The Orthoscopic design was developed by Ernst Abbe in 1880. Since the Orthoscopic eyepiece design is so old, why is it still so popular? Because they can deliver high contrast, distortion free and high resolution images. These characteristics make them the eyepiece of choice for many planetary observers.
Orthoscopics are a 4 element design with a cemented triplet field lens and a single plano/convex eye lens. The field lens is in a convex/convex, concave/concave, convex/convex array. It is the lower amount of glass elements that make these such great eyepieces for planets. Less glass means less distortion, higher through put, and higher contrast. Sometimes simple is best.
This isn't meant to dismiss more complex designs, such as Naglers and other wide field eyepieces. I own and have owned and use many types of eyepieces for different types of observing. But for high-power, planetary and lunar work I still love the Orthos.
In fact the Carl Zeiss Abbe Orthoscopic Eyepieces, which are no longer in production can still fetch $600.00 and more on the used market! They are a true classic and one of, if not the best, example of Ortho oculars ever produced.
But if you are on a budget don't despair. Baader Planetarium has a line they call "Baader Planetarium Genuine Orthoscopic Telescope Eyepieces" and they offer terrific performance especially for their low cost (around $100). Although, like all orthos, they have a narrow apparent field of view, when doing high powered work on planets and the moon I don't care about great vistas. I want clarity, high contrast and no distortion and these do deliver.
