Schmidt-Cassegrain Telescope – Everything You Need to Know!

Every Astronomer and sky gazer is aware of the Schmidt-Cassegrain Telescope. No doubt, it is among the most popular and useful types of telescopes. The reason for its fame is the compactness and versatility of its optical design.

It is actually a catadioptric telescope – the telescopes that use both mirrors and lenses in the formation of an image.

Schmidt-Cassegrain Telescope is primarily a reflecting telescope. However, to eliminate aberrations resulting from the mirror design, a corrector lens is also present in it – making it a catadioptric telescope.

If you try to look into the history of telescopes, you will realize how much the telescopes have evolved in this modern age. Similarly, Schmidt-Cassegrain Telescopes (SCTs) have also evolved to a great extent.

Most new SCTs are easy to use because of the computerized technology they exhibit. These computerized SCTs are best for Photography, terrestrial observations, and stargazing.

History of Schmidt–Cassegrain Telescope

James Gilbert Baker was the first one who proposed a Cassegrain design in 1940 for Schmidt’s camera. The first one was manufactured during World War 2 at the Mount Wilson Observatory.

In this design, a Schmidt corrector plate was used in addition to the primary spherical mirror in order to correct the spherical aberration.

How Does a Schmidt-Cassegrain Telescope Work?

As I mentioned earlier, the SCTs are primarily reflecting telescopes, but they also exhibit a corrector lens to eliminate the aberration. So, this is why they are also considered Catadioptric Telescopes.

Before moving to the working principle of a Schmidt-Cassegrain telescope, you should know that which components participate in the working of a typical SCT. So, the following are the components involved in the working of almost every SCT:

  • Primary Concave Mirror
  • Schmidt Corrector Lens (Corrector Plate)
  • Secondary Convex Mirror
  • Scope (Eyepiece)
  • Camera (In case of Photography)

The Working Principle

The incoming light first passes through the Schmidt corrector lens (known as the corrector plate), which is present at the front of the SCT. The light then reflects from a primary concave.

The scope focuses the reflected light from the primary concave mirror to the front of the telescope. At the front of the SCT, a secondary convex mirror is present, which again reflects the light.

In the end, the light travels back through a hole in the primary concave mirror to the back of the scope, where an eyepiece is located for visual observation – or a camera in the case of Photography.

By folding the light in this manner, it is possible to make an SCT much smaller than an equivalent refractor or Newtonian.

Some people misunderstood that an SCT has a focal length twice as long as the length of the tube because the light travels twice the length of the tube. However, in reality, the SCTs have a focal length five times longer than the length of the tube – interesting, isn’t it?

I know you might be thinking about how it can be possible? Well, it is because of the convex curvature of the secondary mirror.

The convex curvature of the secondary mirror makes the scope to act longer than it is by effectively magnifying the focal length. However, the primary concave mirror of an SCT has a focal length, only about twice the size of its diameter.

For example, the focal length of the primary mirror is about 16” for an 8” telescope – which means it focuses the light 16” in front of the primary concave mirror.

But, if you palace a secondary convex mirror between the focal point and primary mirror, the light will reflect back to the primary mirror at a less steep angle. The secondary convex mirrors of most SCTs provide a 5x magnification factor.

For What Purpose You Can Use Schmidt-Cassegrain Telescopes?

You can use an SCT for the purpose of stargazing, observing astronomical objects, and Photography as well. Most people use it for stargazing because it provides an amazing and clear view of the objects.

However, you might face a problem while buying an SCT because the strength and size of any telescope matter a lot. The problem you may face is related to its price.

The aperture size of lower price scopes is 160mm (6 inches). However, if you have a higher budget, you can also select a higher-end scope with an aperture of 200mm (8 inches) or more.

The size of the aperture matters a lot. There is a simple rule, the larger the aperture size of your scope, the more light it will collect. An aperture gives a clear view of the objects in the sky and those which are farther away.

Pros and Cons of Schmidt-Cassegrain Telescopes

No doubt, SCTs are among the best telescopes for observing celestial objects, stars, planets, and other non-Earthly bodies. However, you may also experience some drawbacks while using it. So, let’s have an overview of the Pros and Cons of SCT.


  • All-purpose telescope exhibiting both lenses and mirrors
  • Best for sky gazing and astrophotography with CMOS, CCD, and DSLR cameras
  • The optics are of excellent quality and provide razor-sharp images
  • Highly portable and compact (You can carry it anywhere, anytime)
  • Easy-to-use and setup
  • Collimation can be done easily without the need for any special tools
  • Durable and highly versatile


  • It is more expensive than the Newtonian telescope of equal aperture but less expensive than refracting telescopes of the equal aperture.
  • Mirror flogging may cause an issue while long image sequences.
  • Significant contrast as compared to refracting telescopes, due to secondary mirror obstruction
  • In humid areas, you may require a dew shield or heaters to prevent the accumulation of dew on the collector plate.
  • Collimation can be distorted during transportation. So, you may need to collimate every time you transport an SCT.

The Bottom Line

A Schmidt-Cassegrain Telescope, which uses both mirrors and a Schmidt lens, is one of the best telescopes you can use for the purpose of sky-gazing, observation of astronomical objects like planets, stars, and non-Earthly bodies.

This powerful telescope is also being used for the purpose of astrophotography by many experts and beginner astronomers.