THE LOW ENERGY IMAGING TELESCOPES

The LEITs and their associated electronics were proposed by the Laboratory for Space Research in Leiden, the Space Research Laboratory in Utrecht and the Mullard Space Science Laboratory of the University College London. Each of the two identical telescopes consisted of a double nested, gold coated, Wolter I optic with a focal length of 1.1 metres and an outer diameter of 0.3 metres. The high-energy cutoff was around 2 keV. The on-axis half energy width (HEW) is 24 arc seconds which degrades to 4 arc minutes 1 degree off-axis. 

In the focal plane of each telescope was either a channel multiplier array (CMA) or a position sensitive proportional counter (PSD) which were interchanged. In addition a Transmission Grating could be inserted behind each telescope and the dispersed spectrum imaged by the CMAs. One CMA failed on 28 October 1983 (apart from a brief revival on 9 February 1984). The other CMA functioned perfectly throughout operations. The mechanism for raising the grating on one of the detectors failed on 15 September 1983, with the grating out of the field of view (FOV) and the imaging was unaffected. Whilst the other mechanism was unaffected, it was unfortunately on the telescope where the CMA had failed. A total of 24 difference sources were observed with the gratings.

The CMA FOV was 2 degrees in diameter and the spatial resolution was limited by the telescope. The CMA had no intrinsic energy resolution, but a number of different filters gave course spectral resolution. The combined telescope and CMA on-axis effective area is shown below for a single telescope and various filters. The most common filter combination was 3000 Å lexan, Al/P and boron as this allowed the interstellar absorption and spectral slope or temperature to be constrained, particularly when used in conjuction with data from the ME or GSPC.

The effective are of the LEIT with a CMA at the focal plane for each of the available filters
An LEIT being assembled

The minimum particle background in the CMA was 8 x 10^-6 count s^-1 pixel^-2 in the central region where each pixel is 4 arc second square. The background counting rate was affected by solar activity for 10% of the time, but for the remaining 90% it was within a factor of 2 of its quiscent rate. The average source detection threshold for a 10⁴ second exposure (considered typical) within the central 12 arc minutes radius region using the 3000 Å lexan filter is 2 10^-3 count s^-1.