A counter is provided on the side of the collimator tube which indicates the position of the collimator [G]. The little lever when pulled out unlocks the dial, and should be pressed up and in again when the change is completed. To focus the camera, the knurled knob moves the collimating mirror vertically and turns the dial counter. Always approach values from the same direction to insure repeatability. A typical focus reading with the standard aperture plate is 010, while the total range of the counter is from 000 to 050.
Using the He-Ar lamp (see Chapter 4), take a series of exposures of consistent duration ( e.g. 10 seconds each), using the narrowest slit (1.5, see Chapter 3). Take a column plot through each exposure to measure the pixel width at FWHM for a single spectral line. The only difference between exposures should be the collimator setting. The smallest width at FWHM is the approximate collimator focus, which is close to 2-3 pixels FWHM at its optimum setting. A similar procedure should be done on a star after the TV and telescope are focused (see Chapter 5).