Telescope Field of View
Calculate the true field of view of any telescope-eyepiece combination using apparent FOV and magnification. Free online astronomy calculator with charts.
About This Calculator
The Telescope Field of View Calculator helps amateur astronomers and stargazers determine the angular size of the sky visible through any telescope and eyepiece combination. Enter your eyepiece's apparent field of view and your telescope's magnification to instantly see the true field of view in degrees, arcminutes, and arcseconds, plus the area of sky covered.
The fundamental formula is FOVtrue = FOVapparent / M, where the apparent field of view is a specification of the eyepiece (typically 30° for simple designs to 110° for ultra-wide eyepieces) and M is the magnification of the telescope-eyepiece combination. The area of the field of view assumes a circular field and is calculated as π × (FOV/2)². The calculator also shows how many Moon diameters fit in your field of view (the Moon averages 31 arcminutes across).
A wider true field of view makes it easier to locate objects and is ideal for observing large targets like the Pleiades, Andromeda Galaxy, and the Moon. Narrower fields help concentrate on fine details on planets and double stars. Most astronomers build a collection of eyepieces at different focal lengths to achieve a range of fields of view for different observing goals.
Regional Notes
Global: Telescope specifications and formulas are universal. Eyepiece apparent field of view is always measured in degrees. The Moon's angular diameter of 31' is an average and varies slightly with its orbital position.
Tips for beginners: If you don't know your magnification, use our Telescope Magnification Calculator to find it from your telescope's aperture, f-ratio, and eyepiece focal length. A good starter eyepiece set includes a 25mm (low power, wide field) and a 10mm (high power, narrow field).
Frequently Asked Questions
What is the field of view of a telescope?
The field of view (FOV) of a telescope is the angular diameter of the portion of the sky visible through the eyepiece. It is calculated by dividing the eyepiece's apparent field of view by the magnification. For example, a 52° eyepiece at 50× gives a true field of 1.04°.
How do you calculate telescope field of view?
Telescope field of view is calculated using the formula: True FOV = Apparent FOV / Magnification. The apparent FOV is printed on the eyepiece (typically 30-110°), and magnification equals the telescope focal length divided by the eyepiece focal length.
What is a good field of view for a telescope?
A good field of view depends on what you are observing. Wide fields of 1-3° are ideal for star clusters, nebulae, and the Moon. Narrow fields under 0.5° are better for planets and double stars. Most astronomers use a range of eyepieces from 25mm (wide) to 10mm (high power) to cover different targets.
How does eyepiece apparent FOV affect what I see?
The eyepiece apparent FOV determines how wide the view appears through the eyepiece itself. Standard Plössl eyepieces have 50-52°, wide-field designs offer 68-82°, and ultra-wide eyepieces reach 100° or more. A wider apparent FOV gives a more immersive view but costs more.
What is the field of view of the Hubble Space Telescope?
Hubble's WFC3 (Wide Field Camera 3) has a field of view of about 160 × 160 arcseconds (2.7 × 2.7 arcminutes), equivalent to 1.98 × 10⁻³ square degrees. This is roughly one-tenth the diameter of the full Moon as seen from Earth.
How do I measure the apparent field of view of my eyepiece?
If the apparent FOV is not marked on your eyepiece, you can approximate it by measuring the diameter d of the field lens (the lens at the bottom) and multiplying by 57.3. For example, a 20mm diameter field lens gives an apparent FOV of about 57.3 × 20 = 57°. This method works best for simple eyepiece designs.
Can I see the whole Moon in my telescope's field of view?
The Moon's angular diameter is about 31 arcminutes (0.52°). With a 52° eyepiece at 50×, your true FOV is 1.04°, so the Moon fits comfortably with room to spare. At 100× the FOV drops to 0.52°, showing the Moon edge-to-edge. At 200×, you see only about half the Moon's diameter.