The Best Telescope Eyepieces

Magnification is the telescope's focal length divided by the eyepiece's. In a 1200mm Dobsonian this gives 1200 / 32 = 37.5x, and in a 650mm f/5 Newtonian it gives 650 / 32 = 20x. Both are lower than the eyepieces most telescopes ship with, which is the entire point.

Exit pupil is the aperture divided by the magnification. On a 203mm f/5.9 telescope at 37.5x that is 5.4mm, which a dark-adapted eye takes comfortably. On a faster f/4.7 telescope the same eyepiece gives 6.8mm, close enough to the eye's roughly 7mm limit that a little light spills past the pupil.

The 27.5mm field stop is the specification that matters most and the one nobody reads. True field is set by the field stop, not by the apparent field, and 27.5mm is the widest a 1.25-inch barrel physically allows. No other 1.25-inch eyepiece will show you more sky than this one.

48 degrees of apparent field is modest by modern standards. The view is a well-corrected circle rather than an immersive window, and on a fast telescope the outer part of that circle will not be as sharp as the middle.

In a 1200mm Dobsonian this gives 1200 / 9 = 133x, and in a 650mm f/5 Newtonian 650 / 9 = 72x. On a 2032mm Schmidt-Cassegrain it gives 226x, which is at the edge of what most nights will support.

Check the result against two times your aperture before buying. On a 130mm telescope the ceiling is 260x and 133x sits comfortably below it; on a 76mm tabletop the ceiling is 152x and this eyepiece is already most of the way there.

Eye relief is the distance your eye sits from the glass, and 16mm is the whole argument for this eyepiece. A budget 9mm Plossl offers around 6mm, which means pressing your eye to the lens and seeing your own eyelashes. Spectacle wearers need roughly 15mm to see the full field at all.

60 degrees of apparent field is noticeably more open than a Plossl's 48 to 52, which matters at high power on a manual telescope: a wider field is a longer wait before the planet drifts out of it.

In a 1200mm Dobsonian the four give 60x, 80x, 133x and 200x. In a 650mm f/5 Newtonian they give 33x, 43x, 72x and 108x. That is a usable spread in both telescopes rather than four steps bunched at one end.

The 6mm is the one to check against your aperture. Two times the aperture is the ceiling, so 200x in a 203mm telescope is fine, while the same eyepiece in a 114mm telescope asks for 200x where 228x is the absolute limit and a typical night supports far less.

17mm of eye relief on every focal length is the genuinely unusual specification here. Eye relief normally shrinks with focal length, which is why cheap short eyepieces are uncomfortable; holding it constant across the set is what a more complex optical design buys.

68 degrees is a wide apparent field, and on a fast telescope wide fields are where budget eyepieces show their limits. Expect the outer part of the field to soften on an f/5 or faster telescope; the middle, which is where you put the planet, holds up.

A Barlow multiplies the magnification of whatever eyepiece sits in it. At 2x, a 25mm eyepiece behaves like a 12.5mm one and a 10mm behaves like a 5mm, so a three-eyepiece case becomes a six-magnification case for the price of one accessory.

That arithmetic is real, and it has a hard edge. On a 700mm f/10 refractor with a 70mm objective, a 10mm eyepiece Barlowed to an effective 5mm gives 140x, which is exactly the 2x-aperture ceiling for 70mm. Anything shorter than that behind the Barlow is empty magnification, and the Barlow will happily produce it.

Three elements in two groups is a better construction than the two-element Barlows bundled in budget kits. The optical cost of a cheap Barlow is real: it is extra glass in the light path, and poor glass shows as softness and false color exactly where you were trying to gain detail.

The filter thread is the quietly useful feature. A moon filter on the Barlow stays put while you swap eyepieces above it, rather than being unscrewed and rescrewed in the dark.

Magnification is telescope focal length divided by eyepiece focal length, so this kit is the variable in that equation. On a 650mm f/5 telescope the 32mm gives 20x, the 13mm gives 50x and the 6mm gives 108x. On a 1200mm Dobsonian the same three give 38x, 92x and 200x.

The 2x Barlow halves every eyepiece's effective focal length, which is where the ten-eyepieces-in-one claim comes from. It is true arithmetic, but a Barlow on a 6mm eyepiece produces 3mm, and on most beginner telescopes 3mm is beyond the useful magnification the aperture supports.

The useful lesson is that the two eyepieces in the telescope's box are usually the two least considered items in it. A 32mm Plossl in a fast Dobsonian gives a genuinely wide, bright field that no bundled eyepiece will match.

A moon filter is not optional at full Moon in anything over about 100mm. The Moon at 130mm is uncomfortably bright, and that is not a figure of speech.