MEGA spectrometer: a monochromator with supermillion resolution

The multiechelle grating arrangement (MEGA) [ Appl. Opt.21, 4363 ( 1982)] is used in a 1.2-m pressure scanned monochromator. The spectral orders from the exit slit of the pressure vessel, containing four 160-mm wide echelle gratings together with a collimating and a focusing mirror, are separated by a prism order-sorter monochromator. Reading the intensities of the spectral orders while varying the pressure gives a read-off of the whole 330–700-nm spectral range (extendable down to 200 nm). A spectral resolution close to 2 × 106 at 632 nm was measured with a He–Ne laser. Operation of the spectrometer is described in detail.