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What is a Cassegrain telescope?

A Cassegrain telescope is a catch-all term used to describe a general telescope design which uses a convex primary mirror and a concave secondary mirror. Light enters through the front of the tube and passes to the bottom where it reflects off the primary mirror, back up to the secondary mirror, and then finally back down out the back of the telescope to the focal plane. This is what's known as "folded optics" in the telescope world, and it allows for a very long focal length design in a compact body.

Schmidt-Cassegrain Telescope

Example of a Cassegrain telescope design (Schmidt-Cassegrain)

Which main category of telescopes do Cassegrains fall under?

Because Cassegrain telescopes use two mirrors to bring an image into focus, many Cassegrain designs like Ritchey-Chretiéns are reflector telescopes. However, in some designs, there is a corrector plate/lens in the front or rear of the telescope. Because of this, some Cassegrains like Schmidt-Cassegrains can be categorized as catadioptric telescopes. So really, the answer is that Cassegrains can be either a reflector or a catadioptric.

What are Cassegrains best used for?

Generally speaking, Cassegrains are best used for observing and imaging targets at long focal lengths, but it depends on the type of Cassegrain:

  • Schmidt Cassegrain Telescopes are best used for just about any target. When paired with other accessories like focal reducers or Barlow lenses, SCTs are some of the most versatile telescopes out there. You can use them for observing and imaging solar system objects like the planets and the moon, as well as deep sky objects like galaxies and nebulae. The SCT is a jack of all trades, with probably its biggest strong suit being planetary imaging.
  • Ritchey Chretién Telescopes are some of the most well-corrected telescopes consumers can buy, as their design allows for a flat field across a large image sensor. They are best used for long focal length deep sky imaging, which tends to lend well for research. However, RCs are ill-advised for planetary images because their optical design lowers contrast dramatically.
  • Maksutov Cassegrain Telescopes make for excellent planetary viewing and imaging, and can provide even better views of the planets than SCTs due to slightly longer focal ratios. However, unlike SCTs, they do not pair well with reducers and therefore aren't nearly as versatile for other applications like deep sky imaging.
  • Dall Kirkham Telescopes are a more expensive telescope design best used for planetary viewing or imaging, and are often used for research. Please note that there are other modified variants of Dall Kirkham telescopes (see below), which are different in that they feature corrective optics for professional imaging with large sensor cameras.
  • Other Cassegrain Telescopes exist as well, such as the Classical Cassegrain, the Corrected Dall Kirkham (CDK), the Riccardi Dall Kirkham (RiDK), Riccardi-Honders, Harmer-Wynne, and more, and are usually reserved for research purposes. 

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.

Explore Schmidt Cassegrain Telescopes

Ritchey-Chretién Telescopes

Ritchey-Chretien Telescope

Best for deep sky imagers who want a long focal length scope with great optics

The Ritchey-Chretién, or RC for short, is a specialized reflector design that is excellent for long focal length, deep sky imaging. If you're hoping to primarily image distant galaxies or just get close-up views of deep sky objects in general, an RC may be right for you. Unlike a Newtonian or many refractors where additional accessories are needed to begin serious imaging, RCs are some of the best out-of-the-box astrophotography telescopes. Featuring a coma-free design, RCs suffer from very few optical aberrations (imperfections), making them ideal for serious imagers. Due to a large central obstruction, which is where the secondary mirror obstructs the light path, RCs tend to have lower natural contrast than other optical designs and therefore aren't as ideal for planetary imaging like Schmidt-Cassegrains are. This also means poor visual performance, and we don't recommend buying an RC for visual use. RCs also feature slow focal ratios, which make them great for distant objects, but generally requiring longer exposure times than faster telescopes. This also means they are rather unforgiving telescopes in terms of tracking errors, and for that reason are not recommended for beginning imagers. The slow focal ratio can, however, be alleviated somewhat with the use of a reducer. Overall, RCs are limited in their use, but excel at what they're made for: imaging distant deep sky objects.

Explore Ritchey-Chretién Telescopes

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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Dall-Kirkham & Cassegrain Telescopes

Dall-Kirkham Telescope

Best for planetary observers & imagers who want a design free of refractive elements

Though less common, telescopes like the Classic Cassegrain and the (non-corrected) Dall-Kirkham designs are excellent reflectors used primarily for high-end planetary viewing and imaging. Since these telescopes use an all-mirrored design, they are generally considered superior to other planetary telescopes like Schmidt-Cassegrains or Refractor telescopes which use glass lens elements and can introduce chromatic aberration. Classic Cassegrains and Dall-Kirkhams do not suffer from any chromatic aberration and have very long native focal ratios, making them the ideal choice for high-end and research-grade planetary observing. Most of these telescopes are very expensive and have this specialized use, but excel at long focal length viewing and imaging.

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FAQs

Still have questions? We have answers.

How Do I Know Which Cassegrain 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 and intermediate users who primarily wish to observe the planets and the moon.

If you're an experienced astrophotographer who primarily wants to use a telescope to photograph deep sky objects with excellent optics, a Ritchey-Chretién may be the right choice for you. Finally, a Dall Kirkham may be a good choice if you're looking for a telescope that will excel at planetary observing or imaging.

Are Cassegrain Telescopes Good For Astrophotography?

Yes! Every type of Cassegrain telescope can be used for astrophotography. Some designs, like the Ritchey-Chretién and Corrected Dall Kirkham are so well corrected that they are often used in professional observatories for research purposes. Since most Cassegrain telescopes feature very long focal lengths, though, it can be like looking at space through a coffee straw. For this reason, we don't normally recommend them for beginner imagers, especially if you're planning to do deep sky imaging. However, most Cassegrains make for great astrophotography telescopes.