Inverting telescopes have a better field of view!

[JamesC]

I've been interested in astronomy for a long time, but for some reason I never really caught the explanation for why the inverting design (i.e. positive eyepieces) was preferred to the opera glass design (negative eyepieces). Wikipedia to the rescue: Galilean versus Keplerian design. This site has ray diagrams for both Galilean and Keplerian arrangements. With the positive eyepiece, off-axis rays are converging back towards the eye, giving a wide field of view at the exit pupil. I guess this is old hat to most astronomers! I guess it also explains why short focal length eyepieces tend to have such poor eye relief. I feel a bit dumb for not catching on before, but better late than never!