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Astron. Astrophys. 336, L9-L12 (1998)

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2. Observations

We have carried out a far-infrared survey by using ISOPHOT (Lemke et al. 1996) which is an imaging photopolarimeter on board the Infrared Space Observatory (ISO; Kessler et al. 1996). Two fields named LHEX and LHNW, each extending approximately [FORMULA], were selected in the "Lockman Hole", a region of the smallest HI column density in the sky (Lockman et al. 1986), to minimize confusion noise due to infrared cirrus. Table 1 summarizes the combinations of the filters and the detector arrays which were used for the observations.


[TABLE]

Table 1. Detectors and filters[FORMULA]
Notes:
[FORMULA] Cited from Klaas et al.(1994)
[FORMULA] C100 is a [FORMULA] Ge:Ga detector array, and C200 a [FORMULA] stressed Ge:Ga array
[FORMULA] The size of each detector pixel is [FORMULA] for C100 and [FORMULA] for C200, so there are very small gaps between the individual detector pixels.
[FORMULA] The FWHM of the Airy disk is [FORMULA] (the Airy diameter): [FORMULA] for C100 and [FORMULA] for C200.


A pilot observation spending 6.6 hours in total was made in revolution 132 (March 28, 1996) in the PHT32 oversampled maps and scans mode (Klaas et al.1994). A quick data analysis showed that the signals were dominated by electronic noise due to the short integration ramp containing only three non-destructive readouts. We therefore selected the PHT22 staring raster map mode for the main observations using 35.4 hours, which were executed between revolutions 194 and 215 (May 28 and June 19, 1996). Each of the two fields is made up of four sub-fields having an approximately [FORMULA] area each. Table 2 shows the parameters used for raster mapping of a single sub-field, together with details of the readout settings.


[TABLE]

Table 2. PHT22 parameters for mapping a single sub-field
Notes:
[FORMULA] [FORMULA] corresponds to half of the C100 array (1.5 [FORMULA] one detector pixel), [FORMULA] to quarter of the C200 array (half of one detector pixel), and [FORMULA] to half of the array (one detector pixel).
[FORMULA] Kindly provided by Reach (1996) in units of (MJy sr-1)
[FORMULA] The duration of each integration ramp is 2 sec, and the number of non-destructive readouts is 63 per ramp. The integration time per raster point is 16 sec.
[FORMULA] The duration of each integration ramp is 4 sec, and the number of non-destructive readouts is 127 per ramp. The integration time per raster point is 20 sec.


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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998
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