Monthly Question...What's Inside an Eyepiece?

The following cross-section diagram illustrates the various parts of a simple eyepiece, also referred-to as an ocular. (The diagram isn't meant to represent any particular eyepiece design.) The lens closest to your eye is the eye lens. The lens at the opposite end -- facing the inside of the telescope tube -- is the field lens.

A field stop narrows the area through which the light passes, and it can serve a number of purposes. It may simply hold a lens that is smaller in diameter than the inside of the eyepiece barrel. Another purpose is to act as a baffle to minimize reflections on the interior of the eyepiece. Most eyepieces have field stops of some sort, but on a good eyepiece they don't adversely affect the field of view. On inferior eyepieces (or cheap refractor telescopes objectives), field stops are intentionally used to interfere with the passage of light; this is to reduce the distortion caused by poor quality lenses that are optically uniform only in the center of the lens. In addition to using the field stops as baffles, some better quality oculars have minute grooves scribed around the inner circumference of the barrel to make the inside anti-reflective.

Most eyepieces have threads on the inside of the barrel at the end near the field lens. This allows filters to be screwed into the barrel; eyepieces should be threaded for standard-size filters. The threads also allow the collar to be screwed into the barrel to hold the lenses in place. Never remove any of the lenses, since repositioning them correctly is a job best left to an optician, and will probably void any warranty on the eyepiece.

Published in the November 1998 issue of the NightTimes