The four cross dispersing gratings currently available are 165 x 220 mm with a slightly smaller ruled area. Here again changing cross dispersers is possible during the night, but should be minimized to prevent loss of observing time due to setup and alignment.
Table 6 contains grating specifications, while figure 7 shows the cross disperser efficiencies vs. wavelength. Figure 3 , figure 4 , figure 5 and figure 6 may be consulted for more detailed order separation data (plot of order separation vs. wavelength), or order separations may be calculated as in §2.11.
In addition to the cross disperser gratings, a silver coated flat is available which can be used in conjunction with a narrow band order separation filter to provide high resolution spectra of a single order for maximum spatial coverage.
Table 6: Cross Disperser Gratings
For gratings 226-1,2 the encoder settings required to center a particular order of the echelle grating at the camera focus are given in figure 8. The cross disperser may be set by hand or by computer. To manually set the cross disperser, loosen the brake under the cross disperser housing (flip the lever ~180 away from the cross disperser access door) before turning the hand crank. Table 7 contains grating settings for loading, flipping, and zero order of the cross dispersers.
Table 7: Cross Disperser Settings