 90 $\mu$m observations (Oliver et al. 2000).
• Nine rasters were done in the so-called FN2 field. These rasters were again re-observed with a shift in position. The field covers the same area as the ELAIS N2 15 $\mu$m and 90 $\mu$m observations. These data are FIRBACK/ELAIS observations. They were done from the ISO revolution 785 to 798.
• One raster was observed in the so-called FSM1 field (formerly called Marano 1 field) during the revolution 593 and three rasters in the so-called FSM234 field (from revolution 739 to 744). Rasters have been observed four times. Displacements between the four independent observations correspond to about 0.5 pixel, except for the FSM1 field where the displacements correspond to 2 pixels (note that the overlapped surface with the "original'' Marano field is less than 15$\%$).

Each raster is performed in the spacecraft (Y, Z) coordinate system which is parallel to the edges of the detector array, with one pixel overlap in both Y and Z direction. The exposure time is 16 s per pixel and thus 128, 128 and 256 s per sky position in the FN1, FN2 and in the FSM fields respectively.

We also have a PHT25 measurement in the FSM1 field. This AOT is the absolute photometry mode for PHT-C, in which photometric calibration is achieved by chopping against the internal fine calibration source. For low fluxes (i.e. in our case), chopping is also done against the switched-off "Fine Calibration Source'', which has a temperature level of about 4 K. This is the temperature of the optical support on which the instrument is mounted. Such a measurement serves to define a zero point. This mode is especially well suited for observations aiming at accurate determination of the absolute brightness of the background emission.