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Astron. Astrophys. 337, L1-L4 (1998)

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2. Observations and Data Reduction

The observations were performed on 15th Dec 1997 with the C200 array of ISOPHOT (Lemke et al. 1996), the photometer on board ISO (Kessler et al. 1996). Raster maps at 120, 150, 180 and 200 µm, each 13[FORMULA]5[FORMULA]13[FORMULA]5 in size and centered on [FORMULA] = [FORMULA], [FORMULA] = [FORMULA], were obtained with a sampling of 1[FORMULA]5. Since this is also the pixel size of the 2[FORMULA]2 array, each pixel provides a complete map of the galaxy (except of the border), thus avoiding flatfield problems, and the 4-fold redundancy increases the stability against artefacts like relicts of cosmic glitches. The total integration time per sky pixel was 64 s at 200 µm and 48 s at the other more sensitive filters.

The data were reduced using the PHT Interactive Analysis tool (PIA 1) V7.1 in standard processing mode, together with the calibration data set V4.0 (Laureijs et al. 1998). This includes correction for the electronics' non-linearity, deglitching (removal of data sections disturbed by cosmic particle events), correction for signal dependence on the reset time interval. The detector responsivity was calibrated using associated measurements of the thermal fine calibration source on board.

The accuracy of the absolute photometric calibration depends mainly on systematic errors like those from detector transients and responsivity drifts, and it is currently known to be better than 30[FORMULA]. In fact, at each filter the deviations between the four independent maps obtained by the four detector pixels reached about 10[FORMULA] in the overlapping areas, and the average of the four maps was taken for the final map. Also, since the observations were performed directly one after another, the relative filter to filter calibration relevant for the shape of the SEDs should better than 10[FORMULA] which is the upper limit of the formal errors as well. Finally, differential details within the maps are accurate to better than 5[FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998