How does a Cassegrain telescope work?

Outside of the two most popular telescope types, reflectors and refractors, there are also other designs that you might want to be aware one. One of the most popular is a Cassegrain telescope, which is a form of reflecting telescope that is well used amongst astronomers.

But how exactly does a Cassegrain telescope work? And how does it differ from a refracting telescope? Well, let’s take a quick look at how exactly a Cassegrain system functions.

Cassegrain’s work in the same way that reflector telescopes do – in fact, you’ll often see Cassegrain’s referred to as Cassegrain reflectors. The light comes through the telescope and reflects off of the primary mirror and onto the secondary mirror, which in turn projects the image through to your eyepiece.

Now, there are different models of Cassegrain that are some of the most popular types of telescopes that are currently available. So, let’s look at them in a little closer detail.

The Schmidt Cassegrain is probably the most popular type of Cassegrain telescope. This type of telescope combines the system of a reflective Cassegrain telescope with that of a refracting lens telescope, which is used to correct the view before it comes through to the mirrors.

We call this combination a catadioptric telescope, which is used for scopes that are a combination of reflective and refracting systems. Not only do they have a spherical primary mirror, but they also have a set of spherical secondary mirrors too. With both primary and secondary mirrors being the same, this gathers a decent amount of light.

The Celestron NexStar 4SE is an example of a good Schmidt-Cassegrain telescope you can find on the market that’s proven to be super popular with amateur astronomers in recent years.

Much of this is down to its pricing, as a Cassegrain reflector tends to provide excellent value for money. Though they have a fairly heavy telescope tube, they tend to be fairly compact. They provide us with the power and aperture of a refractor at the cost of a reflector.

Another popular form of catadioptric telescope is a Maksutov-Cassegrain. They also combine the benefits of both the reflecting mirrors with a refracting corrective lens.

Maksutov-Cassegrains differ from Schmidt-Cassegrains in a few ways. They’re typically smaller aperture telescopes, whilst SC’s can be 10 or 12 inches of aperture. As well as this, Maksutov’s have a thicker corrector lens than Schmidt-Cassegrains. A Mak will also have a secondary mirror too, but typically the secondary mirror will be an aluminized spot on the back of the corrector.

A Maksutov-Cassegrain that I particularly like is the Celestron Astro Fi 127mm. It’s consider to be one of the best goto telescopes under $1000, as it’s an ideal choice for those just getting started.

If you’re thinking abut getting a Cassegrain telescope, then it’s important that you know the pros and cons of them beforehand.

  1. One of the main benefits of Cassegrain telescopes is that they’re very short. This therefore makes them quite light, and very easy to carry around with you. So if you’re looking for a portable telescope, then this might be the best choice for you.
  2. In a practical sense, Cassegrains (or catadioptric Cassegrains) are good because they help to provide you with the best view possible. They get rid of any errors or what we call “abberations” on the image, which is essentially some color distortion that is pretty common within astronomy.
  1. The main downside, especially in comparison to normal reflectors, is that Cassegrains are undoubtedly going to be more expensive. Newtonian reflectors and Dobsonian reflectors are going to be some of the cheapest telescopes that you can buy, and Cassegrains and refractors are typically going to be more expensive.
  2. In general, a Cassegrain telescope will take far longer to cool down than a normal reflector or a refracting telescope – this is because the tubes are closed off.

A Cassegrain telescope is traditionally a type of reflective telescope. However, it is often combined with refractive lens too, which can help to provide an even better view.

Yes, Maksutov Cassegrain telescopes make up some of the best telescopes for both beginners and intermediate astronomers. However, do bear in mind they may be more expensive than a simple reflector telescope.

Generally, you will have to collimate a Schmidt Cassegrain (SCT) when you set it up, like you do with a reflector telescope. You don’t have to collimate a refracting telescope, which is one of the reasons why they’re so popular.

The Cassegrain takes it’s named from French priest Laurent Cassegrain who was first known to write about this style of telescope back in the 1600s.

Yes, the Hubble Space Telescope, and most professional astronomy telescopes, are forms of Ritchey–Chrétien telescope, which is a type of Cassegrain.

To sum it up, it’s clear to see that Cassegrain telescopes are quite similar to reflector telescopes, and they don’t just rely on one objective lens like refractor telescopes do. However, there are many different types of Cassegrain telescope out there too, and some of them combine both the reflective mirrors with a refracting lens, which can help to maintain a high quality image.

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