2 The FIRBACKsurvey: Fields & observations

2.1 Fields

FIRBACKis a survey at 170 $\mu$m covering four square degrees in three high galactic latitude fields, called FIRBACKSouth Marano (FSM), FIRBACKNorth 1 (FN1) and FIRBACK/ ELAIS North 2 (FN2) (see Table 1). They were chosen to have foreground contaminations as low as possible: the typical HI column-density is less than or equal to $N_{\rm H} \simeq 10^{20}$ cm^-2, and the 100 $\mu$m brightness is less than 1.7 MJy/sr on DIRBE maps. In addition, FN1 and FN2 were chosen to match some fields from the European Large Area ISO Survey, ELAIS (Oliver et al. 2000), which had been covered at 15 $\mu$m with ISOCAM (Serjeant et al. 2000) and at 90 $\mu$m with ISOPHOT (Efstathiou et al. 2000). FN2 observation time is a collaboration between the ELAIS and FIRBACKconsortia.

 

 

Table 1: Fields of the FIRBACKsurvey at 170 $\mu$m.

field	$\alpha _{2000}$	$\delta _{2000}$	l	b	S100a
FSM	03$^{\rm h}$11$^{\rm m}$	$-54^{\circ}$45$^\prime$	270$^{\circ}$	$-52^{\circ}$	1.42
FN1	16$^{\rm h}$11$^{\rm m}$	$+54^{\circ}$25$^\prime$	84$^{\circ}$	$+45^{\circ}$	1.17
FN2	16$^{\rm h}$36$^{\rm m}$	+ $41^{\circ}$05$^\prime$	65$^{\circ}$	$+42^{\circ}$	1.19

^a Mean brightness at $100~ \mu$m (MJy/sr) in DIRBE maps

(annual average, zodiacal component subtracted).

 

 

Table 2: Observational characteristics of the FIRBACKfields.

field	FSM	FN1	FN2
area (sq. deg)	0.95	1.98	0.96
rastersb	4	2	2
redundancyc	16	8	8
$t_{\rm int}$d (sec)	256	128	128
raster stepe (pixels)	1,1	1,1	1,1
offsetf (pixels)	0.5,0.5	<1g	<1g
	2,2a
date	Nov.-1997	Dec.-1997	Jan.-1998
	Jul.-1997a
revolutionh	739 to 744	753 to 774	785 to 798
	593a

^a In the case of the FSM1 field only.

^b Number of different rasters mapping the same field.

^c Number of different observations per sky pixel on the

center of final coadded map.

^d Integration time per sky pixel on the center

of final coadded map.

^e Offset in pixel in the Y and Z directions of the spacecraft

between the steps on the raster.

^f Offset in pixel between different rasters.

^g Offset is irregular due to the rotation of the fields.

^h ISO revolution numbers (or number range) of observation.

2.2 Observations

Observations were carried with ISO, using the ISOPHOT spectro-photo-polarimeter. We used the C_200 $2 \times 2$ pixel photometer and C_160 broadband filter centered at $\lambda = 170\, \mu$m. Scanning the sky was done in raster map mode, AOT P22, with one pixel offset between each pointing, to provide the redundancy. Individual rasters were shifted with respect to each other by a fraction of a pixel to provide proper sampling where possible. Table 2 summarizes the observational characteristics of the fields.

The FSM field is composed, for historical reasons, of four individual fields, called FSM1, 2, 3 and 4 (Fig. 6 in Lagache & Dole 2001). FSM1 on the one hand, and FSM2, 3 and 4 on the other, have been observed continuously: transient effects are thus reduced and no rotation of the field occurs between different rasters (same roll angle). FSM1 rasters are offset by two pixels in order to maximise redundancy and establish the ISOPHOT sensitivity for such observations, whereas FSM2, 3 and 4 are offset by a half pixel in both Y and Z directions to increase oversampling.

The FN1 field is composed of eleven individual fields (Fig. 7 in Lagache & Dole 2001), observed twice. Observations were not performed continuously, so that each individual raster has a different roll angle, giving a sampling of the sky that is non uniform.

The FN2 field is composed of nine individual fields (Fig. 8 in Lagache & Dole 2001), observed twice. The other characteristics are the same as for FN1.