Catadioptric Telescope Designed for Astrophotography

The catadioptric optical system is a telescope designed for astrophotography rather than visual observing. In amateur astronomy, Astrographs are used chiefly for obtaining images of various objects. Still, they have also been used for sky surveys and searching for comets or asteroids.

What is a Catadiopritric Telescope?

A catadioptric telescope is an optical system optimized for producing images of objects at an infinite distance, incorporating both refractive optics (lenses) and reflective optics (mirrors). Because both mirrors and lenses are used in catadioptric optical systems, the user obtains certain performance advantages over other types of scopes. It also gives telescope manufacturers advantages in manufacturing, which indirectly benefits the consumer.

As a solid option for a beginner telescope, catadioptric telescopes have been popular with amateur astronomers for a long time. Several different optical designs fall into the catadioptric category, but they all have similar performance benefits. They may provide better aberration correction than other all-lens (refractor) or all-mirror (Newtonian reflector) telescopes over a wider aberration-free field of view.

The Pros of Catadioptric Telescopes

1. Catadioptric telescopes use a folded-path optical system and thus can be shorter than their focal length would imply.

2. They are lighter and more compact compared to telescopes of the same aperture, as is the mount that holds them.

3. Catadioptric telescopes are convenient to transport due to their mechanical size and reduced weight.

4. Their ability to use mirrors with purely spherical figures and easily reproduced refractive elements, usually called corrector lenses, results in lower manufacturing costs. The manufacturer can thus lower the purchase price for consumers.

Refractors and Newtonian Reflectors

Refractors and Newtonian reflectors typically utilize a linear optical path, which implies that if the user wants a long focal length system, the length of the OTA must be similar to the desired focal length. That results in long OTAs, which can become quite heavy, require a large, expensive mount for tracking, and are more challenging to balance on their mount.

Catadioptric telescope designed for astrophotography use folded-path optical systems; thus, their OTAs can be shorter than their focal length suggests. This feature is even more effective than it might seem at first, and that is because of the secondary mirror design of many of these scopes.

Cassegrain reflecting design

The secondary mirror of most catadioptric telescopes not only reflects the cone of light from the primary mirror through the central perforation in that mirror, but it also adds some power to that process. That means it intercepts a cone of light heading toward a “focus” just outside the front of the OTA and reflects it backward as a cone with a smaller apex angle than it would have.

This narrower cone of light “appears” to result from a more extended focal length optic than the actual focal length of that primary mirror! Power has been added. But also, the physical length of the OTA is now much shorter than the resulting focal length of the system would suggest!

Incidentally, this same “trick” is commonly used in classical Cassegrain reflecting telescopes used in professional observatories. In these systems, the primary and secondary mirrors have figures different from the catadioptric scopes, and no corrector lens is required in the classical Cassegrain reflecting design.

Catadioptric telescopes

Catadioptric telescopes achieve savings in both mechanical size and weight due to this shortening method. In addition, a short optical tube can be much less complicated in its design. A simple aluminum cylinder might be adequate construction to prevent flexure and keep the secondary and corrector aligned with the primary mirror, and that means more weight reduction is possible. Compare that with large observatory telescopes, which must use complex, heavy structures to support their secondary mirror hardware to achieve minimal flexure problems.

Heavy telescope

Catadioptric telescope designed for astrophotography is for amateur astronomers who transport their scopes to dark sky sites for observation or photography, or even to their backyard. They will agree that moving a light telescope is more fun than one which requires a significant weight-lifting effort! Talk to any avid amateur who has transported and set up a heavy telescope and mount it at a remote site, and they will concur. Thus, weight reduction under size reduction makes the catadioptric design great for a beginner telescope.

Large aperture

Another advantage of large aperture telescopes is their inherent angular resolution capability is better than smaller aperture scopes. A “large” aperture catadioptric scope can provide the observer with good light-gathering ability. Higher angular resolution, and some weight reduction compared to a smaller aperture refractor!

Cost-to-size ratio

The cost-to-size ratio can favor the amateur when deciding what telescope to purchase. Because manufacturers have discovered the ability to use mirrors with purely spherical figures and corrector lenses which can be mass-produced. Lower manufacturing costs can be passed on to the consumer in the form of lower purchase prices.

The Cons Of Catadioptric Telescopes

1. Catadioptric telescopes become heavier rather quickly as they increase in aperture.

2. They may require more frequent optical alignment when compared to refractors.

3. A catadioptric telescope’s moving parts are more complex than those in a refractor or reflector telescope.

4. Catadioptric telescopes have an inherent optical performance limitation based on the central obscuration of their aperture caused by their secondary mirror. However, classical Cassegrain and Newtonian reflecting telescopes share this problem. The “limitation” is very subtle, so most amateur astronomers do not even notice or care about this detail.