Refracting Telescopes

An astronomical refracting telescope uses two converging lenses:
• the objective lens, with a long focal length f_o, collects light from the distant object
• the eyepiece lens, with a short focal length f_e, acts as a magnifying glassLight from a very distant object arrives as parallel rays. The objective forms a real, inverted image at its principal focus. The eyepiece is placed so that this image is also at the eyepiece's principal focus.In normal adjustment the principal foci of the two lenses coincide, so:
• the distance between the lenses is f_o + f_e
• rays leave the eyepiece parallel, so the final image is formed at infinity. This is restful for the eye, which is relaxed when viewing distant objects.The final image is virtual and inverted. Inversion does not matter for astronomy.To draw the ray diagram, take three parallel rays from a point on the object at an angle to the axis. The ray through the centre of the objective is undeviated and fixes where the image forms in the focal plane. All three rays meet there, then pass through the eyepiece and leave parallel to a construction line drawn from that image point through the centre of the eyepiece.Worked example: a refractor has f_o = 1.20 m and f_e = 0.040 m. In normal adjustment the lens separation is 1.20 + 0.040 = 1.24 m.