Explore Scientific 82° Series 18mm Waterproof Eyepiece

Product Overview

The Explore Scientific 82° Series 18mm is an argon-purged waterproof eyepiece with a 2-inch barrel, a published field stop of 25.3mm and 13mm of eye relief. The body measures 89mm long by 56mm wide and weighs 13.7 oz (390 g).

18mm with an 82-degree apparent field is a medium-power eyepiece that still delivers a genuinely wide true field. The 25.3mm field stop is the figure that governs it: divide 25.3 by your telescope's focal length and multiply by 57.3 for the true field in degrees. In a 714mm refractor that is 2.03° at 40x; in a 1200mm Schmidt-Cassegrain, 1.21° at 67x.

Explore Scientific builds the series around low-dispersion glass and enhanced multilayer coatings, in a precision-machined metal housing with flat-black internal surfaces to keep scattered light out of the field.

Who It's For

• Deep-sky observers: two degrees of true field in a medium-focal-length telescope frames the Veil, the Rosette and the larger open clusters whole while still magnifying enough to darken the background sky.
• Dobsonian owners: 82 degrees of apparent field is a long drift time at 40x to 70x on an undriven mount, which is where the extra field pays for itself.
• Observers who value a flat field: the low-dispersion glass and coating package are aimed at holding contrast and star shape out toward the field stop.
• Damp-climate observers: the argon-purged, sealed housing is built for dew-heavy nights.
• Observers who use focal extenders: Explore Scientific specifically notes compatibility with extenders, so an 18mm can double as a 9mm equivalent behind a 2x.

Key Features & Design

• 82° apparent field of view: a wide, immersive field where the edge of view sits outside the eye's central attention rather than framing it.
• 25.3mm field stop: the published figure that fixes true field exactly, rather than approximating it from apparent field and magnification.
• Low-dispersion glass with enhanced multilayer coatings, the combination Explore Scientific specifies for contrast and flatness across the series.
• Argon-purged and sealed: an inert internal atmosphere that carries no moisture to condense on internal surfaces as the eyepiece cools.
• 13mm eye relief published for this focal length.
• 2-inch barrel taking standard 2-inch threaded filters.
• Precision-machined metal construction with flat-black internal surfaces to suppress scatter.
• Published dimensions: 89mm long, 56mm wide, 13.7 oz (390 g).

Recommended Uses

• Wide-field deep-sky observing of nebulae, large open clusters and star fields.
• Medium-power views of globular clusters, where 40x to 70x starts to resolve the outer stars.
• Lunar observing at moderate magnification with the whole disc comfortably framed in most telescopes.
• Sweeping the Milky Way on an undriven mount, where the 82° field keeps targets in view.
• Use behind a focal extender as a medium-high-power eyepiece with the same wide apparent field.

Compatibility and Accessory Notes

• Requires a 2-inch focuser or diagonal. The 2-inch barrel does not step down to 1.25 inches, so a telescope with only a 1.25-inch focuser is outside its range — the 82° Series does include 1.25-inch focal lengths, and we can point you to those.
• Magnification: telescope focal length ÷ 18. A 500mm Newtonian gives 28x, a 714mm refractor 40x, a 1200mm SCT 67x, a 1500mm Maksutov 83x.
• True field: 25.3 ÷ focal length × 57.3. That is 2.90° at 500mm, 2.03° at 714mm, 1.21° at 1200mm.
• Filters: standard 2-inch threaded filters screw into the barrel. Broadband and narrowband nebula filters are the natural pairing at this focal length.
• Focal extenders: behind a 2x extender the 18mm behaves as a 9mm while keeping the 82° apparent field, which is a compact way to cover two powers with one eyepiece.
• Balance: at 390 g it is a moderate load for a Dobsonian or a small refractor. Counterweights and magnetic balance weights handle any tip, and we stock them.

Good to Know Before You Order

• It is a 2-inch eyepiece. A 2-inch focuser or 2-inch diagonal is what it fits. If your telescope has a 1.25-inch focuser, the shorter focal lengths in the 82° Series come in 1.25-inch barrels and cover similar ground — tell us your telescope and we will point you at the right one.
• 13mm of eye relief is moderate. Comfortable for most observers with the eyecup adjusted; for observing with glasses on, the 72° ED Series is specified for longer eye relief and we stock it.
• An 82° field asks the eye to move. Taking in the whole field means glancing around it rather than seeing all of it at once, which is normal for wide-angle designs and is part of what makes them absorbing.
• At 390 g it can shift the balance of a lightly loaded Dobsonian or a short refractor. A small counterweight settles it, and it is not a heavy eyepiece by 2-inch standards.
• Explore Scientific does not publish an element count or a box-contents list for this eyepiece, so those rows are absent rather than estimated. We can ask our rep if either matters to you.

Frequently Asked Questions

Is it difficult to set up?

No — it drops into any 2-inch focuser or diagonal and is ready to use immediately. There is nothing to assemble or adjust.

What true field will I get?

Divide 25.3 by your telescope's focal length in millimetres and multiply by 57.3. A 1000mm telescope gives 1.45°; a 714mm refractor gives 2.03°.

Will it fit a 1.25-inch focuser?

No — the barrel is 2 inches. A telescope with a 1.25-inch focuser is better served by one of the 1.25-inch focal lengths in the same series.

Can I use filters with it?

Yes. Standard 2-inch threaded filters thread into the base of the barrel, which suits nebula filters particularly well at this field size.

How does it compare with the 100° Series?

The 100° designs give a wider apparent field in a larger, heavier body. The 82° 18mm at 390 g is the more manageable of the two on a light mount, and its 25.3mm field stop still delivers a genuinely wide true field.

Does the argon purging make a practical difference?

On cold, damp nights, yes. Sealed ordinary air carries moisture that condenses on internal surfaces as the eyepiece cools; argon does not.

Bottom Line

In short: an 18mm 82° eyepiece with a 25.3mm field stop, 13mm of eye relief and a sealed argon-purged 2-inch body at 390 g — a medium-power eyepiece that still frames two degrees of sky in a short-focus telescope. If you would like to know exactly what true field it will give in the telescope you own, send us the focal length and we will work it out for you.