Telescope Field of View Calculator

Background.

Before you swing a telescope toward the full Moon, a sprawling open cluster or the Andromeda Galaxy, the useful question is whether the eyepiece will frame the whole thing. The full Moon is about half a degree across; Andromeda, in long-exposure photographs, stretches to roughly six Moon-widths. A 25 mm eyepiece in a 1200 mm telescope shows about one degree, so the Moon fits twice over and the galaxy does not fit at all.

There are two ways to work that degree out, and they rarely agree exactly. The reliable one starts from the eyepiece's field stop, the metal ring inside the barrel that draws the sharp edge of the view. Because that ring sits at the telescope's focus, its diameter divided by the telescope's focal length is the slice of sky in radians. Tele Vue publishes this relation for its own eyepieces using 57.3 degrees per radian; the calculator uses the unrounded 180/π.

The quick way divides the apparent field printed in the eyepiece's advertising (50° or so for a Plössl, 68–82° for wide-field designs) by the magnification. Real eyepieces have some distortion, so this usually overstates the sky by a few percent; the calculator turns its answer back into the field stop it implies, a number you can hold against the maker's specification.

Alongside the field you get the widest view your focuser barrel can ever pass and the number of seconds a star takes to drift across with the drive switched off.

What is telescope field of view calculator?

True field of view is the angular diameter of the circle of sky visible through an eyepiece, in degrees or arcminutes. It belongs to the telescope and eyepiece together. Apparent field is different: it is how wide the bright circle looks to your eye, a property of the eyepiece alone, from about 50° in a Plössl to around 100° in the widest modern designs. The field stop is the physical aperture that sets the edge of the view, quoted in millimetres. Drift time is the number of seconds a star needs to cross the full field when nothing is tracking it.

How to use this calculator.

1. 01Pick the method. Use the field-stop method whenever the eyepiece's specification sheet lists a field stop diameter; fall back to the apparent-field method when the box only gives a degree figure.
2. 02Enter the telescope's focal length in millimetres, not its aperture. It is usually printed on the tube or in the manual as, for example, f = 1200 mm. If you use a Barlow lens, multiply by its factor first.
3. 03Enter the eyepiece focal length engraved on the barrel, then the field stop or the apparent field depending on the method you chose.
4. 04Set the barrel limit to 27 mm for a 1.25-inch focuser or 46 mm for a 2-inch one, and enter the declination of your target if you want its drift time (0° puts it on the celestial equator).
5. 05Read the true field, then glance at the implied field stop or implied apparent field. A large gap between that and the maker's figure tells you the eyepiece has noticeable distortion.

The formula.

The field-stop route is pure geometry. A ring of diameter d_stop sitting in the focal plane of a telescope with focal length f_tel subtends an angle d_stop ⁄ f_tel in radians, and multiplying by 180⁄π (57.2958) converts it to degrees: TFOV = (d_stop ⁄ f_tel) × 180⁄π. Nothing about the eyepiece's lens design enters, which is why it is trusted. Strictly the angle is 2·arctan(d_stop ⁄ 2f_tel), but for real eyepiece fields the difference is under 0.01 % at a 1.3° field and under 0.1 % even at 5°, far smaller than the uncertainty in a published stop.

The shortcut is TFOV ≈ AFOV ⁄ M, where the magnification M = f_tel ⁄ f_eye. It holds only if the eyepiece maps angles onto your eye in strict proportion, so its error equals the eyepiece's angular distortion. Each method reports the other's quantity too: the apparent field a stop implies (TFOV × M), or the stop an apparent field implies.

The barrel ceiling is the same geometry applied to the largest stop the barrel can hold. Drift time is t = TFOV × T_sid ⁄ (360 × cos δ). The sky turns 360° in one mean sidereal day, T_sid = 86400 ⁄ 1.00273790935 = 86,164.09 s (23 h 56 min 4 s), from the solar-to-sidereal ratio published by the U.S. Naval Observatory: 15.041 arcseconds of sky per second at the equator. A star at declination δ follows a smaller circle, so its speed falls by cos δ; +45° and −45° drift alike, and declinations beyond ±89.9° are refused because at the pole the time becomes infinite.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Tele Vue Optics, Inc., 'Telescope Formulas'. Manufacturer engineering note, verified by retrieval to state verbatim: Field Size = '(eyepiece field stop diameter / telescope focal length) x 57.3°', and Magnification = 'Objective focal length / Eyepiece focal length'. This is the independent second authority for the field-stop relation. Open access. Retrieved 2026-07-29.
2. [2]U.S. Naval Observatory, Astronomical Applications Department, 'Computing Approximate Sidereal Time'. Source of the ratio of mean solar to mean sidereal time, 1.00273790935, from which this page derives the mean sidereal day of 86164.0905 s and the sidereal rate of 15.041069 arcsec per second of time — rather than typing either as a literal. Open access. Retrieved 2026-07-29.
3. [3]Urban, S.E. & Seidelmann, P.K. (eds.) (2013). Explanatory Supplement to the Astronomical Almanac, 3rd edition, University Science Books, §6 (Earth rotation and time scales). Definition of the mean sidereal day and its relation to UT1. Print / bibliographic reference.
4. [4]Hecht, E. (2017). Optics, 5th edition, §5.7.3 'Telescopes', Pearson Education. Afocal magnification, field stops and the apparent field of an eyepiece. Print / bibliographic reference.