Catadioptric Telescopes- Is It An Amazing Discovery?

A Catadioptric telescope uses reflective mirrors and refractive optical lenses to focus light near a primary mirror.  Catadioptric telescopes remove chromatic and spherical aberrations that create imaging artifacts related to light bending as it passes through an optical lens.

Should you consider a Catadioptric Telescope when it’s time to purchase one? What’s the difference between Catadioptric, Reflector, and Refractor telescopes? Today we’re going to explore Catadioptric Telescope.

Catadioptric Telescope-Why Was it Needed?

The Catadioptric Telescope allows astrophotography of bigger celestial bodies in the night skies. Each type of telescope has a benefit and a drawback.  The Catadioptric is a merging of the best qualities of the refractive and reflective telescope.

Refracting Telescope 

The Refracting telescope was invented (theoretical, on paper) in 1608 by Hans Lipperhey. In 1609 Galileo Galilei modified the telescopes and began collecting and publishing data. An inherent flaw of the Reflector Telescope is Chromatic Aberration.

Reflecting Telescope 

The Reflecting Telescope was invented (theoretical, on paper) in 1663 by Jame Gregory. Isaac Newton modified the design and built a telescope in 1668. Using mirrors instead of lenses creates a longer focal path than refracting telescopes.

While the Reflecting telescope’s mirror design removed chromatic aberration, it introduced other challenges. In 1609 mirrors were difficult to manufacture correctly.  The ability to accurately shape the mirror stretches technology to its limits.  The mirrors, created from Speculum metal, a combination of approximately 75% Copper (Cu) and 25% Tin (Sn), rapidly tarnished.  As the mirror tarnished, their ability to reflect light decreased.

Casegrain Telescope 

Laurent Cassegrain lived from approximately 1629-1693. There is very little information about Cassegrain, the person. At the time of his death, Cassegrain was a priest who taught science classes at the modern-day equivalent of a high school.

Cassegrain proposed a reflecting telescope with a primary and secondary mirror.  The design modification was that the primary mirror would have a hole in its center to allow for an eyepiece insertion.

Who Invented the Catadioptric Telescope?

Unlike the Reflector and Refractor telescope, the invention of the Catadioptric telescope began with a lighthouse! In 1820 Augustin-Jean Fresnel invented the catadioptric lighthouse reflector.  In 1859 that design was used to build a microscope lens.

In 1931 optician Bernard Schmidt invented the telescope corrector plate to correct chromatic and spherical aberration.

How Does A Catadioptric Telescope Work?

A Catadioptric Telescope (and its variations) incorporate functionality from Refracting and Reflecting telescopes.  It’s quite common to find Catadioptric telescopes classified as “reflector” telescopes, or SE’s.

Refracting Telescope	Reflecting Telescope

Reflecting Telescope

• Light enters the telescope and strikes the Primary Mirror
• The light is reflected from the Primary Mirror to a Secondary Mirror
• The light is reflected toward an optical eyepiece

Refracting Telescope

• Light passes an optical lens as it enters the telescope
• The optical lens “redirects” towards a center focal point.
• The light is captured by the eyepiece

Catadioptric Telescope

Telescopes with a Catodioptric (Cassegrain) configuration, like the Hubble Space Telescope, which “folds” the light as it travels through the telescope. 

Let’s follow the path of the light that light travels when it enters a Catadioptric telescope.  

• Light enters the telescope from the sky (star, planet, moon, etc.) through a corrector lens. Chromatic Aberration may occur. 
• The corrector lens seals the telescope and holds the secondary mirror.
• The light travels to the far end of the telescope and strikes the Primary Mirror.
• The light travels back up the telescope and strikes a Secondary Mirror.
• The light is reflected down the telescope and passes through a hole in the center of the Primary Mirror
• The light is collected/viewed through collection optics (eyepiece, camera) 

Advantages and Disadvantages of Catadioptric Telescopes

Catadioptric Telescopes are good “all around” telescopes, but do have both benefits and disadvantages due to the telescope design.

Catadioptric Telescope Benefits

• The best reflector and refractive telescope properties combined.
• Good “all around” telescopes. Planets, moon, Deep Space Objects. 
• Low maintenance
• Extremely portable up to and including 8” aperture models.  We recommend a stationary mount for telescopes larger than 8”.

Catadioptric Telescope Disadvantages

• Not so easy on the bank balance.  The cost is higher for a Catadioptric Telescope versus other Newtonian telescopes. Less expensive than a similar-sized aperture refractor telescope. 
• Secondary mirror obstruction causes image degradation.  It’s easily seen when images are compared against a reflector telescope.
• Moisture/dew collection on the collector plate.  We recommend a dew shield. 

Different Types of Catadioptric Telescopes

There are two primary types of Cassegrain Telescopes.  The Mkasutov-Cassegrain and the Schmidt-Cassegrain. There are many similarities between the telescopes. 

Maksutov-Cassegrain 

Light enters the Maksutov-Cassegrain (AKA MAK) through a “thick” convex corrector plate. The thick corrector plate is a gigantic heat sink.  It will take time (lots of it for larger aperture telescopes) to acclimate to temperatures. 

The thicker Corrector Plate results in a slower light gathering. (The Maksutov-Cassegrain isn’t your best friend for astrophotography.) 

High focal length and slow focal ratio combine to create a narrow field of view. The narrow field of view makes the Maksutov-Cassegrain ideal for planet and moon observation.

The Maksutov-Cassegrain telescopes have a smaller secondary “mirror” than Schmidt-Cassegrain telescopes. The secondary mirror is actually a small aluminized spot on the inside of the corrector lens. This results in more light entering the Maksutov-Cassegrain telescope.  More light equals a sharper and brighter image than the Schmidt-Cassegrain telescope.

Schmidt-Cassegrain

Light enters the Schmidt-Cassegrain through a thin, almost flat (slightly convex) corrector plate. The mirror is spherical in shape, which induces some imaging (COMA) distortion. At a fundamental level, COMA will result in a crisp, sharp image in the center of the telescope’s field of view but a distorted or blurry image toward the edge of the field of view. 

Schmidt-Cassegrain vs Maksutov-Cassegrain Side-by-Side

	Maksutov-Cassegrain	Maksutov-Cassegrain
Apertures Sizes	6”-14”	2.4”-7”
Best Use	Astrophotography	Visual Observations
Astrophotography	Deep Sky, Planets, Moons	Planets, and MoonShort exposure only
FOV	Large	Tiny
Cool down	Quick	S-l-o-w
Collimation	Yes	No

A side by side comparison of the two telescope types quickly indicates the primary differences between the two telescopes. While maybe you can purchase Maksutov-Cassegrain telescopes with larger apertures, we don’t recommend them.  The cool-down time, or acclimation time, is too great. You end up with a blurry view.

The small FOV of the Maksutov-Cassegrain makes it an ideal choice for viewing planets and the moon, while the large FOV of the Maksutov-Cassegrain makes it a good choice for both local and deep sky object viewing.

Catadioptric Telescopes Wrap up

It’s hard to beat a Catadioptric telescope’s versatility. Combining the best attributes of reflector and refractor telescopes, Catadioptric Telescopes perform almost any task a backyard astronomer could want.  

Within the Catadioptric Telescope category, there are many different variations.  The primary two telescopes are the Schmidt-Cassegrain and the Maksutov-Cassegrain. 

If you have any expectation of any kind of astrophotography, you’ll be most pleased with a Schmidt-Cassegrain telescope.  The Maksutov-Cassegrain has a very small field of view, making it a relatively poor choice for planetary or deep sky astrophotography.  The Maksutov-Cassegrain is an exceptionally poor choice for deep-space astrophotography. 

Maksutov-Cassegrain are great telescopes for those seeking to primary look at planets and the moon.  The aperture size, combined with less expensive optics, create a lot of bang for the buck.