Two decker plates which slide just above the slit (figure 1 ) are available to control the slit length. Table 2 and table 3 list the stellar and comparison dimensions and their projected sizes on the detector. The maximum usable decker size will depend upon the wavelength, the echelle grating, and the amount of cross-dispersion. For the long focus cameras, figure 3 , figure 4 , figure 5 and figure 6 show plots of order separation vs. wavelength which may be used to select a suitable decker, given the dimensions in table 2 and table 3. The values from the graphs may be divided by 2.2 for the UV camera. The selection of a decker length may be determined by the need to do sky subtraction and the order separation. Decker number 1 is usually preferred for stellar observing since it has a larger selection of small openings.
Table 2: Echelle decker no. 1
Table 3: Echelle decker no. 2