Background

The DEIMOS grating tilt stages have a named position called Zeroth_Order which is appropriate for zeroth order imaging, a mode commonly used for aligning slitmasks. This document describes the procedure for measuring the zeroth order imaging position and updating the value in the DEIMOS software.

Measuring the Zeroth Order Angle

The zeroth order angle must be chosen such that it places the image entirely on the detector, but is not so close to the limit of travel that you risk running into the limit (or, more likely, violating the software limit and thus being unable to start the move). As of 2016, the appropriate values for the stages are as follows:

Slider	Limit of travel		Zeroth order position
	G#TLTVAL	G#TLTRAW	G#TLTVAL	G#TLTRAW
3	-18.438	-78300	-18.300	-78212
4	-18.491	-91600	-18.276	-90967

Invoking the Zeroth Order Position

Since the zeroth order position is a named position for the grating tilt stages, one can command the stage to move to the zeroth order position by setting the GxTLTNAM keyword to the special value Zeroth_Order as follows:

• modify -s deimot G3TLTNAM=Zeroth_Order

Changing the Zeroth Order Position

On October 2016 we measured the Zeroth order positions for Slider 3 and Slider 4. Slider 3 was loaded with grating 600ZD and Slider 4 with 900ZD.