Catadioptric telescopes are compact and fairly low-maintenance telescopes popular among visual observers and imagers alike, as well as being heavily marketed to beginner astronomers. Their fairly simple configurations and performance advantages can be extremely appealing, but it’s important to know the pros, cons, and cost before buying, as well as pick out the best models possible both in price and overall quality. Thankfully, we’re here to help you with the process.

Types of Catadioptric Telescopes

Catadioptric telescopes usually come in two types, both of which are variants of the Cassegrain telescope. The Cassegrain uses a concave primary mirror with a hole in it and a convex secondary mirror. The secondary mirror “folds” the light path back through the primary mirror, and the focal point is at the rear of the telescope like a refractor. Catadioptric Cassegrains use a corrector lens to fix the resulting aberrations of one or both mirrors being spherical in curvature, which is easier to manufacture than a parabolic or hyperbolic mirror but induces massive aberrations that would otherwise ruin the image. While various configurations exist, the only two common ones are Schmidt-Cassegrains and Maksutov-Cassegrains. Both use spherical primary mirrors and approximately spherical secondary mirrors with a corrector at the front of the telescope.

The majority of Maksutov-Cassegrain and Schmidt-Cassegrain telescopes have a focuser built into the telescope, where you turn a knob at the back to move the primary mirror and attach threaded accessories to the back of the telescope, which remain in place. Schmidt-Cassegrain telescopes use a universal thread system and size for attaching accessories to the back, while Maksutov-Cassegrains can have various types of threads and adapters.

Maksutov-Cassegrains, also referred to simply as Maksutovs (the Maksutov corrector was invented for the Cassegrain configuration), Maks, or MCTs, use a thick meniscus corrector, and the secondary mirror usually shares the spherical curvature of the corrector, simplifying manufacture. Maksutovs are easier to make to high standards of quality than Schmidt-Cassegrains, and the built-in secondary mirror is more easily fabricated at small sizes to reduce the obstruction overall, which allows for sharper and higher-contrast images. The secondary mirror being integrated with the corrector also greatly reduces the need for collimation. However, Maksutovs are impractical to manufacture in large sizes due to cost and performance reasons. Maks above 5” in aperture sometimes have the secondary mirrors separate from the corrector and are known as Rutten-Maksutovs or RuMaks.

Schmidt-Cassegrains, usually abbreviated as SCTs, use a thin Schmidt corrector plate—which appears flat but actually has a complex curve, sort of like a Fresnel lens—at the front and a secondary mirror attached to a holder on the corrector. Both mirrors are spherical in curvature. SCTs designed for imaging use an aplanatic configuration where one or both mirrors are slightly aspheric and/or corrector lenses are added to flatten the field; the Meade ACF design uses a purely aspherized design, while the Celestron EdgeHD adds corrector lenses.

The largest Maksutov-Cassegrains and Schmidt-Cassegrains ever built are both 22”, though the one and only 22” Maksutov used a 25” primary mirror due to some quirks in the Maksutov optical design requiring a larger primary than the corrector lens, while the Celestron 22” Schmidt-Cassegrain was ostensibly a production unit, and a half dozen were made before the line was discontinued. The largest of either design you will usually see is 16” in aperture.

Other configurations you might see include:

• Schmidt-Newtonian – Spherical primary mirror Newtonian reflector with Schmidt corrector. Designed for very fast f/3 to f/4 imaging or wide-field visual scopes. Less coma than a regular Newtonian but few other advantages, and third-party coma correctors are a more convenient option. Mass-produced by Celestron and Meade in fairly large numbers.
• Maksutov-Newtonian – Newtonian reflector with a spherical primary mirror and Maksutov corrector. These are usually optimised to have a small central obstruction and good planetary performance as a result, but the small central obstruction affects the usefulness of deep-sky imaging, and again, a regular Newtonian is usually just fine.
• Klevstov-Cassegrain – Sold as Vixen VMC, Meade 107D, various Russian and Japanese small brands, and also used by many mirror-lens designs. Small corrector lens attached to the secondary mirror, often poor performance and large central obstruction, and thus not recommended. The Field-Maksutov design is basically the same as the Klevstov in general layout and performance.
• Argunov-Cassegrain – Klevstov-Cassegrain with a slightly different lens/mirror configuration and the same flaws as the Klevstov. Sold as Vixen VC but otherwise uncommon.
• Corrected Dall-Kirkham – Used by many imaging scopes, Dall-Kirkham with coma correcting and field flattening lenses. Usually not economical for amateur-sized scopes.

The low-quality “Bird-Jones” telescope, a Newtonian with a small sub-aperture corrector in the focuser drawtube, is also technically a catadioptric. Other sub-aperture corrector designs such as these exist, though few are common or well-designed, as do a few other front corrector configurations like the Lurie-Houghton.

Advantages & Disadvantages of Catadioptric Telescopes

Since not all catadioptrics are necessarily of the Cassegrain, Maksutov, or Schmidt configuration, the following advantages/disadvantages apply only to SCT and Maksutov telescopes, since those are the only ones readily available in most cases anyway.

Key advantages

• Compactness – SCTs and Maksutovs use a very stubby tube due to the folded configuration of the Cassegrain optical layout, usually no more than 2-2.5 times the diameter of the primary mirror in physical length.
• Long focal ratio and focal length – SCTs are usually f/8 to f/11, while Maksutovs are usually f/10 to f/16 and thus have very long focal lengths as a result. This makes it easy to achieve high magnifications for lunar, planetary, and double star viewing without a fancy Barlow lens or ultra-short focal length eyepiece. Similarly, planetary imaging only requires a modest Barlow lens to achieve the required image scale, whereas an f/4 to f/6 Newtonian might need a 5x Barlow or even stacking multiple Barlows.
• No diffraction spikes – Because SCTs and Maksutovs have the secondary mirror attached to the corrector plate, no spider is needed to hold it in place, which means no diffraction spikes like a conventional Newtonian or Cassegrain-type reflector has.
• No chromatic aberration – While some subaperture corrector lenses can produce chromatic aberration, SCTs and Maksutovs’ corrector lenses don’t produce any chromatic aberration, as with a pure reflecting telescope.
• Infrequent collimation – The stubby tubes and rigid hardware of SCTs and Maksutovs mean collimation is usually very steady. SCTs (if collimated correctly with everything tightened) rarely need collimation more than every few uses, while Maksutovs may never need collimation at all owing to the secondary mirror often being part of the same piece of glass as the corrector.
• Lower general maintenance – The sealed tube keeps debris, airborne chemical contaminants, and other foreign objects out of Schmidt-Cassegrain and Maksutov-Cassegrain telescopes, meaning the mirror coatings are far less likely to corrode or even get dirty.

Key Disadvantages

• Cost – All catadioptric telescopes inherently cost more than a Newtonian. A Schmidt-Cassegrain usually costs at least twice as much as a Newtonian reflector boasting comparable capabilities, if not more, be it a new or used model, while a Maksutov-Cassegrain is more than triple as much. Large Maksutovs above 7” in aperture are essentially custom products and rarely available for a reasonable price.
• Long focal ratio and focal length – The long focal lengths of SCTs and Maksutovs place an incredibly high demand on tracking/guiding requirements for deep-sky astrophotography, while the slow focal ratio prohibits imaging without an extremely long exposure time. Dedicated focal reducers are practically mandatory for imaging nebulae, galaxies, and star clusters with an SCT, while a Maksutov-Cassegrain is usually completely useless for the task.
• Limited field of view – The long focal length of SCTs and Maksutovs also, of course, means you get a higher power and narrower field of view with any given eyepiece. Additionally, 8” and smaller catadioptric scopes typically don’t illuminate the field of view of the widest possible 2” eyepieces and some cameras, even if they technically allow the attachment of 2” accessories, further limiting the field of view possible with such telescopes. Larger catadioptrics will still not allow you to use a 2” eyepiece and a focal reducer without vignetting, which means that the field of view is always cramped owing to the several-meter-long focal length.
• Large central obstructions: The large secondary mirror in the catadioptric obstructing the primary mirror has a negligible effect on light-gathering power but can reduce contrast at the eyepiece by essentially smearing the image slightly. Maksutov-Cassegrains rarely have a central obstruction below 25% or so, while even the most optimized SCTs have obstructions of around 33% or greater, and some have obstructions exceeding 40% when the baffle around the secondary is accounted for. Central obstructions over 30% are noticeably detrimental to the image contrast, particularly with smaller telescopes. A central obstruction above 40% essentially ruins high-resolution planetary viewing with telescopes like the Celestron C5 (which is ironically capable of little else owing to its small aperture and limited field of view). By contrast, a typical Newtonian reflector, such as a Dobsonian intended for visual use, has an obstruction of around 20-30%.
• Cooldown time – Compared to a Newtonian reflector, an SCT or Maksutov-Cassegrain requires more time to cool down due to the large secondary mirror also needing to cool down and the corrector plate trapping warm air inside the tube. Maksutovs don’t have the mass of the secondary mirror to contend with, but the thick corrector can completely trap warm air, requiring some sort of active ventilation system with 7” and larger apertures and basically ruining performance with most larger scopes even with countermeasures.
• Mounting requirements – Older catadioptrics come on fork mounts or manual equatorial mounts that require precise polar alignment in order to be used correctly; newer computerised mounts may have questionable longevity due to cheaply made components. In both cases, the long focal length of these telescopes dictates precision pointing and tracking, and astrophotography requires even more stringent tolerances. A 6” or greater catadioptric pretty much requires motorised tracking to be an enjoyable experience to use, and even a 100mm scope needs some form of fine mechanical adjustment for aiming.
• Collimation difficulty – While collimating an SCT or Maksutov isn’t as frequently required, you need to actually point the telescope at a bright star while making adjustments in order to do so (see our collimation guide). Many users also make the mistake of adding thumb screws or other modifications that do little but make these telescopes shift in collimation more frequently.
• Focusing – The internal moving-mirror focus system used on almost all Schmidt-Cassegrain and Maksutov-Cassegrain telescopes has unlimited travel but can have the primary mirror rock back and forth on the rod it slides on as you focus. This can cause “image shift” where the view appears to bounce at high magnifications, and “mirror flop” which ruins long exposures. Many aplanatic SCTs have mirror locks to solve the mirror flop problem, but image shift affects most catadioptric scopes, at least in a minor fashion.