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Step 4: Fine Collimation:
Use a high-power eyepiece (5mm or less) and follow the same procedure described in Step 1
(adjust the secondary so that the donut hole is in the center of the donut). You may want to
switch to a low-power eyepiece while making adjustments and re-centering the star, and back
to high power to check the collimation.
Finally, use the Ronchi Ocular to re-check the spacing. As long as only small adjustments
were made in Step 3, the spacing should still be fine. If it is not, repeat Step 2 and Step 3.
Back Focus
The CDK design does not have any moving optical components, so the focal point is fixed.
This means that the focal plane of the telescope is a set distance behind the backplate of the
telescope. The back focus for the various telescope models in shown in Table 1 below.
Backfocus from
mounting surface of
telescope backplate
Back focus from fully
racked in focuser
CDK12.5
10.45”
7.2”
CDK14
11.09”
8.09”
CDK17
10.24”
7.24”
CDK20
8.81”
5.81”
CDK24
*14.1”
5.81”
CDK700
**12”
8.5”
Table 1:
Shows the back focus with and without the focuser installed for the various PlaneWave telescope
models.
*CDK24 can also use optional spacers to increase back focus to be the same as the CDK17 or CDK14.
**The CDK700 is referenced, not from the backplate of the telescope, but from the end of the altitude hub
which is the mounting surface for the optical train. Also, the CDK700 is using the Focuser/Rotator 600180,
which uses 3.5” of backfocus on the CDK700.
