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Central wavelength and order number

Having decided which central wavelengths are to be observed, it is then necessary to configure the spectrograph by moving the following mechanisms:

• The echelle angle . Changing this angle changes the position of the echellogram on the detector in the dispersion direction.

• The echelle angle . Changing this angle changes the position of the echellogram on the detector in the cross-dispersion direction.

• The prism translation. Moving the echelle grating changes the direction in which it sees the cross-dispersing prisms, and the prism assembly has to be moved (and rotated) to correct for this.

• The slit angle. In order to keep the projected slit at the same orientation on the detector, the slit angle has to be rotated as a function of wavelength.

• The collimator. There is a choice of two collimators, one coated for the UV (less than 4000 Å) and one with a wideband coating. When changing wavelength it is necessary to select the appropriate collimator.

• The colimator focus. When a different collimator is selected, it is necessary to apply an offset to the collimator focus, this is automatically calulated and applied by the UES control software.

The instrument control software contains a model of the spectrograph, which takes as input the central wavelength and order number required by the observer, and configures the spectrograph as described above. It is possible for the observer not to specify the order number, in which case the software will determine the interference order for which the wavelength selected is closest to blaze.