An eyepiece design invented by C. Huygens c.1662. It consists of two planoconvex lenses with their convex surfaces facing the objective lens. Because of its cheap and simple construction it is often used with small refractors, but will only perform well with large focal ratios. The Huygenian eyepiece suffers from aberrations if used on telescopes faster than about f/10. A variant known as the Mittenzwey eyepiece uses convex meniscus lenses and is usable down to f/8.
Subjects: Astronomy and Astrophysics.