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3.3 Spectral Resolution and Slit Width

Figure 4 is a conservative estimate of the trend of resolution or FWHM (full-width at half-maximum) versus slit width for the instrument. The spatial scale is 0.78 arcsecs/pixel. It should be pointed out that there is a general trend of slightly poorer focus as one goes from the center of the field to the edge.

The procedure for slit width selection has been to try to match the projected slit width to the resolution of the detector with some consideration being given to the slit light losses. For spectrographs with significant collimator-to-camera angle ( 15°), such as the Gold spectrograph, the full expression for projected slit width should be used.

Here w is the projected slit width, W the actual (physical) slit width, and r is the "grating anamorphic magnification". The factor r is a function of the grating tilt and collimator-camera angle, i.e.

where is the collimator-camera angle (55°) and t is the grating tilt. For the Wynne camera design, the camera-to-collimator focal ratio is 0.24 . At large grating inclinations the anamorphic demagnification factor becomes significant. In these cases the slit can be opened wider without degrading the resolution. See F. Schweizer's article PASP 91 , No. 539, 149, 1979 for more information.

Another factor, of lesser importance, for consideration at high grating inclinations is the grating fill factor, or that fraction of the light leaving the collimator that strikes the grating. As the grating tilt increases, a point is reached where the grating becomes overfilled and light is lost above and below the grating. This factor is also shown in Figure 5. Even at tilts as high as 5.00, the loss is only 9%.