Before explaining the choices of eyepieces that are available it is important to discuss: Telescope Performance.
We have all seen the pretty boxes at department stores advertising 650 power (650X) or 1200 power (1200X). Sounds pretty impressive, doesnt it? The fact is the amount of power a telescope can magnify isnt the most important thing when observing.
Most avid amateur astronomers know the truth.
The best telescopes and eyepieces provide good contrast and bright images.
Why is that so important? The reason is that most astronomical objects are really quite large, but usually very faint.
The most critical things to amateur observers is the ability of the telescope to gather as much light as possible, a telescope with good collimation, a dark sky site and of course a clear and steady atmosphere.
Magnification: Some Basic Facts
The focal length of your telescopes mirror or main lens is inscribed somewhere on the tube assembly, or it will be on the focusing mechanism. Each eyepiece will also have its focal length inscribed on them as well.
The exact magnification can be determined by means of the following formula:
Lets say your telescope has inscribed 1000 mm on the tube assembly, and you have an eyepiece that has 10 mm of focal length. Using the formula the magnification is 1000/10 or 100X. If you had a 40 mm eyepiece you would have 25X in magnification.
Very low magnifications are quite suitable to look at deep sky objects. There is a practical lower limit. As magnification becomes smaller, the "exit pupil", ( diameter of the light beam that passes into your eye) becomes larger. When the beam exceeds the size of your pupils dark adapted size (about 7 mm), it will be too large to be seen with a result of loss of light.
You can determine the exit pupil of a given telescope and eyepiece by using the following formula:
lens or mirror has a diameter of 100 mm and you are using a 25X eyepiece,
the exit pupil will be 4 mm. On the same telescope you can effectively
use a magnification as low as 14X, which would yield a 7 mm exit pupil.
Age will also effect the maximum exit pupil size, as you grow older,
your eyes will not open as wide as 7 mm.
One limit of maximum magnification is the earth's atmosphere, as a rule of thumb you should use magnifications lower than 500X as this will lessen the distortion effects caused by the earth's atmosphere.
Another limit is placed on the telescope by the size of the aperature. As a rule of thumb the upper limit on the magnification is the Aperture of the telescope (in millimeters) times 2.0. Magnifications above this limit may be possible, but distortion effects caused by the mirror or lens will greatly effect the image quality. In our 100 mm telescope example the maximum useful limit would be around 200X.
Another useful performance detail is the amount of resolving power the telescope has. This is a function of the aperture of the telescope, and it is the ability to seperate details, such as double stars. In our 100 mm telescope we want to know if we have enough resolving power to see a double star which is seperated by 2 seconds of arc.
The formula for determining resolving power is given by
In our 100 mm telescope we would calculate 11.6/10 or 1.16 arc-seconds of resolving power should be able to resolve a 2-arc second seperation in our double star.
Now that I have explained some of the factors of telescope performance, lets take a look at the choices of eyepieces that are available.
Eyepieces come in several diameters: 0.96 inch, 1.25 inch, and 2 inch. The 0.96 inch eyepieces are generally found on cheap department store telescopes, I would not recommend buying a telescope only capable of using them, as they can be very difficult to find.
The more common sizes are 1.25 and 2.0.
Eyepieces not only vary in size, and focal length but also in design. The simpilest designs give acceptable performance and in some cases even excellent performance.
The cost of an eyepiece is generally determined by its complexity. The market is literally flooded with eyepieces, all of which are geared to achieving the best field of view, magnification without sacrificing image quality.
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