How to Collimate a Schmidt-Cassegrain Telescope

Graphic of Celestron Schmidt-Cassegrain Telescopes with the text how to collimate a schmidt-cassegrain telescope

This article is a step-by-step guide on how to collimate a Schmidt-Cassegrain telescope (SCT). Like any other reflecting telescope, SCTs are vulnerable to shocks and drops causing it to go out of alignment. Therefore, collimating a SCT ensures its optimal performance. Continue reading to learn about the collimation process.

Setting up for Collimation

First, you will need to set up your SCT for collimation. Make sure outside conditions aren’t turbulent and stars can be seen clearly. If there is heavy dew, use a dew heater system as dew can form on the Schmidt corrector when collimating.

Using your most powerful eyepiece (use anything below 10mm for best results), focus on a bright star in the sky. The mount will need to be tracking the star to ensure it doesn’t drift away during the process. You can also track a star using a polar aligned equatorial mount.

Next, you will need to insert the eyepiece. Tighten the thumbscrew thoroughly to secure the eyepiece in to place . This will keep your eyepiece aligned with the optical axis of the telescope. Now you should be all set up to collimate a SCT.

Schmidt-Cassegrain Telescope Collimation Process

Step 1. Once you have located and centered a bright star, defocus the star until you can see a dark hole in the middle. You will also be able to see 2-3 diffraction rings around the hole.

graphic of de focused star
Figure 1. Defocused star

Step 2. Next, you will need to defocus the image on the other side of focus. Turn the focusing knobs the opposite way in order for the image to pass through the focus point.

Graphic of turning focusing knob other way
Figure 2. Turning focus knob to defocus other side of the image

Step 3. Inspect both sides of focus to see which image has clearer light and dark rings or diffraction patterns. Choose the side with the clearer rings for collimation.

Figure 3. Side with the clearer rings will make collimation easier

Step 4. Next, find the direction where the central shadow and the inner bright diffraction rings are skewed relative to the outer bright refraction rings.

graphic of central shadow on defocused star
Figure 4. Outer rings are brightest at the top left

Step 5. Carefully place a finger at the edge of the front cell and point towards the collimation screws. Do not touch the Schmidt corrector. You will now be able to see the shadow of your finger through the eyepiece. Rotate your finger until it’s shadow is close to the narrow part of the rings.

Graphic showing shadow of finger hovering over telescope cell
Figure 5. Shadow of finger seen through the eyepiece

Step 6. Using a Phillips-head screwdriver, adjust the screws of the secondary mirror. The first screw adjusted should be the one closest to your pointing finger. If your finger is in the middle of two screws, choose the screw that is furthest away.

Graphic of choosing which collimation screw to turn
Figure 6. However your finger in front of the secondary

Step 7. Turn the screw 1/10th of a turn to in order to center the dark hole. As you tighten one screw, loosen the other two. This relieves pressure from the aluminium plate allowing you to keep tightening the screw. At this stage the star can move so, move the mount to re center the star.

Graphic of tightening collimation screw
Figure 7. Tightening the collimation screw to center the dark hole
Graphic of loosening collimation screw
Figure 8. Slightly loosening other screws to relieve pressure

Step 8. Keep loosening or tightening each screw until the dark hole is centered in the eyepiece and the secondary mirror is tightly secured with no wobbling. This will require some trial and error. When the dark hole, the central shadow and inner rings are all centered, the telescope should be collimated.


Collimation does not have to be a complicated and arduous task. Just follow these steps on how to collimate a Schmidt-Cassegrain telescope. It will take some trial and error but with a bit of time you will get the hang of it.

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