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Figure 5: An example of an approximately collimated defocused star.
The CDK20 uses three ¼-20 socket head cap screws for collimation, as shown in Figure
6. These screws require a 3/16 Allen wrench. The CDK12.5 and the CDK17 have four
knobs which can be adjusted by hand, as shown in Figure 7. For the CDK12.5/17 it is
easiest if you simultaneously adjust two knobs that are diagonal to each other. Rotate one
knob clockwise and the diagonal knob counter-clockwise in order to maintain tension
between all the knobs. Always finish a move with the secondary preloaded securely
against all knobs.
Collimating is a two-person job: one person looks through the eyepiece while one person
adjusts the collimating screws. Attempting to do this alone can be quite difficult and
frustrating. If necessary, you might try using a webcam with a 1-1/4” nosepiece, such as
the Celestron NexImage, in place of an eyepiece. With this setup, you can watch the
video stream from the camera while you make adjustments.
Note that the central spacing knob is not used until Step 2.
Figure 6: The CDK20 secondary housing. There are three collimating screws and a central knob. The
secondary on the CDK20 is spring loaded, pulling the secondary into the three collimating screws.
Central Spacing Knob
Collimating screw
