Inside a refracting telescope.
Why does a telescope need two lenses?
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Why does a telescope need two lenses?
Fit both lenses, then compare their focal lengths.
The objective brings nearly parallel starlight to a small real image. The eyepiece lets your eye view that image at a larger angle. A Keplerian telescope places the image at the eyepiece focal plane, so light leaves the eyepiece parallel. Its image is inverted. A larger objective improves the diffraction limit but does not change the focal-length magnification.
Paraxial Keplerian refractor with positive thin lenses and a distant target. Wavelength 550 nm sets the Rayleigh diffraction limit. No aberration, atmospheric seeing or exact lens prescription. The open tube is a mounted cutaway. The target plate marks incoming parallel rays; target distance is not to scale.
Make a focused view with 20× to 40× angular magnification.
Focus a magnified view of a distant target.