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4.8 Instrumental Polarisation  

The instrumental polarisation for ISOPHOT without the contribution of the ISO optical elements (primary, secondary and pyramidal mirrors) has been modelled in laboratory, see [26] These measurements have shown that the polarisation of the P detectors is less than 10%. The polarisation of the arrays varies from pixel to pixel, some pixels have measured 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 5 m with a polarisation angle of -63 degrees, and for PHT-SL starting at 12% at 6 m rising approximately linearly to 40% at 12 m with a polarisation angle of -22 degrees. The main source of PHT-S instrumental polarisation is the grating.