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°.
ToolsEyepieces, Useful formulas
Related terms
- True field
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.
- Eyepiece
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.
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