3.4 Polarimetric Imaging

Because the polarimetric imaging was more complex, it was performed with the Calibration Uplink System (CUS) and not through an AOT. Observations using this mode could be performed using one filter wheel position (broadband filter or CVF position), selected by the user, per application of the mode. Only CAM LW was supported. The target was observed through three separate polarisers. The polarisers had orientations with respect to the spacecraft axis of 0, 120 and 240. The angle is defined such that the +y-axis has orientation 0 and angles are measured toward the +z-axis direction (see the ISO Handbook Volume I, [40]).

ISOCAM polarisation observations start with a raster measurement without polariser on the free entrance hole. The observer could then choose:

1. to rotate the polarisers on each successive raster position, or
2. to perform the raster in one polariser followed by a rotation of the entrance/polariser wheel to the next polariser position at which another raster was performed, and so forth.

In both observing procedures, the rasters were repeated in several observing cycles. The second observing mode was in particular recommended for extended targets. The nominal spacecraft pointing was offset to compensate for the known source displacement on the detector which occured while cycling the polarisers (see Section 2.2.3). A detailed description of the performance and capabilities of polarisation measurements with ISOCAM is given in Siebenmorgen 1999, [55]. In the ISO Data Archive (IDA), the polarisation data can be found catalogued under one of the modes: CAM05 (only for point sources) or CAM99 (for point and extended sources).