The telescope objective has a diameter of 20 cm and a focal length of 1 m. What is the maximum focal length that makes

The telescope objective has a diameter of 20 cm and a focal length of 1 m. What is the maximum focal length that makes sense to use the eyepiece when observing with this telescope?

3 cm. With a larger focal length of the eyepiece, some of the light collected by the lens will not enter the observer’s eye.

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