12" RC Collimation

Collimating. Some short preliminary notes.


The Ritchey-Chr├ętien instrument design has two hyperbolic mirrors. They need to be collimated with high precision. Here are some notes for my setup. For this description I use a Hotech SCA laser collimator for the first steps (checking the focuser zero tilt position, and adjusting the secondary mirror). Although this collimator is aimed for Newtonians primarily, it is quite usable for RC scopes as well.


1. Mount the laser collimator in the rotatable focuser. I use a standard Hotech SCA collimator. Make sure you tighten the "expansion adapter" by turning and tightening the front ring.


2. By rotating the focuser, make sure the laser spot on the secondary mirror is stationary and that the rotation is not letting the spot move. I note that the friction clutch (I have 3 friction clutch screws) on the focuser must be slightly tightened, otherwise you get some minor shift of the focuser axis, when the focuser is rotated. It is absolutely crucial that the spot is stationary while rotating the focuser!


3. Then make sure the rotation is locked, also the 3 friction screws.


4. Check the "zero tilt position" of the focuser, and if necessary, adjust it so the laser spot is centered in the small ring in the center of the secondary. Start from a "zero" position of the tilt.


5. Adjust the secondary's screws, so the reflection onto the 45 degree plate on the Hotech collimator is centered.


6. Next, we will start to align the main mirror. I remove the Hotech collimator. Basically you adjust the main mirror so the image of the secondary and the spider is fully symmetric. This can be seen through the eyepiece holder, either directly or through a small inserted hole.


Comment: One can also use dedicated RC collimating units for this last step.

   ... to be continued...