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How Catadioptric Telescopes Work

Catadioptric (or compound) telescopes utilize both refractive (lenses) and reflective (mirrors) elements to bring an image into focus. Many catadioptric designs feature folded optics, which keeps the overall length of the tube very short in comparison to similar-aperture refractors and reflectors. Because of this, catadioptrics can offer a large aperture in a compact size telescope, making them very portable compared to other large aperture telescopes. All catadioptrics will need to be collimated but much less frequently than reflector designs. Additionally, due to their closed tube design, catadioptrics need slightly longer to adjust to ambient air temperature for viewing and imaging. Overall, catadioptrics can make for excellent visual and imaging telescopes of both deep sky and planetary objects. In this guide, we'll go over what qualities to look for in a catadioptric and how to decide on the right one for you.

Schmidt-Cassegrain Telescopes

Schmidt Cassegrain Telescope

Best for observers and imagers who want a highly versatile telescope

Schmidt-Cassegrain Telescopes, also known as SCTs, are by far the most common type of catadioptric telescopes and for good reason. Schmidt-Cassegrains are highly popular with both beginners and advanced amateur astronomers alike for their versatility, compact design, and great optics. Beginner SCTs are usually paired with a go-to mount, which make SCTs long focal lengths a breeze to find objects. Most Schmidt-Cassegrains feature a native focal ratio of around f/10. Schmidt-Cassegrains can be easily paired with a Barlow lens, which make them especially ideal for observing or imaging the planets at around f/20. However, for those who want to use a faster telescope, one can upgrade to a focal reducer that brings the focal ratio to around f/7. Finally, for deep sky imagers who want an ultra-fast and wide-field telescope, some intermediate to advanced Schmidt-Cassegrains are even compatible with Hyperstar products, which brings the focal ratio down to a staggeringly-fast f/2! We can confidently say that the wide variety of focal ratios available make SCTs the most versatile type of any telescope, especially for astrophotography.

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Maksutov-Cassegrain Telescopes

Maksutov Cassegrain Telescope

Best for those who want a dedicated telescope for planetary viewing & imaging

The Maksutov-Cassegrain, or Mak-Cass for short, is very similar to the Schmidt-Cassegrain design but generally features a slightly longer focal ratio that make them ideal for planetary observing and imaging. There are two main differences between a Mak-Cass and an SCT. The first is the corrector plate, which is generally much thicker on a Mak-Cass. This means that Mak-Casses will take a longer time to adjust to air temperature outside than an SCT will. The second is that Mak-Casses usually have longer native focal ratios than SCTs do. While SCTs are usually set around f/10, Mak-Casses are generally around f/12 to f/15. This makes them a poor choice for imaging deep sky objects due to longer exposure times, but a rather excellent choice for viewing and imaging solar system objects like planets and the moon. Unlike SCTs, Mak-Casses are much less compatible with focal reducers, which make them much less versatile overall. However, they are generally great optical performers at their native focal ratio and are ideal for those who want excellent planetary performance on a limited budget.

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RASA Telescopes

Celestron RASA Telescope

Best for deep sky imagers who want an ultra-fast wide field scope

The Rowe-Ackerman Schmidt Astrograph (RASA) design is a modified version of the Schmidt Cassegrain Telescope. Unlike SCTs, there is no secondary mirror in a RASA that bounces the light back down the tube and out the back of the scope. This means that the camera — the RASA design is incompatible with visual observing — is unconventionally mounted on the front of the telescope. This results in an extremely fast focal ratio with a large aperture, around f/2. Although the RASA design does not have a very long focal length, it can take much shorter exposures to achieve stunning astrophotographs. This is similar to an SCT design with a Hyperstar, but since it is optimized for the short focal ratio, it has even better imaging performance. It is recommended to use a cylindrically-shaped astronomy camera when using the RASA (especially the 8” model) instead of a DSLR or mirrorless camera. Additionally, due to the short back focus, filter wheels cannot be used with the RASA 8". However, single filters may still be used for narrowband imaging with a special adapter via the “drop-in” method.

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Maksutov-Newtonian Telescopes

Maksutov Newtonian Telescope

Best for those who want similar performance to an APO triplet on a budget

The Maksutov-Newtonian, or Mak-Newt for short, is similar to a standard Newtonian design, but the difference is that it incorporates a corrector lens plate, which makes it a catadioptric and not a reflector. The use of the corrector plate greatly helps correct for coma compared to a standard Newtonian. An equivalent aperture Mak-Newt has around 350% less coma and makes stars 3.6 times smaller. Unlike Newtonians, Mak-Newts do not need a coma corrector. The corrector plate also replaces the spider vanes holding up the secondary mirror, which means no diffraction spikes on bright stars. Like SCTs, Mak-Newts require occasional collimation, but much less frequently than Newtonians. Of course, these benefits come with two main drawbacks: a higher price tag, and an overall heavier telescope. Like Newtonians, the long tube means they are quite prone to wind. Due to a sealed tube design, Mak-Newts take some time to cool down to ambient air temperature, and the corrector plate attracts dew easily. Overall, though, Mak-Newts are excellent optical performers, and many amateur astronomers even feel they provide similar views to APO refractors of the same aperture for a fraction of the price, though at the cost of slightly lower contrast due to the secondary mirror obstruction.

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FAQs

Still have questions? We have answers.

Do I need to collimate a Schmidt-Cassegrain or other catadioptric telescope?

Yes, you will need to collimate a Schmidt-Cassegrain Telescope (SCT) and other types of catadioptrics. SCTs need to be collimated much less frequently than Newtonians, meaning you can spend less time doing maintenance and more time observing. Other catadioptric designs, such as the RASA, will require infrequent but precise collimation due to their fast focal ratios.

How Do I Know Which Catadioptric Telescope To Buy?

Generally speaking, we recommend that all beginners interested in the catadioptric design go with a Schmidt-Cassegrain Telescope (SCT) like the Celestron Nexstar series. These telescopes come with a go-to mount that can automatically point the telescope to an object you type into the hand controller, making it easy to find objects. A Maksutov Cassegrain can be a good choice for beginners who primarily wish to observe the planets and the moon.

If you're a more experienced observer or astrophotographer, a higher-quality SCT may be up your alley. For those who want extremely fast telescope with great optical performance for imaging, the Celestron RASA is hard to beat for the price. Lastly, the rare but powerful Maksutov Newtonian may be a good choice for imagers with a heavy-duty mount that want APO refractor performance for a fraction of the price. 

Are Catadioptric Telescopes Good For Astrophotography?

Catadioptric telescopes are an excellent choice for astrophotography, but it depends on which kind of astrophotography you're trying to do. Maksutov-Cassegrains are great at planetary imaging, but make for poor deep sky imaging telescopes due to their long focal ratio. RASA telescopes are excellent for deep sky imaging, especially of wide field targets, but for that reason aren't a good match for planetary imaging.

Schmidt-Cassegrains are highly versatile and can image both planets and deep sky objects very well. For planetary imaging, consider adding a quality Barlow lens paired with a planetary camera to an SCT for best results. For deep sky imaging, SCTs can be rather slow at their native focal ratio around f/10. To improve this, you can use a focal reducer that's compatible with your SCT, which can bring the focal ratio down to f/6.3 or f/7. If that's not fast enough, some Celestron SCTs that say they are Fastar compatible can be outfitted with the Hyperstar system. This allows you to image at f/2 along with the other focal ratios all on the same telescope!

Lastly, Maksutov Newtonians offer superb visual and imaging performance of both deep sky objects and the planets that rival some APO triplet refractors. These telescopes are usually heavier, longer, and less versatile than most other telescopes like SCTs, but are still a great choice for deep sky astrophotography in particular.