About telescope lenses

If a bi-convex eyepiece is not used and a plano-convex or meniscus lens is used instead, it should be placed with its plane or concave side towards the eye. That arrangement, which looks like the opposite of the best arrangement for minimizing spherical aberration, is the correct one for an eyepiece. This is because that lens is dealing with small pencils of rays coming from the real image, which the observer is looking at with the eyepiece.

Those narrow pencils are like thick rays, and they are almost parallel to the axis. The eyepiece must deal with these thick rays as well as possible, and for that the eyepiece should have its convex face turned to receive those thick rays. On the other side of the eyepiece the narrow pencils, or thick rays emerging from the plane side of the eyepiece, will pass through the eye ring, so they form a strongly converging group. The eyepiece, arranged this way round, treats those thick rays with less spherical aberration than it would the other way round. Those rays which hit the outer region of the eyepiece lens are bent a little more than the ideal amount, because there is very little spherical aberration. But if the eyepiece were the other way round that extra bending would be much greater, and would have two effects.

• It would give extra magnification to the outer parts of the final virtual image, making distortion.
• It would give curvature of field to the image, so that the outer portions are farther away than the central part.

To see how that curvature of field arises, one must look at the differential ‘extra bending’ between the outer and inner rays of each small pencil. This story of the eyepiece is not something to discuss with students at an introductory level.