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".

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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.