How Do I Collimate My Schmidt Cassegrain Telescope (sct)

How do I collimate my Schmidt-Cassegrain telescope (SCT)?

What would a petite version of Newtonian Refractive Telescope look like? It would look exactly like a Schmidt-Cassegrain telescope.

This telescope overcomes the biggest drawback of the Newtonian telescope. Its size, Newton’s telescopes were plain huge.

The SCT can fold a lengthy optical path into a shorter tube. For this reason, astronomy experts prefer the SCT over its counterpart.

To make the most of any telescope, you need to collimate it. Collimating an SCT is relatively easier than a Newtonian telescope.

Nonetheless, there are certain precautions that you much take. In this article, we will tell all about collimating an SCT.

But before that, let us look at some bonus highlights about an SCT.

What is collimation?

Collimation is a process of aligning the optical components of a telescope. There are two main types of collimation.

First is the optical collimation. This aligns the telescope’s optics in an oriented focal plane.

Second is mechanical collimation. It is necessary to align the physical parts of the telescope.

Why is it essential to collimate a Schmidt-Cassegrain (SCT) telescope?

Collimating an SCT is vital if your goal is to get the sharpest image quality. This is especially true while viewing celestial bodies at elevated magnification.

How to collimate a Schmidt-Cassegrain telescope?

Experts agree that the SCT’s design makes it hold the collimation better. If you do it right once, collimation is a monthly task. If your SCT needs collimation weekly, check the mirror’s lock.

Tools for SCT collimation

There are no special tools required for collimating an SCT. But you need to check the collimation. To do that, refer to the section above.

You can choose not to check the collimation beforehand. If you do that, you need a screwdriver. This helps in adjusting the screws of the secondary mirror.

A general outline for collimation

Collimating an SCT is not an incomprehensible task. You need to adjust the screws of the secondary mirror. This brings a change in the tilt of the same mirror.

This aligns the secondary mirror with the primary axis. The primary axis of the primary mirror is fixed.

Do a tilt test of the SCT.

The thermal stabilization of the SCT is a critical step before collimation. In a heated SCT, the optics are distorted. This can make the SCT look like it is not collimated when it is.

It is advised to do the collimation during night-time. This is because the heat waves from the sun can cause temperature fluctuations.

As the air heats up, it acts as a weak lens. This distorts the image of the celestial body.

Use a high-power eyepiece when you do the star test. A magnification of 200x to 300x will suffice.

Choose a 1st magnitude star (bright star) for doing the test. Align the star in the centre of your field of vision.

Defocusing the star will make a donut-shaped image appear. You will see a tiny hole inside the donut shape. That hole is the reflection of the secondary mirror.

The hole needs to be in the centre of the donut. This means that your SCT is collimated.

If the hole is not in the centre, you need to adjust the collimation.

Fine Tuning the SCT collimation

For this step, you need to figure out which screw needs adjustment. There is an easy way of doing that.

Place your hand in front of the SCT. Stick out a finger in front of the SCT’s aperture. You will see a shadow of your hand in the donut.

Align your hand in such a way that it touches the widest area of the donut. See which screw is the closest to your finger.

Now, you need to decide whether you should tighten or loosen the screw. This typically depends on the focus of the image.

For a simpler method, you can try tightening or loosening the screw. If either of the two options enhances the image, continue doing that.

You need to turn the screw in the right direction. This will cause the star to move closer to the broader part of the donut.

If tightening a particular screw, make the image quality worse. Bring the screw back to its original position.

Try turning the other two screws. If adjustment is made right, the star will appear right in the middle of the donut.

There should be zero asymmetric flarings in the image. However, the sharpness of the image depends on several factors. These factors include magnification and observing conditions.

The key is to have a perfectly round and symmetrical image. Viola! Your SCT is collimated now.

Recommended Reading:


  1. SkyNightMagazine
  2. Cosmic Pursuits