Step 2, Setting The Primary To Secondary Spacing - PlaneWave CDK Collimation Instructions

Figure 7: The CDK12.5 collimation screws and the central secondary spacing bolt.

Step 2, Setting the Primary to Secondary Spacing:

For your CDK optical system to perform as well as it should, the spacing between the
primary mirror and secondary mirror should be set to an accuracy of +/- 1mm!
Fortunately, you won't have to measure this spacing. When the primary to secondary
spacing is set correctly, the focal plane will land at an exact known distance behind the
fully racked in focuser. The secondary spacer places the Ronchi screen of the Ronchi
Ocular at the place where the focal plane is supposed to be. In order to set the primary to
secondary distance, you will be moving the secondary mirror. And by adjusting
secondary mirror distance, you will be moving the focal plane relative to that Ronchi
screen. The job here is to move the secondary mirror such that the focal plane coincides
with the Ronchi screen on the Ronchi Ocular. When you have done this, you have
accurately set the primary to secondary spacing.
With reasonable effort, the primary to secondary spacing can be set to well within its
1mm tolerance.
Now that the collimation is at least close and a bright star is centered, replace the low
power eyepiece with the provided Ronchi Ocular. Rack the focuser all the way in, as far
in as it goes, and make sure the 2" adapter is seated all the way into the focuser and the
Secondary Spacer is seated flush in to the 2" adapter. If the Ronchi is nulled (you can see
no lines, or one ronchi line covers the entire aperture), see figure 8. This means that your
spacing is already set so you can proceed to Step 3. This is not uncommon because the
primary to secondary spacing is set at PlaneWave and you most likely will not have to
adjust it.
If you see two or more lines you should move the secondary toward or away from the
primary. The mechanics for moving the CDK12.5 and the CDK20 secondary mirrors are
a little different.
V112907
Collimation knobs
Central Secondary Spacing Bolt
5