INTRODUCTION

ISOPHOT (PHT) is the imaging photo-polarimeter on-board the Infrared Space Observatory ISO. Its four principal modes are single element photometry (3 - 120 m), array imaging (40 - 240 m), polarimetry (3 - 240 m) and spectrophotometry (2.5 - 12 m). The wavelength range that was covered by IRAS will be extended by a factor of two towards longer wavelengths. This wavelength regime, which is dominated by the emission of very cool objects (15 - 30 K), is not accessible from the ground and only with considerably less sensitivity with airborne instruments.

Since ISO is a satellite observatory designed for pointed observations, objects significantly fainter (by a factor of 100) than the IRAS survey detection limit can be measured with ISOPHOT.

Key features of the instrument are:

• The filter set allows nearly simultaneous and therefore consistent measurements from the NIR to the FIR.
• A range of apertures (5'' - 180'') can be selected to match the point spread function of the selected filter and thus optimize the source-to-background contrast.
• Because the cold space telescope contributes negligible emission, relatively wide beams can be used to detect faint extended emission.
• A chopping mirror in the beam path permits differential measurements to suppress drift effects of the photoconductive detectors when trying to detect faint sources superposed on a much higher background level.
• The chopper mirror also allows for oversampled scans on bright celestial objects at far infrared wavelengths.
• Internal reference sources provide a homogenous calibration relative to the best available sky standards. The ISO spectrometers (SWS and LWS) will be photometrically calibrated with the PHT instrument.
• The PHT spectrometer sub-instrument is optimised in wavelength coverage and spectral resolution for the investigation of dust features.
• The high sensitivity and therefore relatively short observation times permit differential measurements through different polariser settings.

Purpose of this document

This manual provides background information and all details on ISOPHOT which is necessary 1) to select the appropriate observing modes (AOTs) for a particular scientific goal and 2) to calculate approximate observing times. The reader should be familiar with the contents of the ISO OBSERVER'S MANUAL.

In chapter 2, a more detailed description of the instrument is provided. Chapter 3 gives an overview of the capabilities and instrument modes (AOTs). Chapter 4 contains the general performance overview with the limiting sensitivities of each instrument mode, while more detailed descriptions of sensitivity calculations for the specific modes are associated with the individual instrument mode descriptions in chapter 6. Chapter 5 describes the overall calibration philosophy and the accuracy of photometric calibrations. Finally chapter 6 contains a ``cookbook'' giving detailed advice on when and how to use the different observing modes.