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
Each detector is a Xenon-filled multi-wire position sensitive proportional
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
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).
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 email@example.com (Internet).
Go to general Coded Aperture Imaging page
November 30, 1995