The Prostar Orthoscopic eyepiece, also known as the Abbe Orthoscopic, or Ortho for short, is one of the oldest eyepiece designs, but still today is often regarded as one of the better planetary eyepieces along with Plossls.
Similar in design to the Plossl, the Orthoscopic is four elements in two groups. The difference in the Orthoscopic design however the produces a smaller Apparent Field of View (AFOV), typically a more modest around 45°.
Orthoscopic eyepieces are better suited to slower f/ratio Newtonians, refractors and to Cassegrain telescopes.
Prostar 1.25” Orthoscopic eyepieces are all fully multi-coated with blackened lens edges to reduce ghosting/internal reflections and improve contrast.
Note that achromatic refractors show achromatic aberration, and this will be more significant as the eyepiece focal length becomes shorter. To help overcome this blue flaring, a Yellow #12 filter or Baader Fringe Killer filter can be used to reduce this effect.