Each grating has its own scanning mechanism, enabling the use of both parts of the spectrograph at the same time, albeit through the same aperture. Wavelength scanning is achieved by rotation of a flat mirror close to each grating in discrete scan steps .
After reflection from the gratings, the light almost retraces its path
and, by means of small-diameter re-imaging relay optics, the high
resolution spectral image of each wavelength band is re-imaged onto the
detector block. These relay optics have various functions:
- they relay the high resolution image to a more easily-accessible
location on the detector block ,
- they change the relative opening of the beam to a value that is
optimised to the detector dimensions,
- they enable efficient straylight rejection.
To use the F-P's, the radiation returning from the LW grating is
collimated again, transmitted through a tunable F-P interference
filter and imaged onto separate detectors in the detector block.
This second light path is also
shown in Fig. 2.2 (p. 
).
The two F-P's are mounted on a single pair of parallel plates. Their
separation and parallelism can be varied by changing the currents in
three pull coils.