CHECK CENTERING ACROSS THE BOUNDARY BETWEEN OFFSET MIRROR AND SLIT

With any reasonably bright star, center to (x,y) coordinates on the
offset mirror.  Record the resulting coordinates of the star.
Then send the star to (x,y) coordinates on the slit side of the field.
Again record the coordinates.

Repeat this sequence five times.

Repeat the sequence five more times with both ends of the moves on the
offset mirror.