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Don Seltzer wrote on 17 Dec – “I thought that a simple two lens telescope inverted the image, and a three lens telescope produced an erect image.”

I apologize if that was a high-end question because I’m working on the simple end of this subject. I have what I think is a 3x upright scope missing the exit (eye) lens and all I know at this time is that the existing 18 mm objective lens focuses on an object at about 130 mm. I’m trying to select a replacement exit lens without buying every ~18 mm lens that Surplus Shed offers.

My impressions at this time are that a magnifying lens converges the rays of light to a point at the focal length. If you “look” at the rays before the focal length, the image will be erect. If you put a suitable concave lens in before the focal length, you make a Galilean telescope with a erect image. See the attached “Telescope Finders” from an old Edmunds paper. Note that on the first scope the objective focal length is 171 mm and the overall scope length is just over 5” ( 127 mm). The exit/negative/concave lens captures the light rays before the focal point of the objective lens.

If you “look” past the focal length, the image is inverted.

I think that this means that a two lens scope could be either erect or inverting depending on the lens focal lengths and locations.

It gets much more complicated than that. See:
https://www.physicsclassroom.com/Class/refrn/u14l5db.cfm
https://answers.yahoo.com/question/index?qid=20091015204404AAFguIT
https://physics.bu.edu/~duffy/semester2/c28_lenses.html
- And many more.
Regards, Noell

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