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3.3. Cross-Disperser Grating

  The cross-disperser grating operates in a Wadsworth configuration (Strong 1958, p. 612), accepting the parallel beam from the echelle and focusing an image of its first order diffraction on a slightly tilted image plane of the detector 1800 mm away. It has a ruled area of 190×143 mm divided into 8.5 partitions, each having a width of 17 mm and a separate blaze angle to keep the efficiency high over all parts of the parabolic surface. This grating was ruled by Hyperfine Corp. at a frequency of 171 grooves mm-1 and a blaze angle of 0.5° in a direction that reduces the Wadsworth angle so that aberrations are minimized. The decenter distance of the chief ray is 130 mm.

At any one time, the field of view of the detector covers a 200Å range in the cross-disperser direction (nominally 950-1150Å) but only of the free spectral range of a typical echelle order. Thus, to cover the entire spectrum the echelle is mounted on a pivot bearing so that 4 different exposures can be made at slightly different angles of incidence and diffraction.