Explore Scientific 120 Degree Series 9mm Argon-Purged Waterproof Eyepiece, Enhanced Multi-layer Deposition Coatings, 2" Barrel The Explore Scientific 120 9 mm Eyepiece is Argon Purged for an increased life span With 13 mm of eye relief the Explore Scientific Eyepiece is comfortable to view through At a Focal Length of 9 mm the Explore Scientific Eyepiece is a great high to mid power eyepiece The Explore Scientific 120 Degree Eyepiece has 12 elements in 8 groups