Five echelle gratings are currently available; the individual specifications are given in table 4. The ruled area dimensions are 127 x 254 mm. Echelle gratings may be changed during the night, but angle settings should be determined prior to nightime use to minimize loss of observing time.
Both the dispersion and the free spectral range (the wavelength difference between two wavelengths in adjacent orders at the same angle of diffraction) are set by the choice of echelle grating. These two properties, together with the amount of cross dispersion (limited to orders one through three of the cross dispersers), determine the spacing between orders. The 31.6 g mm-1 echelle grating, for example, will give complete spectral coverage (with the UV camera), but since the free spectral range is small, the orders are close together. While this may not be a problem for stellar objects, it could be serious for long slit spectroscopy. Wider spacing between orders can be obtained by using either the 58-63° or the 79-63° echelle. These echelle gratings may require some adjustment along the dispersion in order to center a particular wavelength on the detector. To manually reset the echelle grating, be sure to loosen both clamps (push up on the clamp handles) before turning the tilt knob. Approach the desired setting from higher numbers. Pull down on the clamp handles to clamp the grating motion. The echelle grating can also be set by the computer (§3). Table 4 and Table 5 will be of use in adjusting the echelle grating.
Table 4 contains parameters for each echelle grating that can be used to determine the proper grating setting, dispersion and order separation for a given wavelength. For example, given the 79-63° echelle and a long focus camera, an observer wishes to know the above quantities for observing at H. From table 4, the first order echelle blaze wavelength is given from which the order of interest can be determined:
or H would fall closest to the center of order 34.
The dispersion is given by
where is the angle of the echelle blaze peak and f is the camera focal length (§2.13).
Since the blaze of order 34 is
the echelle grating would have to be tilted
to center H.
An estimation of the spectral coverage perpendicular to the echelle dispersion can be obtained given the amount of cross dispersion (table 6). With the T2KB CCD (49.2 x 49.2 mm) and 226-1 cross disperser the spectral range is
Note that this is independent of echelle grating.
The free spectral range at H would be
which implies that the order separation would be
Another way of determining the spectral coverage and arrangement of specific wavelengths covered in a CCD frame is by use of the Th-Ar maps and overlays kept at the 4-m telescope.
Table 4: Echelle Gratings
Table 5: Echelle Grating Adjustment