The patch of sky actually visible in the eyepiece. As a first approximation it is the apparent field divided by the magnification: a 52° eyepiece at 50× shows about 1°, twice the width of the full Moon. The exact figure comes from the field stop.
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Related terms
- 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.
- Field stop
The ring inside the eyepiece that gives the field its sharp edge. Its diameter gives the exact true field: field (°) = field stop (mm) ÷ telescope focal length (mm) × 57.3. A 27 mm field stop on a 1,000 mm telescope thus shows 1.55°.
- 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.
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