Explore Scientific 68 Degree 40mm Argon-Purged Waterproof Telescope Eyepiece Review (2026)

What this eyepiece is built for

The 68 degree 40mm eyepiece is a wide-field, low-power design meant for sweeping views of large, extended deep-sky objects such as big nebulae, open star clusters, and rich Milky Way star fields where a wide true field of view matters more than magnification. Explore Scientific uses advanced computer optical design with combinations of low dispersion and high refractive index glass, aimed at high contrast, high resolution, and a flat field across the 68 degree apparent field, which reduces the curved, distorted look that cheaper wide-angle eyepieces can show near the edge. Eye relief is listed at 31 millimeters, generous enough to remain comfortable for most eyeglass wearers at this focal length. Explore Scientific recommends it works best with telescopes at f/6 focal ratio or longer, where edge sharpness holds up better than it would in a very fast, short focal ratio scope.

Argon-purged, waterproof construction

This eyepiece is O-ring sealed and purged with argon gas rather than nitrogen, which Explore Scientific notes is a meaningful durability choice since argon atoms are larger than nitrogen atoms and therefore less likely to slowly leak out through seals over the eyepiece's lifetime. The result is a genuinely waterproof design that resists internal fogging from moisture and temperature swings, a real advantage for anyone observing in humid climates or through dewy nights where internal fogging can ruin a session with a non-sealed eyepiece. Fifteen layers of enhanced multi-layer deposition coatings are applied to every glass-to-air surface, which is a thorough coating regimen aimed at maximizing light transmission and minimizing internal reflections and ghosting, especially important on a wide-field eyepiece with many lens elements.

Size, weight, and focuser compatibility

A 40mm eyepiece with a 68 degree apparent field is a physically large eyepiece, and this model uses a 2 inch barrel rather than the smaller 1.25 inch standard, which means telescopes without a 2 inch focuser cannot use it without an adapter, and even with one the extra weight still needs to be accounted for. The eyepiece includes a silicon rubber eyecup for comfortable viewing and threads for 2 inch filters, letting buyers add light pollution or narrowband filters for specific targets. Because of its size and glass content, this eyepiece carries meaningful weight compared with smaller focal lengths in the same product line, and mounting it on a lighter telescope, particularly one with a smaller or less sturdy focuser, can shift balance enough to need a counterweight adjustment on an equatorial mount.

Parfocal design and using it with other eyepieces

Explore Scientific designs its 68 degree series to be parfocal, meaning eyepieces across the line share a common focus point, so swapping from this 40mm to a shorter focal length eyepiece in the same series should require little to no refocusing. That is a practical convenience during an observing session, since finding a target at low power with this 40mm eyepiece and then switching to a shorter focal length for a closer look does not mean hunting for focus all over again each time. This benefit is strongest when paired with other eyepieces from the same 68 degree series rather than mixed with unrelated brands or designs, where parfocal behavior is not guaranteed.

Who should buy it

This eyepiece is a strong match for telescope owners with a 2 inch focuser who want the widest, most immersive low-power view their scope can deliver, especially for large nebulae, open clusters, and wide star field sweeps where a big true field of view is the priority. The argon-purged waterproof build is a real advantage for anyone observing regularly in humid or dewy conditions, where a non-sealed eyepiece risks internal fogging. Buyers with only a 1.25 inch focuser, or with a lighter telescope that cannot comfortably balance a large 2 inch eyepiece, should look elsewhere, either at a smaller focal length in this same series or at a 1.25 inch wide-field alternative. Those focused mainly on high-power planetary viewing will also get more use from a shorter focal length eyepiece than from this low-power 40mm.

Frequently asked questions