This subsystem consists of two low-resolution grating spectrometers
with 
which
measure spectra in the wavelength range 2.5-5 
m (PHT-SS) and 6-12
m (PHT-SL). The spectrometers are non-scanning and each has a linear
array of 64-element Si:Ga detectors; all 128 detectors are operated
simultaneously. PHT-SS and PHT-SL have a common entrance aperture which is
square, 24x24 arcsec.
The size of the detectors in the dispersion direction is the same as that of
the image of the entrance aperture in the detector focal plane. This results
in a triangular spectral bandpass with a spectral range
(Full Width at Half Maximum) for a single detector
of 0.0445 m (= 3560 km/s) for PHT-SS and 0.0949 m
(= 3160 km/s) for PHT-SL.
The resolution is about 85 for PHT-SS and about 95 for PHT-SL,
respectively.
The spectra can be fully sampled at half of the resolution, i.e.
with 0.089 m (PHT-SS) and 0.1898 m (PHT-SL).
PHT-S can be used with the chopper .
Both gratings are operated in first order, and the central wavelength of each pixel has been measured in the laboratory. The wavelength scale will be checked against celestial sources which emit narrow lines.