Measure Flexure vs. (EL, ROTPPOSN)

This test will use a grid of pinholes to check the degree of differential
flexure between the slitmask and the detector.

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1) Depending on which mode (imaging, low-res, or high-res) you are using,
you will choose a particular slitmask and calibration source:

a) Imaging: Use a grid of pinholes and the internal flat field lamp.
b) Low-res: Use a long line of pinholes and the Neon lamp.
c) High-res: Use a single pinhole and either the Neon lamp or the CuAr
lamp.

2) Determine a good exposure level for the images.

3) Make sure that the telescope is clear, and can move in elevation for
a number of hours.

4) Point the telescope at zenith.

5) Run the script ~goodrich/bin/flexure, which will take images at various
rotator positions (typically 0 deg through 360 deg in 30 deg. increments).

6) Repeat step 4 at elevations of 75, 60, 45, 30, 15, and 0 degrees.