Sky-Watcher Equinox-100 PRO 100mm (4") f/900 Fluorite ED Apochromatic Refractor Optical Tube Assembly. The exquisite State-Of-The-Art Optics as used in the Evostar-ED PRO models are now housed in tube assemblies of even higher quality. The Tubes are Precision CNC machined & Hard Anodized. These are exceptionally high Performance telescopes for Planetary & wide field Deep-sky observations. Built with two-element multi-coated air-spaced objectives, the second element is made from the highest-grade extra-low dispersion (ED) Japanese Fluorite glass. Extra-low Dispersion glass allows the optical designer to virtually eliminate chromatic aberration (false colour fringing around bright objects), which results in higher resolution & superior contrast in the images. Each objective lens is finished by hand by skilled opticians & the quality of each telescope is monitored by means of a Ronchi test & an Artificial Star Test. No effort is spared to reach optimal optical quality.