Using advanced computer design, combinations of low dispersion and high refractive index optical glasses, and durable multilayer deposition coatings, Explore Scientific 68° Series Argon-Purged Waterproof extreme wide field eyepieces are optimized to produce high contrast, high resolution, and superior flat field characteristics. The visual effect of these eyepieces with their long eye-relief and their 68° apparent field is truly a full-immersion experience. The 68° Series eyepieces come with a removeable and foldable soft silicone rubber eyecup for the comfort of the observer, and as experienced astronomers know, eye comfort is critical when trying to observe at the visual limits of the telescope. Protection from the Outdoors - By making the eyepiece body Argon-purged, the waterproof eyepiece internal elements will remain as pristine as the day they were assembled. Argon atoms are ? larger? than that of Nitrogen and therefore they are less likely to ?leak? or diffuse their way through the seals over the life of the eyepiece. The atomic radius of Argon is larger than Nitrogen by 0.13 angstroms and has a larger atomic volume by 11.2 cubic centimeters/molecule. Argon also has superior inertness vs Nitrogen that prevents adverse reactions with eyepiece components and therefore extends the longevity of the internal components, and also prevents chemical reactions with other elemental contaminants that may be introduced through a weakened seal. The sealed, dry environment is impervious to internal fogging and contaminants such as fungus growing in between the lens elements. Our waterproof eyepiece is easier to clean, and there is no risk of cleaning solution migrating from the top of the lens and seeping around edges to be trapped in between elements. To insure a perfect seal we test every eyepiece in one meter of water for 30 minutes.