Grating Tilt

Grating tilt angles can be adjusted automatically by grating tilt mechanisms, one per grating. The tilt mechanisms carry the gratings in their mounts, and must limit image motion errors according to the image motion requirements above. Grating exchange out of tilt mechanisms is not automated, but must be accessible while the instrument is installed. The minimum tilt range is $\pm 4$ degrees for the 300 line/mm grating, and $\pm 12$ degrees for the 1200 line/mm grating. Grating tilt must be sensed at the 0.1 arc-sec level to maintain grating angle.