Ken Hough's Website
Non-astronomers often ask something along the lines of "How much magnification has this telescope got?"
A simple, but probably not very helpful answer would be "It depends....". Astronomical telescopes are not rated in terms of magnification. This is because the magnification that will be seen via the eyepiece depends on both the focal length of the telescope optics and on the focal length of the eyepiece being used, and on any Barlow lens or focal reducer that might be included.
Magnification is taken to be the ratio of the linear size of an object as seen through the eyepiece compaired with the linear size of that object when viewed with the naked eye and is calculated as :
Magnification = Effective focal length of telescope / Focal length of eyepeiceThe effective focal length of a telescope takes into account the inclusion of any Barlow lens or focal reducer. For example, an f=1000mm telescope used with a X2 Barlow lens, has an effective focal length of f=2000mm.
By using an eyepiece of short focal length, it is possible to obtain high magnification. But how high a magnification is useful?
Again, it depends! Under rare conditions when the atmosphere is very stable, the maximum useful magnification of a telescope is approximately twice the diameter of the objective lens or mirror expressed in millimetres. Beyond this limit, no more detail will be seen by increasing magnification. This means that under ideal conditions, a 10"/250mm telescope could be used at a mgnification of up to 500. It also means that a telescope with a 60mm objective is limited to a useful magnification of only 120.
In practice atmospheric turbulence will often limit useful magnification to only 100 to 200 even for a large telescope.
Nomatter what you might read in an advert, a catalogue, or on the side of a box containing a telescope, claims like magnification = 500 from a small telescope are at best very misleading.