Collimating a Schmidt-Cassegrain

Commercial SCTs are easier to collimate than Newtonians. The manufacturer has set and sealed the primary mirror inside the optical tube. This leaves only the secondary mirror for you to adjust. Also, SCTs tend to hold collimation better than Newtonians. Still, it's a good idea to check and fine tune the collimation of your SCT at the beginning of each observing session.

The first step is to check the collimation. Remove the star diagonal and place a sight tube in the focus tube. You should see your eye centered within the reflected image of the secondary mirror. However, if your eye and the secondary are off-center, as illustrated at left, then you need to tweak the collimation.

The secondary mirror is mounted in the front corrector plate. There may be three screw heads exposed. If not, remove the secondary cover. The three small screws are used to adjust the secondary. Do Not Loosen Any Large Screws. This may cause your secondary mirror to fall and damage the primary mirror. Make small adjustments to the three collimation screws. Adjust one screw at a time and check the results after each adjustment. Your SCT is collimated when the view through the site tube shows the reflected image of the secondary centered within the reflected image of the primary mirror.

You can check the collimation by doing the star test.