Chandra, the most advanced

Print edition : September 11, 1999

AT the heart of Chandra is the High Resolution Mirror Assembly (HRMA). Since high-energy X-rays would penetrate a normal mirror, special cylindrical mirrors were created. The two sets of four nested mirrors resemble tubes within tubes. Incoming X-rays wi ll graze off the highly polished surface and will be focussed on the detectors at the end of the telescope (with a focal length of 10 metres). Such grazing incidence is necessary because at greater angles of incidence, X-rays will just pass through mater ials such as mirrors. Chandra's mirrors are the largest of their kind and the smoothest ever created. The largest of the eight mirrors is about 1.2 m in diameter and 1 m long. The mirror assembly itself weighs more than a tonne.

At the end of its 13.8 m telescope, Chandra has two sets of focal plane detectors, which analyse and render the radiation focussed on to them by the front end of the telescope into images and spectra. Both the instruments are "two-in-one" devices consist ing of an imager or a camera and a spectrometer. The spectrometer comes into play when a transmission grating is swung into place behind the telescope mirrors to spread the X-rays into separate energies, much in the manner a prism breaks visible light in to its colours.

One of the devices on board Chandra is the AXAF CCD Imager and Spectrometer, or ACIS, which uses 10 charge-coupled devices (CCDs). AXAF itself stands for the observatory's original name - Advanced X-Ray Astrophysics Facility. Four of the CCDs, in a 2 x 2 array, constitute the imager (ACIS-I), which produces images 2048 pixels across, much like video camcorders and digital cameras, but detect X-rays, covering about 16.9 arc-minute of the sky. The moon has an apparent diameter of 31 arc-second, which gives an i dea of the area in the sky that the instrument images at a given time. The spectrometer (ACIS-S) uses a strip of six CCDs that take X-rays from an object and splits them into 12,288 discrete channels corresponding to 50 different X-ray energies.

The second instrument is the High Resolution Camera (HRC) which uses micro-channel plates (MCPs), a completely different detection method, with a resolving power of 0.5 arc-second, which is equal to reading the letters of a traffic sign from a distance o f 20 km or a newspaper from a distance of half a km. The MCPs are ultra-thin microscopic lead oxide tubes carrying electric charge. To achieve this kind of resolution, 69 million MCPs are used. The tubes are about 1.25 millimetres long and just one-eight h the thickness of the human hair. An X-ray entering a tube liberates an electron that bounces off the wall and releases several more and so on, until an avalanche of 30 million electrons arrives at the bottom where the discharge is interpreted as a meas ure of the X-ray's energy. The imager part of the device (HRC-I) is a single array 9 cm x 9 cm with a 31 arc-second x 31 arc-second field of view. The spectrometer, HRC-S, is a single 2 cm x 11.8 cm strip with special metal foils masking some parts to help spectral analyses.

Indian scientists too have contributed to the HRC. Dr. Somak Raychaudhury of the Inter-University Centre for Astronomy and Astrophysics (IUCAA) in Pune was part of the team at the Harvard-Smithsonian Centre for Astrophysics, which was responsible for the design and calibration of the HRC from September 1992 to February 1995 when the design was completed. The fabrication of the camera and the actual calibration were, however, completed in early 1996. According to Dr Raychaudhury, his contribution was to simulate the working of the HRC, given the spectra of the typical stellar objects that Chandra would be used to observe. This involved studying the energy response of various materials and deciding on the material to be used in the instrument, such as th e material for its UV (ultraviolet) filter, the coating for the micro-channel plates, the camera shutter, the window and so on. Dr. Raychaudhury also designed an optimal set of ground calibration tests.

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