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Choosing a Telescope: Cassegrains

There are three main types of telescopes that are available to the video astronomy community: Refractors, Reflectors (newtonians), and Cassegrains. Each have their strengths and weaknesses. This post will focus on Cassegrains.

Cassegrains are a good compromise telescope between newtonians and refractors. They tend to be more expensive than newtonians but less than the higher end refractors. They have some disadvantages but many can be overcome with accessories. They come in different types. The most common is called a Schmidt Cassegrain.

A cassegrain has a large sperical mirror at the back of the tube which bounces the light to a central mirror. The central mirror bounces the light out the back of the tube. It's view port is similar to how a refractor works. The front of the cassegrain has a glass plate which holds the central mirror in place. Some higher end models use a corrector plate glass to hold the secondary mirror. The corrector plate adjusts the light so you get a uniformly clear view across the entire field.



Though Cassegrains are slow for video astronomy, there are accessories you can purchase which will speed it up. Many models have 6.3 reducer (which reduces an f/10 to f/6.3). There is also a corrector called a Hyperstar which can make the tube into an f/2 telescope. I don't have a personal image of a cassegrain but if you search on "Celestron SCT" you'll see an example of a cassegrain.

#videoastronomy #cassegrain


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