There are seven post-slit filter positions (figure 1 ) and one open position. Table 1 contains a list of these 1" x 1" (25.4 x 25.4 mm) filters, and their transmission curves can be found in figure 2 . All filters (except CuSO4) are anti-reflection coated for maximum throughput. The focus change required is -10 collimator (encoder) steps per millimeter of filter thickness if the spectrograph is focused without a post-slit filter in the beam. Proper collimator focus is important to avoid astigmatism in the image.