Telescopes are effective because they have a huge pupil compared to the ones in your eyes. Light comes in through our pupils, which are around 5mm — 7mm in diameter, so their light gathering power is poor. Our pupils dilate in the dark to gather in more light, but that is in no way enough for astronomy.
Objects in space, from planets to galaxies, are so far away that only a tiny amount of the light the produce reaches us on Earth. That faint light is such a small proportion of everything our eyes take in that most objects are invisible to us, and those we can see — even the bright and nearby ones like mars — have no detail that we can make out.
The number one thing any telescope does to improve night sky observing is gather significantly more light than our eyes. This means a small increase in objective size gives a big increase in light gathering power. If you take away nothing else from this article, remember this: first decide which type of scope is for you see below and then get the biggest objective diameter you can afford.
Everything else in a telescope is secondary to its light-gathering power, especially magnification, which only becomes relevant after the light is gathered.
To use higher magnification requires more light, so the same rule still applies. If you want more magnification, get a bigger aperture. The two main types of telescope are reflectors and refractors, they both follow the same three steps shown below to deliver their night sky images, but they use fundamentally different processes to achieve it.
A third design, the Catadioptric cat , or hybrid, uses a combination of mirror and lens to deliver some advantages in size. All telescopes, from the smallest starter scope to the Hubble Space telescope, work on the same three principles:.
Reflectors use mirrors as their light-gathering objective and refractors use a glass lens to collect and focus the light. A refractor is the classic telescope design of movies. It simply consists of a long tube and relatively narrow tube also known as an optical tube assembly, or OTA with an objective lens at one end and an eyepiece at the other.
Refractors are so called because they use a lens to refract bend incoming light rays to a focus. The detailed diagram below shows light rays orange coming in through the objective lens on the left and being focussed down to a point on the focal plane at the right.
The distance light travels from the objective the focal plane, i. The longer the focal length, the larger the image is created — but bigger images need more light and so longer focal planes generally need larger apertures.
But the reverse is also true: bigger lenses need a longer focal length to bring the light they gather to a focal point. The biggest refracting lens in the world is at Yerkes Observatory , Wisconsin. The limiting factor with refractors is the size of the objective lens and bigger refractors quickly become too expensive for backyard astronomers.
This is because the glass used to make the lens has to be of very high quality. There can be no imperfections on its surface or interior, and it must be perfectly shaped to focus the light it collects.
The physics behind a refractor telescope makes them brilliant at resolving fine detail, such as binary stars , lunar craters and wisps of nebulae.
This quality means that even a smaller, cheaper scope can give good views. Refractor scopes are also perfect for astrophotography. However, lens quality is much higher in these scopes, which means much more expensive too. This depends on the focal length of both lenses. Magnification might seem like the most important aspect of a telescope, but there are limits to how sharp an image a telescope can produce because of the blurring effects of the Earth's atmosphere.
Magnifying a blurred image makes it bigger, but not clearer, so the priority when telescopes are built is to have the greatest light-gathering power possible. Gathering more light makes brighter images, and brighter images make it easier to see faint details. Galileo is credited with being the first person to use a telescope to make observations of the night sky. After hearing of the invention of the telescope in , he built one of his own, called a Gallilean Telescope , in using a convex objective lens and a concave eyepiece lens.
His telescope could magnify objects 3 times. Telescopes he made later magnified objects up to 30 times. Lenses create a type of image distortion known as chromatic aberration. This occurs because as light passes through a lens, different colors are bent through different angles like in a prism and brought to a focus at different points. Because of this, stars viewed through a simple lens are surrounded by rainbow colored halos. This can be corrected for by adding a thin lens of a different kind of glass behind the objective lens.
Lenses present other optical problems including how difficult and expensive it is to make large lenses completely free of defects. Glass also absorbs most ultraviolet light, and visible light is substantially dimmed as it passes through a lens. In addition, lenses in telescopes can only be supported around the outside, so large lenses can sag and distort under their own weight.
All of these problems affect image quality and clarity. A small refracting telescope has an objective of focal length cm. If the eyepiece has a focal length of 4. Refracting Telescopes. Lenses Lenses form images by refraction and are typically made of either glass or plastic. Refracting telescopes use lenses to focus the light, and reflecting telescopes use mirrors. Refracting telescopes work by using two lenses to focus the light and make it look like the object is closer to you than it really is.
Convex lenses work by bending light inwards like in the diagram. This is what makes the image look smaller. The biggest refracting telescope in the world is located at the Yerkes Observatory of the University of Chicago at Williams Bay, Wisconsin.
Instead, they use mirrors to focus the light together.
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