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).