ASTROSAT Soft X-ray Telescope CCD camera:
ASTROSAT SXT will carry as its focal plane instrument a thermoelectrically cooled X-ray CCD camera, based on the e2V Technologies CCD-22 chip flown in the XMM and SWIFT missions. The CCD has 600x600 pixels each of 40 micron square. It is a frame transfer device - an image transferred from image to store section can be read out while a new image is being acquired.
The CCD will operate in single photon counting mode. Each X-ray photon, depending on its energy, will liberate about 100 to 1000 electron-hole pairs. Preserving this total charge information for each photon will lead to the measurement of its energy, thus enabling spectroscopic studies. The energy resolution is strongly degraded by system noise. To reduce thermal noise in the CCD it will be thermoelectrically cooled to an operating temperature of -80oC, which is expected to yield an energy resolution of about 2% at 6keV.
Thermoelectric cooler and CCD assembly
Lab test result: isolated pixel events histogram on the CCD showing its energy resolution
The Focal Plane Camera Assembly will consist of the CCD and its cooling arrangement housed in a cryostat, which will also contain four Fe55 calibration sources, an optical blocking filter for the CCD and an aluminium proton shield to protect the CCD from proton damage while passing through the South Atlantic Anomaly region. The optical blocking filter is made of a single fixed polyamide film of thickness 184 nm, with a 48.8 nm thick aluminium coating on one side. This yields an optical transmission of about 0.25%, limiting the background light reaching the detector. The entire cryostat body is made of aluminium alloy, gold plated for thermal insulation.
A drawing of the SXT Focal Plane Camera Assembly
The Optical Blocking Filter mounted
inside the SXT Cryostat
SXT FPCA Cryostat made of gold-coated aluminium