Eyepiece field of view simulator
How much of the Moon fits in your eyepiece? How small is Jupiter at 48×? Pick a telescope and eyepiece, or binoculars, and an object: the circle below is your view, drawn to scale.
The Sun is left out on purpose: never point a telescope or binoculars at it without a proper solar filter covering the front.
How to read the circle
The ring is the edge of what you see in the eyepiece, the true field. For a telescope it is the eyepiece's apparent field divided by the magnification, or, if you enter the eyepiece's field-stop diameter, the field stop divided by the telescope's focal length × 57.3, which is exact. For binoculars, enter the field printed on them: in degrees, in feet at 1,000 yards or in metres at 1,000 metres.
Objects are drawn at their real angular size. The Moon is about half a degree across, so a low-power eyepiece shows all of it with room to spare; the planets are tiny, and only magnification makes them look big. The "looks as big as" readout multiplies the object's size by the magnification: Jupiter at 48× appears a little wider than the full Moon does to your eye. For the deep-sky objects the dashed outline is their size in the SEDS Messier catalogue; what you actually see is often smaller, because their faint outer parts fade into the sky.
The view in a telescope may be upside down or mirrored, and it rotates as the night goes on, so orientation here is only illustrative. Saturn's rings are drawn tilted 20°; the real tilt changes from edge-on to about 27° over Saturn's 29-year orbit.
Questions
Which eyepiece shows the whole Moon?
Any eyepiece whose true field is wider than the Moon, which is 29.4′ to 33.5′ across. A true field of about 0.6° or more frames the whole disc with a little dark sky around it; on a 1200 mm telescope that means a 52° eyepiece of 14 mm or longer.
Why do the planets look so small?
Jupiter is at most 50.1″ across, less than a thirtieth of the Moon's width, and Mars only 3.5″ to 25.6″ depending on its distance. At 100×, Jupiter at an average opposition (46.9″) looks like a disc about 78′ across, roughly two and a half times the width of the full Moon to the naked eye.
Can binoculars show the whole Andromeda Galaxy?
Its catalogue size is 178′ × 63′, almost 3° long, which fits inside most binocular fields: Nikon lists 5.4° to 7.8° for its HG binoculars, for example. Most telescope eyepieces show only its bright core and part of the disc at once.
Sources
- NASA Space Science Data Coordinated Archive fact sheets: Moon (apparent diameter 1896″ at 378,000 km, radius 1738.1 km, distance about 357,000–407,000 km, from which 29.4′–33.5′ is computed), Mars (3.5″–25.6″, 17.8″ at a mean opposition), Jupiter (30.5″–50.1″, 46.9″; equatorial and polar radii), Saturn (14.5″–19.9″, 18.8″; radii, 29.4-year orbit) and the Saturnian rings (A ring edge at 2.27 Saturn radii).
- SEDS Messier Database (H. Frommert): M42 85′ × 60′, M45 110′ and its centre, M31 178′ × 63′.
- SIMBAD astronomical database (CDS, Strasbourg): positions and V magnitudes of the eleven brightest Pleiades stars.
- Nikon Sport Optics catalogue (Nikon Vision, 2011): real fields of its HG L binoculars, 5.4° to 7.8°.
- Tele Vue Optics, Choosing Eyepieces (Basic) (field-stop true field) and Al Nagler, Choosing Your Telescope's Magnification (apparent field ÷ magnification).
- Binocular field conversions: plain trigonometry, a field w wide spans 2 × d × tan(w ÷ 2) at distance d (3,000 ft at 1,000 yd).