Coded Mask Imaging on ASTROSAT
Electromagnetic radiation in the X-ray band – nanometer
and shorter wavelengths, cannot be focused as easily as
for optical, radio and other lower energy bands.
It is possible to use the grazing angle incidence technique to
focus these higher energy photons, as in the case of the
Telescope on ASTROSAT, but this focussing technique can
be used only for fields narrower than ~ 1°.
Coded mask imaging is one possible way of performing wide
field imaging with photons of energy greater than a few keV.
It comprises of utilising the shadows of a multiple pinhole
mask plate cast on the detector, with the shift in the
shadows encoding the location of the source in the sky.
For the literature on the coded mask concepts one can refer to
the webpage on GSFC, NASA site
Coded Aperture Imaging in High-Energy Astronomy by
Jean in 't Zand.
Two of the four X-ray instruments aboard ASTROSAT are based on
the coded mask imaging concept:
Scanning Sky Monitor (SSM)
Cadmium Zinc Telluride Imager (CZTI)
The SSM uses a one-dimensional imaging system with a position
sensitive proportional counter as the detector. The CZTI comprises
of a two dimensional mask plate mounted on top of a pixellated CZT
The coded mask plates for both the SSM and the CZTI have been
designed based on pseudo-noise Hadamard set Uniformly Redundant
This page was last updated on 31st Jan 2008.