For measuring small polarization degrees of celestial sources subtraction of the instrumental polarization becomes crucial. For PHT-P and PHT-C the polarization of the instrument is about 10%. The exact values of the PHT instrument and the ISO telescope can only be measured in-orbit and will be included in the calibration. The angle of the instrumental polarization projected on the sky depends on the actual observing date as the ISO telescope cannot be rotated around its optical axis because of the spacecraft's pointing constraints.