The Visual Astronomy Assistant is an eyepiece field of view and telescope magnification tool. Pick a Telescope and an Eyepiece and it gives the magnification, the true field of view and the exit pupil, then draws that field as a circle over real sky survey imagery, the Moon, the Sun or a planet at true size. Use it to plan a night at the eyepiece and to see whether the Moon will fit.
How to use it
- Set the Telescope. Press Pick the telescope, or type its Aperture and Focal length in millimetres.
- Choose a Barlow / reducer if you use one, from a 0.33× reducer to a 5× barlow. It multiplies the focal length, and the magnification with it.
- Set the Eyepiece. Press Pick the eyepiece, or type its Focal length in millimetres and its Apparent field in degrees.
- Press Add to view. The Combination gets a coloured circle, its Outline, labelled with the Eyepiece name and magnification. Add several to compare; Clear equipment empties the view, and the × on a table row removes one.
- Under Which view, choose Sky View for survey imagery. Type an object such as M31 in Go to an object and press Enter, and pick a Survey. Adding a Combination zooms to fit every Outline; removing one leaves the zoom alone.
- Choose Solar System View to see the Moon, the Sun or a planet at true angular size, with each Outline drawn to the same scale. A scale bar in the corner says what an angle measures on screen.
- Read the table at the foot: one row per Combination, with Magnification, True field of view, Exit pupil, Focal ratio and the resolution limits in columns. Press Save image to download the view as a JPEG.
How it works
The Telescope’s focal length, multiplied by any barlow or reducer, gives the effective focal length; divided by the Eyepiece’s focal length, it gives the magnification. The Eyepiece’s apparent field, the angle it fills at your eye, divided by the magnification gives the true field of view, the angle of sky you see. The aperture divided by the magnification gives the exit pupil, the width of the beam leaving the Eyepiece.
This is the standard approximation. It takes the apparent field at face value and ignores the distortion of any one Eyepiece, so read the true field as a close planning figure. The table also gives the Dawes limit, 116 ÷ aperture in mm, and the Rayleigh limit, 138 ÷ aperture in mm, both in arcseconds.
The Solar System View uses one size per body: 30′ for the Moon, 32′ for the Sun, and for each planet its size at its widest, when it is closest to Earth. The real Moon runs from about 29′ to 34′, so leave a margin on a tight fit. Survey images are long exposures and show far more colour and detail than the eye does: the tool plans a field, not the view.
Never look at the Sun without a proper solar filter, and never through a Telescope or an Eyepiece unless a solar filter covers the front of the Telescope. The tool prints a warning beside the Sun, and a similar one beside Mercury, which stands close to it in the sky.
Formula
M = F ÷ fe ; T = Ae ÷ M ; P = D ÷ M
M is the magnification (×). F is the effective focal length in mm, the Telescope’s focal length times the barlow or reducer factor. fe is the Eyepiece’s focal length in mm. T is the true field of view and Ae the apparent field, both in degrees. P is the exit pupil and D the aperture, both in mm.
Worked example
A Telescope of 200 mm aperture and 1000 mm focal length, a 25 mm, 68° Eyepiece and no barlow: M = 1000 ÷ 25 = 40×. T = 68° ÷ 40 = 1.70°, or 102′. P = 200 ÷ 40 = 5.00 mm. The Moon, 30′ across, takes up about 29% of that field’s width (30′ ÷ 102′).
Common questions
How do I calculate telescope magnification and eyepiece field of view?
Magnification is the Telescope’s focal length over the Eyepiece’s: 1000 mm ÷ 25 mm is 40×. The true field is the Eyepiece’s apparent field over that: 68° ÷ 40 is 1.70°. A barlow multiplies the focal length first, so set it under Barlow / reducer and both columns include it.
Will the Moon fit in my eyepiece?
The Moon is about 30′, or 0.5°, across, so it fits whole when the true field is larger. Choose Solar System View, then Moon, and add your Combination. On a 1000 mm Telescope, a 10 mm, 60° Eyepiece gives 0.60° and holds the Moon; a 6 mm, 60° Eyepiece gives 0.36° and does not.
What is the exit pupil?
The width of the beam leaving the Eyepiece: the aperture divided by the magnification, in mm. If it is wider than your own pupil, often 5 to 7 mm in the dark and smaller with age, some light never enters your eye.
How accurate is the field of view?
It is exact arithmetic on the values you give it, using the standard approximation. It ignores Eyepiece distortion and uses fixed sizes for the Moon, the Sun and the planets.
Does it need an account, and is anything saved?
No account, and nothing is saved. Everything runs in your browser and the Combinations you add last only while the page is open. The Sky View fetches survey imagery and object positions from public astronomy servers; the values you type stay in your browser.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: