Which eyepiece to buy first

Three numbers, and that is all

• Magnification: the telescope's focal length divided by the eyepiece's. A 10 mm eyepiece on a telescope of 650 mm focal length gives 650 ÷ 10 = 65×, on 130 mm of aperture, an exit pupil of 2 mm.
• Exit pupil: the width of the beam leaving the eyepiece, which is the telescope's aperture divided by the magnification. It says how bright the image is. Shortcut: it is also the eyepiece focal length divided by the telescope's focal ratio, so a 10 mm gives a 2 mm exit pupil on any f/5 telescope.
• True field: the eyepiece's apparent field (engraved on it or given by the maker, in degrees) divided by the magnification. It is the patch of sky you see; the full Moon is half a degree across.

Magnification on its own says nothing: it is the number that sells, and the number that misleads. What decides whether an image is useful is the exit pupil it leaves. Too large, larger than your eye's pupil, and some of the light lands beside it. Too small, under 0.5 mm, and the image gets bigger without showing one more detail, only dimmer and softer.

At the other end, the dark-adapted eye's pupil opens to about 7 mm in a young adult, and less with age, often 5 to 6 mm past fifty. Beyond that, the extra light falls on the iris. That is why a 'very low power' eyepiece is not always a gain.

What each exit pupil is for

• About 5 to 7 mm: the widest field, for finding things, large clusters, the whole Pleiades, extended nebulae under a dark sky.
• About 2 to 3 mm: the all-round deep-sky exit pupil, for galaxies, globular clusters, small nebulae.
• About 1 mm: the Moon in detail, planets, double stars, when the air is steady.
• Below 1 mm, down to 0.5 mm: only on nights of very steady air, for planets and close doubles.

Three telescopes, worked out

A 130/650 tabletop Dobsonian (f/5), with the 25 mm and 10 mm from the box. The 25 mm gives 26× on 130 mm, an exit pupil of 5 mm, and with a 50° apparent field a true field of 1.9°, nearly four full Moons. The 10 mm gives 65× on 130 mm, an exit pupil of 2 mm. The gap is near 1 mm: a 6 mm gives 108× on 130 mm, an exit pupil of 1.2 mm, and a 5 mm gives 130× on 130 mm, an exit pupil of 1 mm.

A 150/750 tabletop Dobsonian (f/5), with the same 25 mm and 10 mm. The 25 mm gives 30× on 150 mm, an exit pupil of 5 mm; the 10 mm gives 75× on 150 mm, an exit pupil of 2 mm. Same focal ratio, so the same exit pupils: a 5 mm gives 150× on 150 mm, an exit pupil of 1 mm, and a 6 mm gives 125× on 150 mm, an exit pupil of 1.2 mm.

A 200/1200 Dobsonian (f/6), with a 25 mm and a 10 mm. The 25 mm gives 48× on 200 mm, an exit pupil of 4.2 mm; the 10 mm gives 120× on 200 mm, an exit pupil of 1.7 mm. A 6 mm gives 200× on 200 mm, an exit pupil of 1 mm. At the low end, a 32 mm gives 38× on 200 mm, an exit pupil of 5.3 mm, and a wider true field than the 25 mm.

Which to buy first

• First purchase: an eyepiece giving about 1 to 1.2 mm of exit pupil, so a focal length of 5 to 6 mm on an f/5, 6 to 7 mm on an f/6. It is the one that shows Jupiter's belts, the Cassini division, the craters along the terminator.
• Choose it with comfortable eye relief (how far from the glass your eye sits): short focal lengths in older designs make you press your eye to the lens, which is tiring, and impossible with glasses on.
• A wider apparent field, around 60° or more, helps on a Dobsonian without tracking: the planet crosses the field more slowly, and you nudge it back less often.
• Second purchase, later: a wide field at low power, if the 25 mm feels narrow. In a 1.25-inch focuser the true field tops out around a 32 mm at 50° or a 24 mm at 68°; beyond that you need a 2-inch focuser.

Try the eyepieces in the simulator →

The Barlow, honestly

A Barlow lens slides in ahead of the eyepiece and multiplies the telescope's focal length, most often by two. It adds neither light nor resolution: it only gives each eyepiece one more magnification. Well chosen, it doubles the range. On a 130/650, a 2× Barlow turns the 10 mm into the equivalent of a 5 mm, 130× on 130 mm, an exit pupil of 1 mm, and the 25 mm into the equivalent of a 12.5 mm, 52× on 130 mm, an exit pupil of 2.5 mm.

Badly chosen, it only serves to overshoot the maximum useful power. On a 200/1200, the same 2× Barlow turns the 10 mm into the equivalent of a 5 mm, 240× on 200 mm, an exit pupil of 0.8 mm: useful only on very steady nights. And the 3× Barlow in cheap kits, with its 4 mm, mostly serves to print a big number on the box.

What not to buy

Every model in this guide

Cheapest first. Each view is the Hercules Cluster through the model, at the same exit pupil, under the same suburban sky.

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