The design of the MES is discussed in some detail by Meaburn et al. (Monthly Notices of the Royal Astronomical Society, vol 210, p463, 1984) so the description given here is brief. In its most common operating mode, a lens designed by Charles Wynne both collimates the incoming beam from the telescope and refocusses the spectrum after the nearly Littrow dispersion by the echelle grating. No cross-dispersion is employed. The separate echelle orders, each containing one or more nebular emission lines, are isolated with highly efficient, three period interference filters. CCD detectors or the CCD-IPCS on the WHT can be used with the MES, these are described in Chapter
The simple optical layout, transmission optics with three layer anti-reflection coatings throughout and the quasi-Littrow configuration of a reflection echelle grating all combine to achieve an exceptionally high efficiency for the spectrometer. One penalty for this optimization is the restriction to 3900--7500 Å of the operating wavelength range.
The efficiency is enhanced further for particular astrophysical problems by having a variety of options for the entrance slits. Either a single long slit or a multi-slit with up to five long slits transmitting simultaneously are routinely employed. Separate line profiles are obtained for each resolvable element along these slits. Where emission and absorption line sources are distributed randomly within the entrance aperture, multi-image masks, prearranged to accept the light from the separate sources, can be used.
Fibre-optic format changers are also available. Initially, six separate fibre arrays have been assembled for use in this mode (MATADOR). Here the entrance ends of the fibres (with up to 529 being employed in one array) are arranged to match the shapes of various sources in the focal plane of the telescope. The output ends of the fibres are then distributed along up to five entrance slits of the spectrometer.
In its primary mode, the MES fills a distinct niche between more traditional, cross-dispersed, echelle spectrometers, which are primarily useful for small sources over very large wavelength ranges, and the stepped Fabry-Perot devices such as TAURUS-2 which are most usefully applied to very extended sources but over very small wavelength ranges. The fibre-optic format changers extend the MES's application to some extent into TAURUS-2 territory.
The imaging mode of the MES, which was originally introduced as a minor alternative, has also proved very useful in practice. Here the slit assembly is replaced by a clear aperture and the plane mirror is inserted. A deep, filtered IPCS image of any source whose line profiles have been measured, can be obtained at the touch of a button. Moreover, an image of the slits superimposed on a quick-look image of the source, produces a precise positional record of the echelle data.
Two other modes are included but have not been used, since they do not compete very well with conventional echelle and intermediate dispersion spectrometers. In these a grism can produce cross-dispersion or, with the mirror in the beam, a low dispersion spectrum.