Chandra is the third observatory in the series of Great Observatories in the NASA programme after the Hubble Space Telescope and the Compton Gamma-Ray Observatory. Chandra was launched in July 1999. It contains a Wolter-Type I high-resolution telescope, two insertable gratings -- the Low-Energy Transmission Grating (LETG) and the High-Energy Transmission Grating (HETG) -- as well as two imaging detector systems. The LETG was developed by SRON in coöperation with the Max Planck Institute für Extraterrestrische Physik (MPE) in Garching.
Chandra/LETG instrument description
The LETG is the transmission grating for the LETG Spectrometer (LETGS), covering the wavelength range from about 0.5 to 14 nm (0.08 - 2 keV).
540 individual grating elements are mounted onto a toroidal ring structure with a diameter of 110 cm. Each of the elements consists of a freestanding gold grating with 1 micrometer grating period. The fine gold wires are held by two different support structures, a linear grid with 25.4 micrometer and a coars triangular mesh with 2 mm spacing. The whole grating ring can be inserted into the convergent beam just behind the High Resolution Mirror Assembly (HRMA) thereby dispersing the light of any X-ray source in the field of view into its spectrum. Spectral resolving powers of more than 1000 are possible. The efficiency of the grating spectrometer is of the order of 10% on average but is enhanced by a factor of two around 2 keV due to partial transparancy.
The LETGS consists of three elements: the High-Resolution Mirror Assembly of four nested paraboloid-hyperboloid grazing incidence mirror pairs, the LETG, and a dedicated position-sensitive detector in the focal plane.
The Chandra mission addresses a large number of astrophysical subjects, ranging from deep sky surveys to detailed spectroscopy of plasmas. The spectral coverage of Chandra ranges from 0.1 to 10 keV. The Low-Energy Transmission Grating (for which SRON is the PI-institute) addresses high-resolution spectroscopy at the longer wavelengths in this range, up to about 140 Angstrom. Studies of astrophysical plasmas in point-like sources form the core of its scientific objectives.
Prime candidates for study are stellar coronae, white dwarf atmospheres, X-ray binaries, cataclysmic variables and active galactic nuclei. Measurements of plasma emission lines allow the determination of physical parameters such as gas temperature and density, ionization state, elemental abundances, velocities, and red shifts.