What it it like, using a Galilean telescope?
The Galilean telescope has its eye lens before the focal plane of the objective. The image of the entrance pupil (the objective) lies in front of the eye lens (by nearly the eye lens focal length) where you cannot place your eye. This means, in effect, the exit pupil forms a porthole into which you have to peer. There is a central portion to the field of view, and a vignetted outer periphery, the width of which depends on the size of your eye pupil. The closer you can get your eye behind the eye lens, the wider the field of view. The edge of the field is blurred because your eye is focussed to infinity, but the exit pupil lies only a short distance from your eye. In low power Opera Glasses, when you are very young and have something like 12 Dioptres visual accommodation amplitude you can focus both the image and the exit pupil. In middle age, when due to Presbyopia you have little accommodation amplitude, the field edge is blurred, and there is nothing you can do about it. Wearing corr
