Problem(s) Encountered:

When rotating the slit angle, the rotator angle suddenly went to numbers around -600 when I was rotating to +70 degrees. After rotating all the way to +90, then back again, the problem fixed itself.

Solutions:

This rotation-angle reading glitch occurs from time to time and has never been traced.  It''s a known bug.  Rotating to a limit and de-rotating back out is known to typically solve the issue.