Apparent vs. True Field
An eyepiece's
apparent field of view is the angular diameter, expressed in degrees (°), of the circle
of light that the eye sees. It is analogous to the screen of a television (not
the picture seen through it). Eyepiece apparent fields range from narrow (25°
- 30°) to extra-wide angle (80° or more). True field = Apparent field / Magnification For example, suppose you have an 8" Schmidt-Cassegrain telescope with a 2000mm focal length, and a 20mm eyepiece with a 50° apparent field. The magnification would be 100X (2000mm / 20mm). The true field would be 50 / 100, or 0.5° - about the same apparent diameter as the full Moon or the Sun. updated: |
The
true field (or real field) of view is the angle of sky seen through the eyepiece
when it's attached to the telescope. The true field can be approximated using the
formula: