The Flying Astronomer's Guide to Choosing your First Telescope.
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Types of Telescopes
There are three main types of telescope for astronomy, differing in how they gather and focus the light. The first, and perhaps what most people think of when they imagine a telescope, uses a lens to bend the light to focus, and is called a refractor.
A typical refractor - long and thin.
A refractor uses a lens to gather and focus light... The bigger the lens, the brighter the image.
The second type uses a combination of a large primary mirror and a smaller, secondary mirror, so is called a reflector, or sometimes a Newtonian after Sir Isaac Newton who invented it. If a reflector is mounted on a very simple (and so cheap) mount, it's called a Dobsonian, after its american inventor.
A typical Newtonian reflector telescope – big and fat!
A reflector uses a large (primary) mirror to gather and focus light, with a smaller (secondary) mirror at the top of the tube sending the image to a more convenient position for observing.
The last type uses a mix of mirrors and very thin lenses (known as 'corrector plates') and are called catadioptrics. There are Maksutov and Klevtsov catadioptric telescopes, but they are all generally the same.
A typical catadioptric telecope.
Note the small silvered spot on the corrector plate, and the position of the eyepiece.
A catadioptric is basically the same as a reflector, but it uses thin 'corrector' plates to slightly bend the light. A primary mirror then reflects the light onto a small reflective patch on the back of the corrector plate, which in turn reflects the light back down the tube through a hole in the main mirror, and then to the observer.
Reflectors typically provide a good balance of light gathering capability and price, and are often preferred by those wishing to look at very faint objects like galaxies and nebulae.
Refractors are often preferred for observing planets, as they tend to resolve finer detail in these brighter objects, where light gathering ability is not so important.
Catadioptrics have become a popular choice, because of their compact sizes and good light-gathering ability. However, catadioptrics tend to be much more expensive at medium to large sizes, and despite their compact size, they can still be very heavy to move around, especially above about 8" (200mm) primary mirror size.
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