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Astron. Astrophys. 361, 407-414 (2000)

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2. Observation and data processing

The FIR survey of the Lockman Hole, which was executed as a part of Japan/UH cosmology program using the ISAS guaranteed time, is described in Taniguchi et al. (1994) and Paper I. Two [FORMULA] fields named LHEX and LHNW were mapped with two filters: C_90 (centered at 90 µm) and C_160 (170 µm). Each of the two fields is made up of 4 sub-fields.

Each sub-field map was produced from the edited raw data by the PHT Interactive Analysis (PIA; Gabriel et al. 1997) version 7.1 or 7.2, and is hereafter referred to as an AAP (Astronomical Analysis Processing of PIA) map. Each AAP map is either [FORMULA] pixels ([FORMULA]) for a 90 µm sub-field, or [FORMULA] pixels ([FORMULA]) for a 170 µm sub-field. To correct the drift in the responsivity of the detectors, we applied the median filter smoothing (see Paper I for details) to the AAP data.

Together with the final AAP maps we also produced the uncertainty maps, and found that the typical 3[FORMULA] noise is as low as 0.012MJy/sr (0.60mJy/pixel for a [FORMULA] pixel), indicating that the instrumental noise is negligible in the following results.

As was done in Paper I, the observed fluxes as well as the brightness of the images are scaled based on the fluxes of the brightest source F10507+5723 (UGC 06009) measured with IRAS. We found that the flux calibration based on the FCS1 measurements underestimates the 90 µm flux of the IRAS source by a factor of 2.6, although it gives the mean brightness of the images consistent with the COBE/DIRBE brightness within 25 per cent. The 170 µm flux of the IRAS source is assumed to be 1133 mJy as described in Paper I. The flux calibration based on the FCS1 measurements again underestimates the 170 µm flux by a factor of 1.5. For the 170 µm flux, we found that the discrepancy is mostly due to an underestimate of the effective solid angle of the ISOPHOT detector. Puget et al. (1999) and Lagache & Puget (2000) used an effective solid angle at 170 µm derived from Saturn footprint measurements, which is significantly larger than that used in the PIA. Here we assume theoretical PSFs of a telescope with a 60 cm primary mirror and a 20 cm secondary mirror which agree well with those measured during the ISOPHOT calibration observations (Müller 2000). With this assumption, the factors of 2.6 (90 µm) and 1.5 (170 µm) discrepancies reduce to factors of 2.0 (90 µm) and 1.2 (170 µm). The origin of the residual discrepancies is still unknown. However, this is not problematic for the main results and conclusions of this paper as described in Sects. 4 and 5 as long as the fluxes of F10507+5723 determined by the IRAS faint source survey are correct.

Fig. 1 shows the images of LHEX and LHNW used for the fluctuation analysis, each of which is the largest square area extracted from the mosaiced map (see Fig. 3 of Paper I) made up from four sub-field AAP maps. Each image is rebinned into [FORMULA] pixels2 (C_90), [FORMULA] pixels2 (C_160 LHEX), or [FORMULA] pixels2 (C_160 LHNW). The plate scale is [FORMULA] for 90 µm maps, and [FORMULA] for 170 µm maps.

[FIGURE] Fig. 1. The left column shows 90 µm images of LHEX (top) and LHNW (bottom). Each image is [FORMULA] wide ([FORMULA]). The right column shows 170 µm images ([FORMULA]). The LHEX image is [FORMULA] wide, while the LHNW one is [FORMULA] wide. The brightness shown in each image is offset from its median brightness. Each image is made of four [FORMULA] sub-fields. Roughly the north is left and the west is top.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000