Explore Scientific 52° Series 40mm Waterproof Eyepiece (2-Inch)

Product Overview

The Explore Scientific 52° Series 40mm is the long focal length end of the 52° range: a 2.0″ O.D. double-taper barrel eyepiece with 27.1mm of eye relief, a 36.3mm field stop and four elements in two groups, edge blackened, behind EMD (Enhanced Multi-Layer Deposition) coatings on all lens-to-air surfaces. Published dimensions are 96,6mm long and 61.8mm wide, at 385.5 grams; 13.6 oz.

At 40mm this is a low-power, wide-true-field eyepiece. The 36.3mm field stop is the figure that matters: divided by the telescope's focal length and multiplied by 57.3, it gives the true field in degrees. In a 988mm refractor that is 2.11°; in a 1216mm apochromat, 1.71°; in a 1200mm SCT, 1.73°. Magnification follows from focal length divided by 40 — 25x, 30x and 30x respectively.

The housing is O-ring sealed and argon gas purged, tested under 1 meter of water for 30 minutes. Argon does not carry moisture the way trapped air does, so the internal surfaces stay clear of fogging as temperature drops through a night.

Who It's For

• Wide-field deep-sky observers: 2.11° of true field in a 988mm refractor frames the Pleiades, the Double Cluster and the Veil in a way a shorter eyepiece cannot.
• Eyeglass wearers: 27.1mm of eye relief is enough to take in the whole 52° field with spectacles on, which is rare.
• Observers who use a finder eyepiece: at 25x to 30x in most telescopes, this is the eyepiece for locating a target before switching to power.
• Nebula filter users: the 2-inch thread takes an O-III or UHC filter, and low power with a large exit pupil is where those filters do their best work.
• Anyone observing in dew: the sealed, argon-purged body is built for damp nights.

Key Features & Design

• 4 elements in 2 groups, edge blackened: a simple, low-element-count design, which keeps the number of air-to-glass surfaces down and transmission up — useful at low power where every photon is going into a large exit pupil.
• 27.1mm eye relief: unusually generous, and the single most comfortable figure in the 52° Series.
• 36.3mm field stop: a large true field for a 2-inch eyepiece, and the number to use for calculating field coverage.
• 2.0″ O.D. double-taper barrel: the taper resists the eyepiece sliding out of a diagonal if a thumbscrew loosens, which matters more with a 385.5 gram eyepiece than a light one.
• EMD coatings on all lens-to-air surfaces: raises transmission and holds contrast against scattered light.
• O-Ring sealed, argon gas purged, tested under 1 meter of water for 30 minutes.
• Standard 2.0-inch filter thread (48mm x 0.75mm): takes 2-inch broadband, narrowband and lunar filters directly.
• Removable flip-down soft silicone rubber eyecup and slide-on caps top and bottom.
• Optical glass, aluminum, steel and silicone rubber construction, laser engraved with focal length, series, serial number and brand.

Recommended Uses

• Low-power sweeping through the Milky Way and large open clusters.
• Extended nebulae with a 2-inch narrowband filter fitted.
• Finding and framing targets before moving to a higher-power eyepiece.
• Comet observing, where a large true field keeps head and tail together.
• Full-disc lunar viewing at low magnification.

Compatibility and Accessory Notes

• Focuser: the 2.0″ O.D. barrel fits any 2-inch focuser or diagonal directly. It does not fit a 1.25-inch focuser, and there is no adapter that makes a 2-inch eyepiece fit one.
• Filters: standard 2.0-inch thread, 48mm x 0.75mm, which is the common astronomy filter standard.
• Magnification: telescope focal length ÷ 40. A 750mm Newtonian gives 19x; a 1216mm apochromat, 30x; a 2000mm SCT, 50x.
• True field: 36.3 ÷ focal length × 57.3. A 750mm Newtonian yields 2.77°; a 2000mm SCT, 1.04°.
• Exit pupil: 40 ÷ focal ratio. An f/10 SCT gives 4mm, an f/8 apochromat 5mm, an f/6.5 refractor 6.15mm and an f/5 Newtonian 8mm.
• Weight: 385.5 grams; 13.6 oz. On a small refractor or a Dobsonian this is enough to shift balance, so a counterweight or a retensioned altitude bearing is sometimes worth having.

Good to Know Before You Order

• Exit pupil sets a practical limit at fast focal ratios. At f/5 the 40mm produces an 8mm exit pupil, which is wider than most dark-adapted eyes accept, so part of the light is lost at the iris and a central obstruction can show as a shadow. Between f/6 and f/10 the eyepiece is in its element. If your telescope is faster than f/6, a shorter focal length in the series holds the same field of view with a more usable exit pupil, and we can suggest one.
• The 52° field is a corrected field rather than a wide one. Immersion is what the 68°, 82° and 100° Series are built for; the 52° Series concentrates on edge sharpness, eye relief and value.
• It needs a 2-inch focuser or diagonal. At 2.0″ O.D. this eyepiece has no 1.25-inch path. Telescopes with 1.25-inch focusers are better served by the 1.25-inch focal lengths in the series, which we stock.
• 385.5 grams sits at the heavy end for an eyepiece. In a small refractor's diagonal or a Dobsonian's focuser it changes the balance point, which is usually solved with a magnetic counterweight or a friction adjustment.
• Waterproof means sealed against immersion, not built for use underwater. The 1 meter / 30 minute rating exists so that dew, rain and a drop into wet grass are non-events.

Frequently Asked Questions

Is it difficult to set up?

No — it drops into any 2-inch focuser or diagonal and is ready to use. There is nothing to assemble, collimate or adjust, and the eyecup flips down for spectacle wearers.

What magnification will I get?

Divide your telescope's focal length in millimetres by 40. A 1000mm telescope gives 25x.

How much sky will it show?

Divide the 36.3mm field stop by your telescope's focal length and multiply by 57.3. A 1000mm telescope gives 2.08° of true field — about four times the width of the full Moon.

Can I use it with glasses on?

Yes. 27.1mm of eye relief is more than most spectacle wearers need to take in the full 52° field, and the silicone eyecup folds down out of the way.

Will it fit my 1.25-inch focuser?

No — the barrel is 2.0″ O.D. For a 1.25-inch focuser, the shorter 1.25-inch focal lengths in the 52° Series are the ones to look at, and we can point you to the closest equivalent.

Does it take filters?

Yes, standard 2.0-inch threaded filters (48mm x 0.75mm) screw into the base of the barrel. Low power and a large exit pupil is exactly where an O-III or UHC filter shows its value on nebulae.

Bottom Line

In short: a 40mm, 2-inch, argon-purged waterproof eyepiece with 27.1mm of eye relief, a 36.3mm field stop and a flat 52° field — the low-power, wide-true-field, spectacle-friendly end of the series, with a 2-inch filter thread for nebula work. If you would like us to work out the magnification, true field and exit pupil it will give in your particular telescope, send us the aperture and focal length and we will run the numbers.