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This 2" x 2" (50.8 mm x 50.8 mm) filter holder has 2 filter positions, plus open, as determined by detents on the filter slide. When the filter bolt lever is pushed all the way in, the left filter is in the beam. Pulling the filter bolt lever out to the first detent places the right filter in the beam. When the filterbolt lever is pulled all the way out to the stop, no filters are in the beam. Transmission curves for some of the more commonly used order separation filters are shown in figure 2. Measured transmission curve plots for each filter available at the coudé spectrograph are shown in a manual kept at the telescope. Other special filters are available on advance notice, or observers may bring their own.
Figure 2: Commonly used coudé filters.