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EYEPIECES - MAGNIFICATIONS, FIELD OF VIEW, EYE RELIEF

There so many different eyepieces available for telescopes. Each one has something different in it's characteristics even if it just a change in magnification. First of all there are the sizes. These are the diameters of the eyepieces. They are 1" (25.4mm), 1.25" (31.7mm) or 2" (50.8mm). The most popular are the 1.25"'s. The 1" eyepieces generally come with smaller telescopes as standard and can be updated to the larger 1.25" size when you have worked out which ones would suit what you like to look at in the sky. It works well this way as the standard 1" eyepieces make the telescope affordable and are enough to get you started. Updating your eyepieces is the most significant way to improve your telescopes performance. 2" eyepieces are not widely used in Australia as of yet mainly because of their huge price tags and most telescopes aren't designed to accept them. An adapter may be purchased, but the 2" eyepiece must still suit the telescopes optics.
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A minimum of three eyepieces is needed to compliment a telescope. This is usually made up from:

1) a good quality, fairly high magnification eyepiece for the planets and Moon. It's the high magnifications that will see the intricate details, so it may as well be a good one.

2) a low magnification eyepiece which has a wide field. These are great to fit in the extra wide objects like nebula and star fields. Since this type of eyepiece is not used for studying close up detail, it doesn't have to be as expensive as the high magnification eyepiece. These eyepieces are also excellent for beginners as they show a large area of the sky to search for the objects your trying to find.

3) a Barlow. A Barlow is an eyepiece which goes between the telescope
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and the normal eyepiece. They change the focal ratio of the telescope.
Barlows come in 1.5x, 2x, 3x, 4x, and 5x. If you have a a focal ratio telescope of F/6, using a 2x Barlow will make it a F/12 (2x6). Barlows are especially useful for getting high magnifications out of short focal ratio telescopes as eyepieces can only be made so short in focal length (4mm is
the shortest). If the focal length of your telescope is only 1140mm long,
and it's a 254mm diameter mirror, using a 6mm eyepieces will only give you 190x magnification (1140/6 = 190) and you will have to put your eye so close to the eyepiece (eye relief) it will be uncomfortable. The Barlow will make it comfortable for your eye to view and double the magnification to 380x.
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