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.

How does a Cassegrain Telescope work?

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.

cassegrain-diagram

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.

Schmidt-Cassegrain

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.

Maksutov-Cassegrain

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. The Celestron NexStar 4SE is an example of a good Maksutov-Cassegrain telescope.

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.

Pros and Cons of Cassegrain Telescopes

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

Pros

  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.

Cons

  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.

Common Questions about Cassegrain Telescopes

What type of telescope is a Cassegrain telescope?

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.

Are Maksutov Cassegrain telescopes good?

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.

Do you have to collimate a Schmidt Cassegrain 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.

Why is it called a Cassegrain?

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.

Is the Hubble telescope a Cassegrain?

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

Conclusion

In conclusion, 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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About Derek

Hey! I'm Derek, I've been interested in astronomy since.. well, forever! I'm an engineer by trade, but I've been playing around with telescopes for many years. I hope to impart some of the knowledge I've learned over the years onto you!

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