4.8 Instrumental Polarisation

The ISOPHOT instrumental polarisation without the contribution of the ISO optical elements (primary, secondary and pyramidal mirrors) was investigated in laboratory, see Schlötelburg 1992, [48]. The laboratory measurements indicated a degree of linear polarisation of less than 10% for the P detectors. The polarisation degree of the arrays varies from pixel to pixel, some pixels showed polarisations as high as 10%.

For PHT-S the instrumental polarisation degree is measured to be considerably higher: starting at 10% at 2.5$\mu$m rising approximately linearly to 45% at 5$\mu$m with a polarisation angle of $-$6$\pm$3degrees, and for PHT-SL starting at 12% at 6$\mu$m rising approximately linearly to 40% at 12$\mu$m with a polarisation angle of $-$2$\pm$2degrees. The main cause of PHT-S instrumental polarisation is the grating.

PHT was equipped with a filterwheel in which linear polarisers are mounted for both the PHT-P and PHT-C subsystems (Klaas et al. 1999, [25]). The polarisers were used to measure polarisation of the sky at 25$\mu$m using the P2 detector and filter P_25 and at 170$\mu$m using the C200 detector array and filter C_160, see Section 3.10.4 for a detailed description of the observing modes. The instrumental polarisation in these filter bands was determined in-flight by measurements of unpolarised sources like stars or a large sample of background fields. The instrumental polarisation degrees and angles measured in-orbit are listed in Table 4.9, the values have been taken from Klaas et al. 1999, [25] for the 25$\mu$m band and Laureijs & Klaas 1999, [33] for the 170$\mu$m band.

Table 4.9: Polarisation degrees and angles at 25 and 170$\mu$m measured in-orbit, from Klaas et al. 1999, [25] and Laureijs & Klaas 1999, [33].

$\lambda_{ref}$	pixel	P	$\Theta$
[$\mu$m]		[%]	[$^{\circ}$]
25	-	3.0$\pm$0.2	-19$\pm$2
170	1	3.6$\pm$0.6	-62$\pm$5
170	2	7.8$\pm$0.7	83$\pm$2
170	3	4.0$\pm$0.6	-37$\pm$4
170	4	4.6$\pm$0.7	71$\pm$5