3 Data reduction, instrumental effects, calibration, maps

The complete process of data reduction and calibration is described in Lagache & Dole (2001). Here, we merely summarize the different steps.

3.1 Interactive analysis

We made use of the PHT Interactive Analysis package (PIA) version 7.2.2 (Gabriel et al. 1997) in the IDL version 5.1 environment, to process the raw data (named ERD: Edited Raw Data) into brightnesses (named AAP: Astronomical and Application Product). After linearizing and deglitching the ramps, we applied the orbit-dependent dark and reset interval corrections. We calibrated the data with the two bracketing FCS lamps (Fine Calibration Source) values, using the mean value in order not to induce baseline effects.

3.2 Glitches, long term transients, flat fielding

Cosmic particles hitting the detector are easy to detect at the time of their impact, but they may cause response variations. On 224 different measurements (that is 56 independent rasters observed by 4 pixels), we report only 13 such cases, which are corrected. Furthermore, thanks to the high redundancy of each raster, a glitch cannot mimic a source because the same piece of the sky is observed independently by the four pixels of the photometer at different times.

Some long term transients (LTT) are seen in the data, and are understood to be the consequence of step fluxes seen by the photometer. During the FIRBACK observations, ISOPHOT was looking at relatively flat fields with low background, but was on more complex fields during the preceding observations. Our best data occur where the observations were made continuously. We correct for the LTT by forcing all the pixels to follow the time variations of the most stable pixel, which is assumed to represent the sky. This correction is found to be linear, and never exceeds 10%.

We then compute a flat field using the redundancy and apply the necessary corrections. The detector behaviour is highly reproducible, leading to constant flat field values: $1.04 \pm 0.02$, $0.91 \pm 0.02$, $1.09 \pm 0.02$and $0.94 \pm 0.02$for pixels 1, 2, 3 and 4 respectively.

3.3 Photometric correction

There is a difference of 11% between the solid angle value of the PHT footprint at 170 $\mu$m used by PIA and the value derived by calibration observations around Saturn and the model. We thus apply a multiplicative correcting factor of 0.89 to the brightness values given by PIA to take into account the real profile of the footprint.

3.4 Maps

For a given raster measurement, we project the signal from each pixel on a regular grid defined by the raster. Between each pointing, we make an interpolation and check that the photometry is not changed by more than 1%. Then we sum all these signals on a celestial coordinate grid to get the final map.

3.5 Calibration of extended emission

Using the knowledge of the average interstallar dust emission spectrum, the zodiacal light emission at the time of the observations, and the Cosmic Infrared Background values derived from COBE, together with HI data on our fields, we derive a brightness value at 170 $\mu$m for each of our fields. This extrapolated brightness at 170 $\mu$m for the three fields is in remarkable agreement with the measured ISOPHOT brightness. Furthermore, the rejection level of straylight up to $60^{\circ}$ off-axis observed by ISO during total solar eclipse by the Earth, is better than 10^-13, implying that there is no significant contribution to the measured flux coming from the far sidelobes. This confirms that ISO is able to make absolute measurements of the extended emission and gives a high degree of confidence to our photometric calibration.