The dual coded mask telescope for Spacelab 2

XRT was developed at the Space Research Department of the University of Birmingham (UK) and was operated between July 29 and August 6, 1985 as part of Spacelab 2 onboard the US Space Shuttle ( STS 51-f). It was the first coded aperture instrument to be flown in space in stead of in the stratosphere. The instrument consists of 2 coded aperture cameras with the same 6 square degrees FOV but with different angular resolutions (12 arcmins FWHM for the 'coarse' and 3 arcmins for the 'fine' telescope).


Each detector is a Xenon-filled multi-wire position sensitive proportional counter. Readout of the two coordinates of a detected photon is by identification of the centroid of the induced charge on two orthogonal sets of cathode wires, a position readout of better than 1 mm being achieved over the 320X320 mm sensitive area.

Egg-crate collimators with 3.2X3.2 degrees FWHM response in front of the detector windows prevent sources just outside the 6.4X6.4 degrees FOV from forming an incomplete shadow of the mask pattern on the detectors.

The aperture pattern and imaging

The coarse mask pattern has 31X33 elements per each of the 2X2 cycles, the fine mask pattern 127X129 elements. The basic patterns are URAs.

Image of the coarse pattern.

Image of the fine pattern (courtesy Rob Rideout, University of Birmingham).

Scientific objective

The principal scientific objective of SL2-XRT was to map the X-ray emission of galaxy clusters, extending the work as performed with the Einstein satellite to higher photon energies. Other targets of SL2-XRT included supernova remnants and the Galactic Centre region.

Characteristics per telescope

Detector type Multi-Wire Proportional Counter Detector gas Xe (90%), Methane (10%) Detector gas pressure at 1.5 atm Active detector area 1000 cm**2 Distance Aperture-Detector 2.9 m Field of View 6.4 X 6.4 degrees (FCFOV) Mask element size 2.5 X 2.5 and 10.0 X 10.0 mm**2 Mask open fraction 0.5 Angular Resolution (FWHM), on-axis 3 and 12 arcmin Active photon energy range 2.5-25 keV Photon energy resolution < 20% at 6 keV Sensitivity typical 0.4 mCrab in 5 hrs (5 sigma)

The above info is an excerpt from "A coded mask telescope for the Spacelab 2 mission" by Willmore, Skinner, Eyles and Ramsey, NIM 211, page 284 (1984).

For further information on SL2-XRT, we suggest to contact Dr. G. Skinner at email address (Internet).

Go to general Coded Aperture Imaging page

November 30, 1995