The Low Energy Transmission Grating Spectrometer (LETGS) on board CHANDRA (AXAF)


    Instrument description:

    The LETG is the transmission grating for LETGS (see below) covering a wavelength range of about 0.5 - 14 nm (0.08 - 2 keV).

    540 individual grating elements are mounted onto a toroidal ring structure. 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 grating spectrometer is of the order of 10% on average but will be enhanced by a factor of two around 2 keV due to partial transparancy.

    The Low Energy Transmission Spectrometer:

    The Low-Energy Transmission Grating Spectrometer (LETGS) consists of three elements; the High-Resolution Mirror Assembly, consisting of four nested paraboloid-hyperboloid grazing incidence mirror pairs, the LETG, and a dedicated position-sensitive detector in the focal plane.

    The LETG itself consists of a great many grating elements mounted on a toroidally shaped support structure with a diameter of 1100 mm, positioned in the mirror beam. Each of the elements, grating, detector and mirror, the latter with a focal length of 10 m and with unprecedented X-ray optical quality, yield a resolving power of better than 1000 at the longer wavelengths.

    The production and qualification testing of LETG was finished in the second half of 1996. In december 1996 a test campaign started at the long beam X-ray test facility at Marshall Space Flight Center, Huntsville, Alabama.

    Click on picture to see larger image + description

    Preliminary test results (status summer 1997) are summarized in Brinkman et al. and Predehl et al.