Cassegrain Telescopes

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.

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.