2.2 Overview

Figure 2.1: Schematic view of the PHT instrument. After passing the field mirror and the focal plane chopper the light beam coming from the telescope reaches change wheel I, which allows for the choice of one of the various sub-instruments and the polarimetric observing mode. The dual channel design of the PHT-S sub-instrument is indicated in the lower part of the drawing by showing the dichroic element and the two PHT-S detector arrays. Both PHT-C detectors are illuminated via change wheel II, which contains the filters for these sub-instruments. This wheel also carries the apertures for the PHT-P photometers. Finally, change wheel III contains the filters for the PHT-P sub-instruments. The fine calibration sources and the third PHT-P detector have been omitted for clarity (see also Figure 2.2).

Figure 2.2: Schematic view of the PHT instrument set-up for calibration measurements with the internal fine calibration sources (FCSs). Each FCS contained two thermal radiation sources (TRS). Tuning of the heating currents of the TRS elements in combination with the beam splitter allowed to match a wide range of calibration fluxes.

ISOPHOT, or PHT, was composed of 3 subsystems, or sub-instruments, optimised for specific photometric modes. Only one sub-instrument could be used at a time. The sub-instruments were:

PHT-P, a multi-band, multi-aperture photometer with three single detectors covering the wavelength range 3 - 120 m from the near-infrared (NIR) to the far-infrared (FIR).

PHT-C, two photometric far-infrared (FIR) cameras for the wavelength range 50 - 240 m.

PHT-S, two grating spectrophotometers, operated simultaneously, for the wavelength ranges of about 2.5 - 5 m and 6 - 12 m.

The modular subsystem concept minimised the number of mechanisms and resulted in a compact and rigid instrument. By appropriately setting three ratchet wheels a certain observational set-up could be selected from a large choice of useful combinations of filters and apertures. Figure 2.1 shows a schematic view of the instrument. For a more detailed description of the cryo-mechanics see Lemke et al. 1993, [35].

A focal plane chopping mirror was included for beam switching and modulation within the field of view of the instrument (Lemke et al. 1993, [35]). For absolute photometric measurements, the fine calibration sources (FCSs) were used as standard reference sources.

In addition, PHT was equipped with two sets of three polarisers, one set for the PHT-P detector group and one for the PHT-C detectors, covering the whole wavelength region from 3 to 240m. During the mission polarisation observations were only done at 25m and 170m.