Explore Scientific 52° Series 25mm Waterproof Eyepiece

Product Overview

The Explore Scientific 52° Series 25mm is the long-focal-length, low-power end of the 52° range: an argon-purged, O-ring sealed eyepiece with a 1.25-inch double-taper barrel, built around four elements in two groups with EMD (Enhanced Multi-Layer Deposition) coatings on every lens-to-air surface and blackened element edges.

The published figures are unusually complete for an eyepiece at this price. Eye relief is 16.4mm, the field stop measures 18.2mm, the body is 67.7mm long and 43mm wide, and it weighs 181.4 grams (6.4 oz). The 18.2mm field stop is the number that matters most in practice: it fixes the true field at 57.3 multiplied by 18.2 and divided by your telescope's focal length. In a 1000mm telescope that is 1.04°; in a 663mm refractor, 1.57°.

At 25mm this is a finder and framing eyepiece — the one that locates the object, shows the whole of a large cluster, and hands over to something shorter when detail is the goal.

Who It's For

• Deep-sky observers working at low power: open clusters, large nebulae and the Milky Way star fields all want a wide true field and a bright image more than they want magnification.
• Anyone who starts a search at low power: 25mm gives the widest sweep in the series, which is what makes star-hopping to a faint target practical.
• Eyeglass wearers: 16.4mm of eye relief at 25mm is generous, and the flip-down silicone eyecup folds back to let spectacle lenses sit close enough to take in the field.
• Observers in damp climates: the O-ring sealed, argon-purged body is tested under one metre of water for thirty minutes, so dew and drizzle are not events.
• Observers filling out a focal-length set who want the same coatings, barrel and eye position across the range.

Key Features & Design

• 4 elements in 2 groups, edge-blackened: a simple, high-transmission layout with few internal surfaces to scatter light, and blackened edges to keep stray light out of the field.
• EMD coatings on all lens-to-air surfaces: every air-glass boundary is coated rather than just the outer ones, which is where transmission and contrast on faint extended objects are won or lost.
• 16.4mm eye relief: comfortable for long sessions and workable with glasses on.
• 18.2mm field stop: the physical aperture that defines the true field — a known figure rather than something to be inferred.
• 52° apparent field: a corrected, flat field rather than a panoramic one, which is the design priority of this series.
• 1.25″ O.D. double-taper barrel: the taper gives a thumbscrew something to bite against, so the eyepiece resists being pulled out of a diagonal.
• O-ring sealed and argon gas purged: tested under 1 metre of water for 30 minutes. Argon inside the body does not condense the way trapped humid air does.
• Standard 1.25-inch filter thread (1.125-inches x 42 TPI) at the base of the barrel.
• Removable flip-down soft silicone eyecup and slide-on caps top and bottom.
• 67.7mm long, 43mm wide, 181.4 grams (6.4 oz): light enough that it does not upset the balance of a small refractor or a Dobsonian.
• Laser-engraved markings: focal length, series, serial number and brand.

Recommended Uses

• Low-power deep-sky observing — open clusters, large emission nebulae, the brighter galaxies.
• Locating objects before switching to a higher-power eyepiece.
• Wide-field sweeping of the Milky Way and rich star fields.
• Damp-site and coastal observing where a sealed eyepiece keeps working.

Compatibility and Accessory Notes

• Fits any 1.25-inch focuser or diagonal directly. In a 2-inch focuser it takes a 2-inch to 1.25-inch adapter, which most 2-inch diagonals already include.
• Magnification: telescope focal length ÷ 25. A 663mm refractor gives 27x, a 1219mm Newtonian 49x, a 2000mm Schmidt-Cassegrain 80x.
• True field: 57.3 × 18.2 ÷ telescope focal length. That is 1.57° at 663mm, 0.86° at 1219mm and 0.52° at 2000mm.
• Exit pupil: 25 ÷ the telescope's focal ratio. An f/5.9 Dobsonian gives 4.2mm, an f/6.5 refractor 3.8mm, an f/10 Schmidt-Cassegrain 2.5mm. At f/4 the exit pupil reaches 6.25mm, which is at the edge of what a dark-adapted eye takes in.
• Filters: the 1.125-inch x 42 TPI thread accepts any standard 1.25-inch filter — broadband, narrowband, lunar or colour. Narrowband nebula filters pair particularly well with a 25mm on a rich field.
• Focal extenders: a 2x extender turns this into an effective 12.5mm while keeping the eye relief of the 25mm, which is a genuinely useful pairing at the long end of a series.
• Within the series: the barrel, filter thread and eyecup are shared across the 52° Series, so a matched set behaves consistently as focal lengths change.

Good to Know Before You Order

• The 52° field is a corrected field, not a wide one. The design priority here is edge sharpness and value rather than immersion. The 68°, 82° and 92° ranges cover the panoramic end, and we stock those if that is what you are after.
• Four elements in two groups is a simple layout. That is a strength for transmission and weight, and it is also why the apparent field stays at 52° rather than pushing wider — wide fields need more glass.
• In a fast telescope the exit pupil gets large. Below about f/5 the 25mm produces an exit pupil wider than many adult eyes dilate to, so part of the light cone falls on the iris rather than entering the eye. A shorter focal length recovers it, and we can suggest one that matches your focal ratio.
• Waterproof means sealed against immersion, not built for use underwater. The 1 metre, 30 minute rating exists so that dew, rain and an eyepiece dropped in wet grass are non-events.
• The double-taper barrel is a compression-friendly shape. It works with both thumbscrew and compression-ring holders, so no adapter change is needed for either.

Frequently Asked Questions

Is it difficult to set up?

No — it drops straight into any 1.25-inch focuser or diagonal and is ready to use. There is nothing to assemble, collimate or adjust, and the caps and eyecup come off by hand.

What magnification will I get?

Divide your telescope's focal length in millimetres by 25. A 1000mm telescope gives 40x; a 1219mm Dobsonian gives 49x.

How wide a piece of sky will I actually see?

The 18.2mm field stop settles that exactly: 57.3 × 18.2 ÷ your telescope's focal length. In a 1000mm telescope that is 1.04°, or about twice the width of the full Moon.

Can I use it with glasses on?

Yes. 16.4mm of eye relief is enough for most spectacle wearers, and the flip-down silicone eyecup folds out of the way so the lens can sit close to the eye lens.

Can I use filters with it?

Yes. Any standard 1.25-inch threaded filter screws into the base of the barrel — the thread is 1.125 inches x 42 TPI, which is the common standard.

How does it compare to a Plössl of the same focal length?

Both are four-element designs, so transmission is in the same territory. The differences are the sealed argon-purged housing, EMD coatings on every lens-to-air surface, the double-taper barrel and the published 16.4mm eye relief, which is more than a 25mm Plössl typically offers. In practice the sealing and the eye comfort are what you notice night to night.

Bottom Line

In short: a 25mm argon-purged eyepiece with a 52° corrected field, 16.4mm of eye relief, an 18.2mm field stop and a standard 1.25-inch filter thread, at 181.4 grams — the low-power, wide-true-field end of a series built around edge sharpness and durability. If you would like help working out which focal lengths would round out your set for the telescope you own, send us its aperture and focal length and we will map it out with you.