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A choice of two echelle gratings is provided, both supplied by Milton Roy (formerly a division of Bausch & Lomb). The gratings are the largest available, with a nominal ruled area of 204408 mm.
The two gratings have different rulings (31.6 and 79.0 lines/mm) but have the almost identical blaze angles (nominally 63.43 degrees). This means that at a given wavelength the two gratings give nearly the same spectral resolution. This point is worth emphasising, since it is very different from the situation with a conventional intermediate dispersion spectrograph (e.g. ISIS or IDS) using first order gratings. The choice of grating for UES is determined not by the resolution required, but by the slitlength and wavelength coverage required (see Section 12.1 for more detail).