Background to Chandra

The X-ray telescope aboard Chandra consists of four nested Wolter type 1 mirrors with a total geometric area of 1100 cm² (780 cm² effective area). The high quality of the mirrors means a spatial resolution of 0.5 arcseconds at the focal plane can be achieved. The mirrors have excellent performance at high energies, and can focus photons with energies up to 10 keV.

There are several main instruments at the focal plane of Chandra. The High Resolution Camera (HRC) consists of two large-format micro-channel plate (MCP) detectors). The first is designed for imaging work, and has dimensions of 10 cm by 10 cm. The second is optimised for spectroscopy with the transmission gratings (see below) and therefore has dimensions of 30cm by 2cm. The very small pore size of these MCPs provides for very good spatial/spectral resolution observations. The sensitive energy range of these detectors is 0.1 to 10 keV.

Chandra also carries two arrays of CCDs; the AXAF CCD Imaging Spectrometers (ACIS). The CCDs of the ACIS-I are arranged to follow the focal plane of the telescope, and therefore are optimised for imaging work. The ACIS-S CCDs are aligned to obtain the best focus from the transmission gratings, and are therefore optimised for spectroscopic work.

The two transmission grating spectrometers are each optimised for work at a particular energy band; the LETG works best at lower energies (0.08 to 2 keV), and the HETG works best at higher energies (0.4 to 10 keV). Both instruments can achieve energy resolutions of (delta E)/E = 100 - 2000.