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How Do Telescopes Work?

The Refractor

The first, and most simple type of telescope is called the REFRACTOR. This is the sort of telescope most of us think of when we imagine what a telescope should look like. They are a bit like the sort we see in films, where the old ship's captain is looking for pirates. They are generally more portable and easier to set up and use than the other types. It was this type of telescope which Galileo used to first see the moons of Jupiter, or which Charles Messier used to observe and catalogue his famous list of astronomical Objects. These days we are able to make lenses of a much higher quality than was available to those early scientists, and there are some relatively inexpensive, good quality instruments available to the amateur. In a refracting telescope light enters the tube through a main "Objective Lens".  This objective lens in most budget telescopes is usually made of glass, but it is possible to also purchase sophisticated, top-of-the-range telescopes which use a material called calcium fluorite. These telescopes produce superior images with no chromatic or other aberrations, but they are also very expensive. In all refractors, irrespective of how they are made, the incoming light is bent (refracted) by the lens in such a way that after it has travelled down the tube to the other end, it all gathers together at one point - called the "focus" of the telescope. This is where the eyepiece is placed. The eyepiece is a device made up of smaller lenses which takes the focused light and organizes it so that we can see the final image by placing our eye against it. One of the most important things the eyepiece does is to magnify the image - which leads us on to the topic of "magnifying power".

To calculate magnifying power you divide the "focal length" of the object lens by the focal length of the eyepiece used. The focal length of the telescope is of course "fixed", whereas the focal length of the eyepiece can be change simply by changing the eyepiece for a different one. So the longer the telescope the greater the magnification for a given eyepiece. For example, a telescope with a focal length of 1000mm (remember you can't change this) will produce a magnification of 100 times if you use a 10mm focal length eyepiece, or 50 times if you use a 20mm focal length eyepiece.  Typical magnifications used in "serious"  amateur astronomical telescopes are between 50X and 500X (a typical pair of binoculars would be 8X or 10X). Many people ask about the "power" of a telescope but there is much nonsense talked about "magnifying power", especially in sales advertising. Telescopes are not really measured by their power, but by things such as "resolution" (the ability to separate objects which are very close together), light gathering capability (the total amount of light which the telescope can gather before magnifying it) and focal ratio (a ratio obtained by dividing the diameter of a telescope into it's focal length). Really, the most important thing to remember is that the larger the diameter of the Object Lens the greater it's light gathering power and the brighter the image for a given magnification. This is because when you magnify an image you are taking a fixed amount of light, which has been collected by the object lens, and spreading it out over a larger area. As you continue to spread it out there is less light available for each unit of area and eventually the image becomes too dim to see properly. So if you don't collect much light to start with, you are never going to have a good, bright image when it is magnified. Refractors are normally fairly small in size (2" diameter to 6") because larger sizes become very large and heavy and the lenses very expensive to make. They are ideal "first" telescopes, are good for planetary work because they provide high contrast images, and can be made to fold up to fairly small sizes for transport.

The Reflector

The next main class of telescope used in astronomy are REFLECTORS. This type have an open end to the telescope tube and do not use a lens at all. They use instead a system of mirrors which are cheaper to make and also much lighter than lenses. In a reflector, the light enters the open end of the telescope tube and travels unchanged down the tube to the main mirror which is located at the bottom. The mirror is convex, and this reflects the light rays at an angle so that they are reflected back up the tube to a point of focus near the top, where they come to focus at a small, flat mirror, which is positioned at a 45 degree angle. This mirror then reflects the light out through the side of the telescope tube to an eyepiece which is fastened to the side of the tube near the top. This is where the image is viewed by the observer. The eyepieces used are exactly the same as for a refracting telescope, but it is quite normal for the tube of a reflecting telescope to be made of cheap material such as plastic or even cardboard in some cases. This type of telescope is relatively inexpensive, partly because cheaper materials can be used to make the main pieces, and partly because it is easier to make quite large mirrors than it is to make glass lenses. Typical main mirror sizes you will find for sale to the general public are from 6" up to as much as 24" diameter, although it is unusual to find anything larger than about 16" unless you go to specialist suppliers.

What we said before about magnification and light gathering power for refractors applies to reflectors as well. This means that because mirrors are easier and cheaper to make than lenses, you can usually afford to buy a larger telescope if you buy a reflector. However, although the larger mirrors gather more light, they also have longer focal lengths, so in order to obtain reasonable magnification you would need a very long telescope. This can run into many feet for the larger mirrors, and they can be tough to carry around. For this reason, the larger reflecting telescopes used by amateurs tend to come disassembled and you have to put them together every time you want to use them. The main mirror assembly forms one piece, a cage assembly holding the secondary mirror another piece, and the base on which everything stands is a third. Luckily, because it is not necessary to have a solid tube for the main piece of the telescope, these instruments normally use a series of connectable rods to join the main mirror assembly to the secondary mirror cage, making everything just that bit easier to carry around.

The Catadioptric

The last main type of telescopes are called CATADIOPTRICS and the most common version of them is the SCHMIDT-CASSEGRAIN design. This type of telescope uses a combination of both refractor and reflector technology to try to capitalize on the advantages of both. Light enters the telescope tube at the top, but this time the top of the tube is not open. Instead, there is an initial "correcting lens", which slightly adjusts the light as it enters the telescope tube. The light then passes down the tube to the main mirror, located at the bottom of the tube as for a reflector, and is then reflected back up the tube, again just as with a reflector. The difference in this case is that there is no flat mirror at the top to reflect the light out of the side of the tube. This time the light is focused on a CONVEX mirror located right in the center of the correcting lens. This convex mirror redirects the light back down the tube again to a focus in the middle at the bottom, where it exits the telescope through a small hole in the middle of the main mirror. The eyepiece is located here, and the observer stands or sits at the bottom. There are advantages and disadvantages to this type of design. One advantage is that because the light passes up and down the tube three times, the final FOCAL LENGTH of the telescope is long, which allows you to obtain high magnification without the need for a very long tube. This also means that SCTs (Schmidt Cassegrain Telescopes) are more compact and easier to carry around. They also produce very high quality images and are sealed to prevent the ingress of dirt, dust, bugs etc. You have to clean your mirror regularly if you own a reflector because the tube is open, but an SCT mirror stays clean for a long time.

Disadvantages of this type are that they tend to be more expensive than the other designs, and also the presence of obstructions in both the corrector plate (secondary mirror) and main mirror (exit hole for the light) reduces the effective diameter, and hence light gathering capability. You will remember that this was one of the most important factors you need in a telescope, but usually this disadvantage is more than compensated for by the advantages, particularly the portability.

Sizes for amateur instruments of this type again vary from typically 6" diameter for the main mirror, to 16", although most are around 8" or 10" due to the increasing difficulty of carrying and assembling the larger instruments.

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