Magnification

How many times a telescope can magnifiy an object is often misleading and most of the time irrelevant to a telescope's design. The magnification can be changed simply by changing the eyepiece and the amount that you need depends on the object you're viewing. To calculate the magnification all you need to do is divide the focal length of the telescope with the focal length of the eyepiece. Because of diffraction effects the maximum amount of magnification that is useful is determined by the size of the main objective lens/mirror (the aperture). This is roughly 50x for every inch of aperture. Unless the object being viewed is a planet in our own solar system this limitation is almost never reached. In fact some objects in the night sky are bigger than the full moon and thus require very little magnification. It is also worth noting that the more you magnify an image the dimmer the image will appear.

A simple yet effective article describing the magnification ranges useful for visual astronomy can be found here.



Barlow lens
With a barlow lens it is possible to increase the magnification without changing either the focal length of the eyepiece or the focal length of the telescope. It is positioned between the eyepiece and the telescope, with the change in magnification determined by the power of the barlow and the distance of the eyepiece to the barlow lens. For example a telescope with a focal length of 500mm and an eyepiece of 10mm has a magnification of 50x. Using a 2x barlow this is increased to 100x. One of the advantages of using a barlow lens rather than an eyepiece with a shorter focal length is the increase in eye relief. The eye relief depends on the focal length and design of the eyepiece, the shorter the focal length the closer your eye has to be to the eyepiece. For telescopes with fast focal ratio's this can become problematic as the focal length can be as low as 2.5mm. A barlow lens can solve this problem.

Like refractors, some are made up of multiple lenses that correct for color fringing and are called apochromatic barlows.



Focal reducer
Works in a similar way as a barlow lens but as the name suggests it reduces the focal length rather than increases the focal length of the telescope. This is often used by astrophotographers to increase the photographic speed of a telescope and to increase the field of view.