The Echelle spectrograph enables us to obtain high-resolution spectra by a comparatively wide entrance slit using the higher orders of an echelle grating. Because of the high order numbers, light from different orders overlaps in the beam dispersed by the echelle grating. In order to resolve this overlapping, a lower dispersion grating (a so-called cross disperser) that disperses light in the direction perpendicular to the dispersion direction of the echelle is used (see Figure ).
One strong point of the echelle spectrograph is its wide wavelength coverage, as the spectrum is ``folded up'' and recorded in a two-dimensional detector. Examples of the spectrum format are shown in Figure , and in the Appendices (Section ).
In general, a given wavelength appears in several spectra of different echelle orders. However, as the spectrum of each order has a blaze profile similar to a sinc function, the wavelength region which can be efficiently measured is limited. Usually, the efficiency is significantly different from one order to another. The wavelength range in one spectrum which does not overlap the similar range in the neighboring order spectra is called as the free spectral range, and can be represented by λ/m, where λ means wavelength and m means order number. The spectrum formats in Figure is shown for the free spectral range. If the free spectral range is covered by the detector, a complete spectrum with full wavelength range can be obtained without any gaps.
For a cross disperser with higher dispersion, the interval between orders is wider and the number of the orders observed with a detector is smaller. Thus, the maximum slit length without any overlap of orders is larger but the wavelength coverage is narrower when the higher dispersion cross disperser is used. On the other hand, for the lower dispersion cross disperser, the interval between orders is narrower and the slit length is more limited. As a result, the observable region in the sky is narrower when the lower dispersion cross disperser is used. Thus, the choice of the appropriate cross disperser should consider these characteristics.