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The Low Energy Telescope

The two LE telescopes and detectors assemblies were identical. The telescopes consisted of double nested gold coated Wolter type I grazing incidence optics, with a focal length of 1.1 meter and an outer diameter of 0.3 meters. The telescope parameters give a high energy cut-off of ~2 keV.

The on-axis half energy width of the point spread function is 24 arc seconds, which degrades to 4 arc minutes 1 degree off-axis. Vignetting in the telescopes reduces the off-axis effective area to 45 % of its peak value 1 degree off-axis. In the focal plane of each telescope either a CMA, or a PSD detector was interchanged. Also a transmission grating spectrometer (TGS) could be inserted behind each telescope and the dispersed spectrum imaged by each CMA. The gratings were 500 lines mm-1 in one telescope (LE2+CMA2 8-400 Å) and 1000 line mm-1 in the other (LE1+CMA1 8-200 Å) with a spectral resolution of 2 Å and 1 Å respectively for energies > 0.25 keV, and 5 Å at 304 Å in both telescopes.

The CMA detector has no intrinsic energy resolution in the X-ray band, but a number of different filters gave coarse spectral information (analogous to UBV photometry). The choice of filter dictates the energy response with the overall energy window covered by the filter combinations ranging from 0.05 to 2.0 keV. The detection efficiency of the CMA 's decreases of 30% at 0.15 keV to 7% at 1.5 keV. The combined telescope and CMA on-axis effective area is shown in figure as function energy for each filter.

The CMA's are sensitive to ultraviolet photons and this caused contamination by bright O and B stars, pointed or serendipitous, in the field. Filters were used to determine the degree of contamination. The boron filter was free of UV contamination and relatively safe, as was the Al/P, except for very brightest and earliest stars. Also, the sum signal distribution of events in the CMA provided a crude method of differentiating between X-ray and UV sources. The most commonly used filters during the mission were 3000 Lexan , Al/P and boron.

The CMA particle background counting rate was typically 8 X 10-6 counts-1pixels-2 (a pixel is 4 arcsec) in the central region. The background counting rate depends on the strength of the solar wind. In 90% of the observations the average background counting rate is within a factor of 2 of the quiescent value. The average source detection threshold for a 104 second exposure (a typical minimum observation time) within the central 12 arc minute radius region of the detector, using the 3000 Lexan filter, was 2 X 10-3 counts per second. For observations longer than a few thousand seconds the sensitivity of the CMA is background limited. Source positions can be estimated to 6 arc seconds and 8 arcsec error radius at 67% and 90% confidence levels, respectively within the central 12 arc min.

There were a number of notable problems with the instrumentation associated with the LE's. Both PSD's failed early on during the performance verification phase. One of the CMA (CMA2) failed on 1983 October 28. The mechanism to insert the grating behind the other telescope (LE1) failed in 1983 September 15, eliminating this spectroscopic capability. The surviving CMA functioned well until the end of the mission.

Last Update: 24 August 2005

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