The ISO Photopolarimeter (ISOPHOT)

PI: D. Lemke, MPI für Astronomie, Heidelberg, D

The ISO Photopolarimeter (ISOPHOT) will cover the entire wavelength region between 2.5 and 240 m by means of three different sub-instruments:

• PHT-P, a multi-aperture photometer with three detectors. The observer can select from 14 different filters and 12 apertures ranging from 5" to 180" to measure the infrared spectral distribution in the wavelength range from 3 to 125 m.
• PHT-C, a camera with a 3 3 array operating with 7 filters between 45 and 130 m, and a 2 2 array with 5 filters between 90 and 240 m.
• PHT-S, a 2 64 array grating spectrometer with / 100. This sub-instrument with a fixed 25 arcsec entrance aperture operates in the ranges 2.5 to 5 m and 6 to 12 m.

As an example, the sensitivities extended sources in the broad band filters are presented below. It is possible to detect faint structures of order a few mJy in only a few minutes of integration time.

ISOPHOT surface brightness sensitivity

A number of short wavelength filters of PHT-P are centered on ``unidentified'' dust features such as those at 3.3, 7.7 and 11.3 m, and at 9.7 m to cover the silicate band. There are 4 filters with spectral responses identical to the 4 IRAS wavelength bands for follow-up observations or cross-calibrations. All filters can be used in the wide beam mode (FOV 180") allowing detection of the faintest extended sources.

In parallel to the work related to the design and building of the instrument, the ISOPHOT team has investigated the detector properties in detail. As a result, a quantitative description of the drift behaviour has been established and efficient detector curing procedures have been defined. These procedures are necessary when ISO passes the Earth radiation belts.

PHT-P is designed for sensitive, high precision photometry. Most of the PHT-C filters are completely new to infrared astronomy and will provide a wealth of new data. PHT-S has been designed to perform sensitive measurements on ice and silicate features, as well as on the unidentified dust bands. ISOPHOT also has sets of long and short wavelength polarizers that can be put in the optical path of the PHT-P and PHT-C sub-instruments.

To improve the photometric accuracy of the observations, ISOPHOT is equipped with two internal black-body calibration sources and a focal plane chopper. The chopper can deflect the beam onto the calibration sources, can be used to point at off-positions, or can be used to point at off-positions. All observations, except for those with PHT-S, will include a calibration measurement with the internal black body. In addition, an observing mode will be available where the chopper is oscillating between black body and astronomical source thereby providing absolute photometry of the source.

In order to improve the instrumental efficiency it is possible to request sparse maps. This observing mode keeps the photometry instrument settings unchanged while pointing at different positions within a area of 1.5 degrees in radius. ISO also offers the possibility to observe extended structures by means of raster mapping. A dedicated mapping mode is available which combines raster pointing with chopper movement to obtain oversampled scans or maps.

The PHOT instrument gave clear and useful results from the recent straylight tests. However, due to anomalies with the filter wheels during these tests, the focal plane unit will be replaced by a spare unit.