The small interchangeable magnifier you look through; it sets the magnification. Its focal length is engraved on it: the shorter it is, the higher the power. The standard barrels are 1.25 inch (31.75 mm) and 2 inch (50.8 mm) wide.
ToolsEyepieces
Related terms
- Magnification
How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.
- Apparent field
The angle the eyepiece’s circle of view covers for the eye, as stated by the maker: about 50° for a Plössl, 68° to 82° for a wide-angle design. The wider it is, the more it feels like looking through a big window. It is used to work out the true field.
- Plössl
A classic four-lens eyepiece with an apparent field of about 50°, sharp and contrasty. It is often the eyepiece supplied with telescopes, and a very good starting point. Its eye relief is about 70% of its focal length, which makes it uncomfortable below 10 mm.
- Wide-angle eyepiece
An eyepiece with an apparent field of 65° to 100° or more, using more lenses. At the same magnification it shows more sky, which helps when finding objects and following a planet with a Dobsonian. It is heavier and bulkier than a Plössl.
- Eye relief
The distance between the eyepiece’s last lens and the spot where your eye must be to see the whole field. Glasses wearers need at least 15 to 20 mm. Short focal length eyepieces of older designs often have very short, uncomfortable eye relief.
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