Table of Contents
What Is Telescope Field of View?
The field of view (FOV) of a telescope describes the angular extent of sky visible through the eyepiece. There are two types: apparent field of view (AFOV), which is a property of the eyepiece alone, and true field of view (TFOV), which is the actual sky coverage when the eyepiece is used with a specific telescope.
A wider TFOV is desirable for viewing large objects like nebulae and star clusters, while a narrow TFOV at high magnification is better for planetary detail and double stars. The TFOV depends on both the magnification (set by the ratio of telescope to eyepiece focal lengths) and the eyepiece's apparent field.
FOV Formulas
Common Eyepiece FOV Values
| Eyepiece Type | Apparent FOV | Eye Relief |
|---|---|---|
| Plossl | 50-52° | Moderate |
| Erfle | 60-68° | Moderate |
| Nagler | 82° | Good |
| Ethos | 100° | Good |
| XWA | 110° | Long |
Observing Tips
- Deep sky: Use low magnification and wide AFOV eyepieces (82-100°) for large nebulae and clusters.
- Planets: Higher magnification (150-300x) with sharp optics. TFOV of 0.1-0.3° is typical.
- Exit pupil: Keep exit pupil between 1-7 mm. Over 7 mm wastes light; under 1 mm is too dim for most objects.
- Moon fits: Full Moon is ~0.5°. A setup giving 1° TFOV will frame the Moon with surrounding space.
Frequently Asked Questions
How do I calculate TFOV from drift timing?
Turn off the telescope drive and time how long a star takes to drift across the eyepiece field. TFOV (arcmin) = drift time (seconds) / 4 * cos(declination). This gives the true field measured empirically.
Does the Barlow lens affect FOV?
Yes. A 2x Barlow doubles the effective focal length, doubling magnification and halving the true FOV. However, it preserves the exit pupil-to-magnification relationship and can be useful when you want to reach higher power with a comfortable eyepiece.
What is the maximum useful magnification?
Approximately 2x the aperture in mm (50x per inch). For a 200 mm telescope, the maximum useful magnification is about 400x, limited by atmospheric seeing and diffraction. Beyond this, images appear dim and blurry with no additional detail.