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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.