Ideal for low magnification observations or afocal photography. These 2" eyepieces from GSO provide wide field of view despide the long focal length. The 5 elements privde well corrected field of view. All air-glass surfaces are fully multi coated. The longest focal length model with 50mm can be used in F/8 or slower telescopes.
The ERFLE eyepiece - this eyepiece has an extra positive element, and the form of the lenses is different. It has a concave surface near to the intermediate image. This surface reduces Petzval sum, and it also helps to achieve a longer eye relief by moving the chief rays away from the axis. The semi-field angle for this design is 30 deg. In this example, the lateral color is not corrected, because the eyepiece is intended for use with a prism. This is a typical eyepiece for use in afocal systems of reasonable quality.