The Short Wavelength Spectrometer (SWS)

PI: Th. de Graauw, Lab. for Space Research, Groningen, NL

The Short Wavelength Spectrometer (SWS) allows users to obtain medium or high spectral resolution data at infrared wavelengths. It has two grating mechanisms, a Short Wavelength section and a Long Wavelength section, feeding light to four detector arrays. Two Fabry-Pérot (FP) filter systems can be inserted into the light path of the Long Wavelength and these feed two smaller detector arrays. The SWS gratings can observe over the wavelength range 2.4 to 45 m, split into four wavelength regions, with spectral resolutions of between 1000 to 2500 depending on wavelength, or a velocity resolution of between 300-150 km/s. The grating detectors have a linear response to input illumination over a large range of input illumination, 0.5 - 100000 Jy. The FPs can observe between 15 and 35 m with a nominal spectral resolution of 30000, or a velocity resolution of about 10 km/s. The FP wavelength coverage can be extended down to 11.4 microns with a resolution reduced to 20000 at the shortest wavelength. Limiting fluxes for a ten minute grating observation at a S/N of 20 range from 0.1 to 1 Jy depending on the wavelength, while the FP can reach W/m in the same time at the same S/N. Absolute photometric calibration is expected to be accurate to 30% for both the FP and the grating. Wavelength calibration is expected to be accurate to 1/5 of a resolution element for the grating and 1/3 for the FPs.

One of the SWS detector blocks was recently changed. The original Si:Sb detector material exhibited very strong detector non-linearity caused by memory effects. It has therefore been changed to an array of Si:As Back Illuminated Blocked Impurity Band (BIBIB) detectors that do not exhibit such effects. The Flight Model (FM) has been delivered to ESA and has been integrated into the cryostat, with the Flight Spare currently being re-assembled after tests of the FP unit. Cold tests of the FM are underway and it is performing nominally.

Development of the SWS data reduction system is well advanced. Parts of the software have been delivered to ESA and are undergoing maintenance while the remainder is under development.