Three slit widths are available for spectrograph+CCD imaging, which are positioned with the silver crank handle [R] on Side 3 of the spectrograph. To change the slit, first release the little rectangular lever on the bottom left of the slit assembly, that engages and disengages the handle from the mirror. The lever must be pushed all the way in, i.e. flush with the base of the assembly to properly release the slide assembly. Turning the crank handle will now pull the assembly in or out, moving to different slit widths as follows:
Standard Slit Plate
Make sure to lock in the new slit by pulling out the clutch lever again.
Each slit has a roughly 4 arcmin length. Unfortunately, the edges of the apertures have some irregularities, which will be noticeable in the resulting spectra. It is therefore crucial that the slit not be moved between the flat field and target observations. The throughput of the slits for various seeing conditions is analyzed in Figure 3.1.
Figure 3.1: Efficiency as a function of slit width and seeing.
The echellette decker is an alternate slit plate for the aperture assembly which contains 2 combinations of holes and 6 short slits, labelled A--H in the table below. Successive decker locations are separated by exactly one turn of the aperture plate lead screw (motor axis). It is suggested that the rather delicate adjustment necessary for proper selection and setting of the echellette decker be verified visually with the guiding TV.