Telescope Field of View Calculator

Background.

This calculator tells you how much sky an eyepiece actually shows in your telescope, using either of the two methods observers use — and it deliberately lets you compare them, because they frequently disagree.

The accurate method uses the eyepiece's field stop. That stop is a physical ring of metal sitting in the focal plane, and its diameter divided by the telescope's focal length is the field in radians, no assumptions required. Tele Vue publishes exactly this relation for its own eyepieces, written with the degrees-per-radian constant rounded to 57.3; this page uses the unrounded 57.29577951, which is 0.0074 % smaller.

The shortcut method divides the eyepiece's marketed apparent field by the magnification. It is the only method available when a manufacturer publishes a degree figure and no field stop, and it assumes the eyepiece maps angle to image height without distortion. Real wide-field designs do not. In the worked example on this page the same 25 mm eyepiece gives 1.0265° from its 21.5 mm field stop and 1.0833° from its marketed 52° apparent field — a gap of 5.5 %, which is not an error in either number but a measure of the eyepiece's distortion. Whenever the two disagree, believe the field stop, and treat the implied field stop this page reports as a cross-check on the marketing figure.

Two outputs make the number useful rather than abstract. The first is the widest field your focuser can physically deliver: a 1.25-inch barrel tops out at a field stop of about 27 mm and a 2-inch barrel at about 46 mm, so on a 1200 mm telescope no 1.25-inch eyepiece can ever show more than about 1.29° no matter what its apparent field claims. The calculator says so directly when your chosen field exceeds the barrel limit. The second is the drift time — how many seconds a star takes to cross the field with the drive switched off. That is a genuinely astronomical quantity rather than a geometric one: the sky turns 360° in one mean sidereal day of 86164.09 seconds, and a star at declination δ traces a smaller circle, so its rate across the field falls with cos δ. Timing a drift is also the classic way to measure a field stop nobody published.

Units are declared throughout. Focal lengths and field stops are in millimetres; all fields and the declination are in degrees; the field is also given in arcminutes because that is the unit deep-sky catalogues use for object sizes; and the drift time is in seconds of time, not seconds of arc. Only the cosine of the declination is used, so a field at −45° and one at +45° drift at the same rate. The field-stop relation is the linear small-angle mapping that is universal for eyepieces; the exact tangent form differs by less than 0.01 % at a 1.3° field and about 0.1 % at 5°, always smaller than the uncertainty in a published field stop. Three significant figures is the honest precision. A declination of exactly ±90° is refused rather than returned as infinity, because a star at the celestial pole genuinely never drifts out of the field.

What is telescope field of view calculator?

The true field of view is the angular diameter of the piece of sky you can see through the eyepiece at one time, measured in degrees or arcminutes. The full Moon is about 0.52° or 31 arcminutes across, so a 1° field shows roughly two Moon-widths.

It is not the same as the apparent field of view, which is how wide the illuminated circle looks to your eye — typically 50° for a Plössl and up to 100° for the widest modern designs. The apparent field is a property of the eyepiece alone; the true field depends on the eyepiece and the telescope together.

The field stop is the physical aperture inside the eyepiece that defines the edge of the field. Because it sits in the telescope's focal plane, its diameter converts directly into a sky angle through the telescope's focal length, which makes it the most reliable route to the true field.

How to use this calculator.

1. 01Choose the field-stop method if you know the eyepiece's field stop diameter — it is the accurate one.
2. 02Choose the apparent-field method if the manufacturer only publishes a degree figure.
3. 03Enter the telescope's focal length in millimetres.
4. 04Enter the eyepiece focal length, and then either its field stop diameter or its apparent field.
5. 05Set the barrel limit: about 27 mm for a 1.25-inch focuser, about 46 mm for a 2-inch one.
6. 06Enter the declination of what you are observing if you want a meaningful drift time; leave it at 0 for the celestial equator.
7. 07Compare the implied field stop against the manufacturer's published one. A large gap means the eyepiece has significant distortion and the shortcut method is over-reporting.

The formula.

The field stop sits in the telescope's focal plane, so a ring of diameter d there subtends an angle d/f_tel radians at the objective. Multiplying by 180/π converts that to degrees. Nothing about the eyepiece's internal design enters, which is why this route is trustworthy. The shortcut route instead assumes the eyepiece expands a true angle θ into an apparent angle Mθ exactly, which is only true for a distortion-free design.

Work through the default configuration. A 1200 mm telescope with a 25 mm eyepiece magnifies 48×. A 21.5 mm field stop gives 21.5 × 57.29577951308232 ⁄ 1200 = 1.0265493829°, or 61.5929629766 arcminutes. Multiplying back by the magnification, that field implies an apparent field of 49.2743703813° — noticeably less than the 52° such an eyepiece is usually sold as, which is exactly the discrepancy the page exists to expose. A 27 mm barrel limit corresponds to 1.2891550390°, so nothing is being clipped. And on the celestial equator a star crosses the field in 1.0265493829 × 86164.0905 ⁄ 360 = 245.699 seconds, four minutes and six seconds.

The sidereal day is not typed in as a literal. It is derived as 86400 divided by 1.00273790935, the ratio of mean solar to mean sidereal time published by the U.S. Naval Observatory, which gives 86164.0905 seconds — equivalently 15.041069 arcseconds of sky per second of time at the equator. Deriving it this way means this page and the Sidereal Time calculator cannot drift apart.

ROUNDING STAGE. Nothing is rounded part-way through; all arithmetic runs at forty significant digits and the single rounding is at the return boundary, to ten decimal places. The numbers inside the reading sentence are display-rounded from those same unrounded values.

SIGNIFICANT FIGURES. Three. Field stops are published to a tenth of a millimetre at best, apparent fields are marketing round numbers, and the drift time assumes a perfectly stationary, well-aligned telescope.

APPROXIMATION REGIME. The field-stop formula is the linear mapping y = f·θ, universal for eyepiece field stops. The exact tangent mapping y = f·tan θ differs by about θ²/3 — under 0.01 % on a 1.3° field, roughly 0.1 % at 5°. No telescope-and-eyepiece combination reaches a true field where this matters. The shortcut mode's assumption of zero eyepiece distortion is the one that actually fails, sometimes by several percent.

INVALID DOMAIN. Every focal length and stop diameter must be strictly positive. The declination is restricted to ±89.9°: at exactly ±90° the cosine is zero and the drift time is infinite — physically correct, since a star at the celestial pole never leaves the field, but not a number, so the calculator refuses it with a message that says so.

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.