Miscellaneous, Notes and printed matter, 1944-1968

              /l/a
COLLIMATION AND ADJUSTMENT OF CASSEGRAIN OPTICS
by Clyde W. Tombaugh
With the telescope directed upward toward the sky, the primary is
the large concave mirror at the bottom or lower end of the tube.
The secondary is the smaller convex mirror suspended by a spider
web and centered in the upper end of the tube.
Primary mirror light shield is removed to facilitate centering.
Sight through the hole in the center of the primary mirror toward
the secondary mirror, in which the image of the primary mirror
is seen. Assuming that both mirrors are out of collimation, one
may see something similar to that portrayed in Figure 1.
Tilt the secondary mirror with the adjusting screws until the
image of the primary mirror is centered in the secondary mirror,
as shown in Figure 2. The secondary mirror is now aligned with
the primary mirror cell.
By turning the screws or bolts on the back of the primary mirror
cell, the primary mirror is "squared on" with the system. Tilt
the primary until the Image of the secondary, seen in the image
of the primary by lookina at the secondary, is centered.
If the secondary mirror happens to be a small one, with lesser
intercept distance to yield high secondary amplification, the
image of the secondary may disappear into the central hole of
the primary. If so, fasten a cardboard disk larger than the
secondary to the back of the secondary mirror cell, and concentric
with the secondary. This may be called the "collimating ring".
In this way, the image of the cardboard disk is made large enough
to extend over the image of the hole in the primary. T1lt the
primary mirror until the col 11mating ring image appears centered,
as in Figure 3.
Install the light shield 1n front of the primary. Look through
'the system again to see if the light shield 1s centered. If not,
it will need some adjustment.