The most important thing that needs to be considered before purchasing any telescope, especially if you are new to astronomy, is what you are going to use the telescope for. With this in mind, it is not always obvious that the physical properties of a telescope should be closely related to your requirements.
Two of the most important parameters associated with astronomical telescopes are the aperture (the diameter of the main objective lens or mirror) and the focal length (the distance from the objective lens or mirror to the point where the image is formed). With a little simple mathematics, you can easily calculate the values of these parameters to help you choose the best telescope for your needs.
Telescope Aperture
The telescope aperture is representative of what is usually described as the physical 'size' of the telescope. The bigger the aperture, the more light gathering ability the telescope has. For example, a recommended aperture for a really useful first telescope would be at least 100mm for a refracting telescope, or 150mm for a reflecting telescope. Using these figures, we can then calculate how faint an object an observer would be able to see:
The entrance pupil of the human eye can reach approximately 8mm in diameter when fully adapted to the dark. This is equivalent to an area of 50 square millimetres (50mm 2). The refracting telescope having an aperture of 100mm in diameter has an area equivalent to approximately 7850mm 2.
The 100mm aperture telescope is therefore capable of collecting 7850/50 = 157x more light, which is then available through the telescope eyepiece. In other words, looking through the telescope will enable an observer to see objects which are 157 times fainter than could be seen with the unaided eye.
Following the same calculation, the 150mm aperture reflecting telescope would enable the same observer to see objects which were 353 times fainter than could be seen with the unaided eye.
Clearly then, if you want to observe faint star fields or galaxies and nebulae, then a bigger telescope aperture is certainly better. If you have a particular object in mind which you would like to see, then knowing how faint it is should allow you to then 'work backwards' with the above calculation, to determine if the telescope you have would be suitable.
Telescope Focal Length and Magnification
The focal length of the telescope is representative of either the physical 'length' of the telescope, or its optical configuration. A short focal length will give a wide field of view (the area of night sky that can be seen), with the objects in that field of view appearing small, whereas a long focal length will give a narrow field of view, but with the objects appearing larger.
The magnification of the telescope is the result of a combination of the focal length of the telescope itself, and focal length of the telescope eyepiece. A telescope of a certain focal length will produce an image of a certain size, which is fixed and will not vary. The eyepiece, effectively used as a microscope, then views that image. A larger image to start with allows the eyepiece to produce a higher magnification. So, by changing the eyepiece, the magnification of the telescope can also be changed.
For example, a recommended focal length for either a refracting or reflecting telescope would be at least 1000mm.
Using this figure, we can then calculate the following:
An eyepiece of long focal length, say 25mm, will produce a magnification of 1000/25 = 40x.
An eyepiece of short focal length, say 10mm, will produce a magnification of 1000/10 = 100x.
Using the same calculation, the same eyepieces used on a telescope having a focal length of 1500mm would give magnifications of 60x and 150x respectively.
Telescopes with long focal length are ideal for observing the craters on the moon or the features of the brighter planets.
About the author:
Steve West is a systems engineer, astrophysics graduate and lifelong enthusiast for all things to do with space. For more advice on telescopes, telescope accessories and educational gadgets including home planetariums, visit: http://www.squidoo.com/telescopes-for-sale
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