Teaching · For Teachers · Astronomy

How much sky each
eyepiece shows,
before dark.

Type the telescope’s aperture and focal length and the eyepieces in the box. The calculator gives the magnification and the slice of sky each one shows, and says whether the full Moon will fit.

Check my eyepiecesWhat you get

The field of view calculator

The worked example is made up: a 130 mm, 650 mm focal length reflector, the kind many schools and libraries lend, with three eyepieces. Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
For each eyepiece, the magnification, the true field of view in degrees and arcminutes, the exit pupil and whether the full Moon fits; for the telescope, the focal ratio, the Dawes resolution limit and the useful magnification range, with warnings for eyepieces outside it. It prints as a table for the observing session and downloads as a spreadsheet.
Who it is for
Science and astronomy teachers, astronomy clubs, and students and families with a first telescope or one borrowed from a library.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Up to 12 eyepieces, with or without a Barlow lens. Prints a table; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Type the aperture and focal length from the tube or the manual.
  2. Add each eyepiece with its focal length and apparent field.
  3. Choose a Barlow lens if you use one.
  4. Read the magnification and true field for each, and check that the Moon or your target fits.
  5. Print the table to take outside.

Check my eyepieces

How the numbers are worked out

Three divisions for every eyepiece.

The 10 mm eyepiece on a 130 mm, 650 mm telescopeMagnification65×650 ÷ 10True field0.80°52° ÷ 65Exit pupil2.0 mm130 ÷ 65The 10 mm eyepieceMagnification65×650 mm ÷ 10 mmTrue field0.80°52° ÷ 65Exit pupil2.0 mm130 ÷ 65
Three divisions describe an eyepiece: the 10 mm gives 65×, a true field of 0.80° (the full Moon is about 0.52°) and an exit pupil of 2.0 mm.

The arithmetic

Magnification is the telescope’s focal length, times the Barlow factor, divided by the eyepiece’s focal length. The true field, the patch of sky you see, is close to the eyepiece’s apparent field divided by the magnification. The exit pupil, the beam of light leaving the eyepiece, is the aperture divided by the magnification. The Dawes limit, 116 divided by the aperture in millimeters, gives the closest pair of equal stars the telescope can split, in arcseconds.

Rules of thumb

About twice the aperture in millimeters (50× per inch) is the most magnification a telescope can use, and the air on most nights allows less. An exit pupil wider than about 7 mm, the most a young dark-adapted eye opens, wastes light, which sets the lowest useful power. The widest field is best for finding objects and for large ones such as the Pleiades; higher power is for the Moon’s craters, planets and double stars.

Sources

  1. Sky & Telescope. Telescope magnification and How to calculate your eyepiece’s field of view (skyandtelescope.org): magnification, true field and exit pupil.
  2. NASA Night Sky Network (nightsky.jpl.nasa.gov): activities and toolkits for telescope observing with students.
  3. Dawes, W. R. (1867). Catalogue of micrometrical measurements of double stars. Memoirs of the Royal Astronomical Society, 35: the empirical resolution limit.
  4. NASA Goddard Space Flight Center, Moon Fact Sheet (nssdc.gsfc.nasa.gov): the Moon’s apparent diameter, about 0.52° on average.

The worked example is made up. The true field is the common apparent-field approximation, close to within a few percent; field-stop measurements are more exact. Your figures stay in this browser. Support: team@caveatacademic.com.