Explore Scientific 72° ED Series 40mm Waterproof Eyepiece (2")

The widest view in the series, with 31mm of eye relief

The Explore Scientific 72° ED Series 40mm Waterproof Eyepiece is the lowest-power, widest-field eyepiece in the series. It has a 72° apparent field, a 2" barrel, 31mm of eye relief and a 48.3mm field stop, and its optics include H-FK61 ED glass to keep color fringing low and contrast high. Explore Scientific builds the series to be waterproof, and this eyepiece weighs 870g (30.7oz).

Key specs: 40mm focal length · 72° apparent field · 2" barrel · 31mm eye relief · 48.3mm field stop · 6 elements in 4 groups · H-FK61 ED glass · waterproof · 870g (30.7oz)

What you'll see

• The Andromeda Galaxy with M32 and M110 in an 8-inch Dobsonian.
• The whole Pleiades and the Hyades in a short refractor.
• Huge nebulae: the full Veil Nebula complex and the North America Nebula in a short refractor with a UHC or OIII filter, plus sweeping Milky Way fields.

In an 8" Dobsonian (1200mm) the 40mm gives 30x and a 2.31° true field. In an 80mm f/6 refractor (480mm) it gives 12x and a 5.77° field. For comparison, the full Moon is about 0.5° across.

Why the 72° ED series

• Wide without the bulk: 72° is wide enough to frame objects with plenty of sky around them, while keeping the eyepiece smaller and lighter than 82° and 100° designs.
• ED glass: an H-FK61 fluorophosphate element (Abbe number 81.6) reduces color fringing on bright stars, the Moon and planets. Coatings are multi-layer.
• 31mm eye relief: comfortable for most eyeglass wearers.
• Waterproof: dew and damp nights are less of a worry, and the eyepiece is easier to keep clean.

What to add for the best results

• A 2" diagonal or focuser (required): refractors and Schmidt-Cassegrains need a 2" diagonal such as the Explore Scientific 2" Diagonal or Celestron 2" Dielectric Star Diagonal. Most Dobsonians with a 2" focuser take it directly.
• A nebula filter: a 2" UHC or OIII filter threads onto the barrel and makes the Veil and North America Nebulae stand out from suburban skies.
• A Barlow: the Tele Vue 2x Big Barlow doubles the magnification and turns this eyepiece into an effective 20mm.
• More of the series: pair it with the 30mm for a matched set with the same 72° field and feel.

Which 72° ED eyepiece is right for you?

See all our eyepieces.

What's in the box

• Explore Scientific 72° ED 40mm eyepiece with dust caps

Diagonals, Barlows and filters are sold separately.

Explore Scientific 72° ED 40mm FAQ

Will it fit my telescope?

It has a 2" barrel, so it needs a 2" focuser or diagonal. It won't fit a 1.25"-only focuser.

What magnification will I get?

Divide your telescope's focal length by 40. In an 8" f/6 Dobsonian (1200mm) that's 30x; in an 80mm f/6 refractor (480mm) it's 12x; in a Celestron 8" SCT (2032mm) it's 51x.

Can I use it with glasses?

Yes. 31mm of eye relief is enough for most people who observe with glasses.

40mm or 30mm?

The 40mm gives the widest true field in the series (2.31° in an 8" Dob). The 30mm is a little more magnified (40x) and lighter. In very fast telescopes (f/5 or faster) the 40mm's exit pupil exceeds 7mm, so the 30mm may be the better choice.

Is it really waterproof?

Explore Scientific sells the 72° ED series as waterproof eyepieces, which helps on dewy nights and when cleaning. Dry it before putting the caps back on.

Specifications