OVERVIEW OF SCIENTIFIC CAPABILITIES

This section gives an overview of ISO's scientific capabilities. It serves as a guideline to the proposer to make a first selection on which capability of which instrument will be most suitable for the proposer's scientific case.

The capabilities provided by the spacecraft are:

• single pointing
• raster
• slews

Observations can be carried out at one single position - single pointing - or in a series of sky positions arranged in a regular grid, where the observation is repeated at each position - raster map. Slewing is only used for initial target acquisition and to move the spacecraft to the next raster map point. The spacecraft observing modes are explained in detail in Part III.

ISO's payload consists of four instruments:

• a camera: CAM, 2.5 - 17 m
• a long wavelength spectrometer: LWS, 43 - 196.7 m
• an imaging photo-polarimeter: PHT, 2.5 - 240 m
• a short wavelength spectrometer: SWS, 2.4 - 45 m

The capabilities provided jointly by these instruments can be summarized in the following categories:

• imaging
• photometry
• polarimetry
• spectroscopy.

Direct IMAGING is achieved by using arrays in broad and narrow spectral bands across much of the entire wavelength range from 2.5 to 240 m with CAM and PHT. Imaging can be performed on a single position or in a raster map. Spatial oversampling is possible. Table 3 summarizes the imaging capabilities including the available pixel fields of view. Images can also be constructed, e.g. in the wavelength range from 18 to 50 m, with a single detector when used in raster mode (Sect. 13.3). Naturally, the efficiency of a single detector raster mapping is much lower than with an array even when the array is small as is the case in the far infrared.

 

Instrument No. of Pixels Pixel field of view ['']

2.5 - 5 CAM 1.5, 3, 6, 12

5 - 17 CAM 1.5, 3, 6, 12

50 - 120 PHT 43.5

120 - 240 PHT 89.4

 

PHOTOMETRY and POLARIMETRY can be achieved in broad and narrow spectral bands across the entire wavelength range with CAM and PHT. Observations using multiple apertures can be performed out to 125 m. Both instruments are equipped with three polarizers which can be used with various pixel fields of view (CAM) or spectral band/aperture combinations (PHT). Photometry and polarimetry can be made on a single position or in a raster map. Additionally, a sparse map, that is a map with arbitrarily spaced pointings within a given area, can be obtained with PHT.

Figure 9 Spectral resolution for photometry and spectroscopy.
The abscissa shows increasing wavelength, the ordinate the resolution .

For SPECTROSCOPY, resolving powers from 40 to 20,000 are available (see Table 4). The sampling of the spectra depends on the instrument. LWS has a number of single detectors each one dedicated to a specific wavelength range. Within a wavelength range a spectrum is obtained spectral point by spectral point. SWS has a number of linear detector arrays, where each samples a small portion of the spectrum - a subspectrum - which has a length of eight resolution elements. The spectrum in the required wavelength range is obtained by observing the appropriate number of overlapping subspectra.

Spectrophotometry can be carried out with the PHT-S subinstrument, where the entire spectrum is obtained simultaneously, or with CAM which uses a Circular Variable Filter (CVF) and where spectral images are taken at one CVF position at a time.

Spectroscopic and spectrophotometric observations can be made on a single position or in a raster map.

 

Instrument Resolution Optical Element

2.5 - 12 PHT-S 90 Grating

2.5 - 17 CAM 40 Circular Variable Filter

2.5 - 45 SWS 2000 Grating

15 - 35 SWS 20000 Grating + Fabry-Pérot

43 - 197.6 LWS 200 Grating

43 - 197.6 LWS 15000 Grating + Fabry-Pérot