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
m rising approximately linearly to
45% at 5m with a polarisation angle of 
6
3degrees, and for
PHT-SL starting at 12% at 6m rising approximately linearly to 40%
at 12m with a polarisation angle of 22degrees. 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 25m using the P2
detector and filter P_25 and at
170m 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 25m band and Laureijs & Klaas 1999,
[33] for the 170m band.
]