Measure focus across detector as a function of collimator position

This test is designed to provide data to calculate the best collimator 
focus position as a function of position across the face of the CCD.
It can be run in any of the three ESI modes, with some changes to the
procedure:

IMAGING mode: Use a star field which is fairly rich in stars across the
full imaging field.  This could be a globular cluster, an open cluster,
or simply a region near the Milky Way.

LOW-RES mode: Use an emission line source such as the night sky or an
extended emission line object like the Orion Nebula.

HIGH-RES mode: Use a smaller emission line source (20 arcsec is fine).
This could again be the night sky, or it could be a large planetary 
nebula.

1) Take images/spectra of the source at various collimator positions.
(List of positions, or recommended center position and step size TBD.)