Whats the harm if we use concave lens as eyepiece instead of another convex lens? We just need to make the foci of eyepiece and objective coincide.
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$\begingroup$ Is that not the same thing as a Galilean telescope? $\endgroup$– mike stoneCommented Aug 25 at 15:32
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As Mike Stone implies in his comment, a telescope with a diverging lens for its eyepiece is a Galilean telescope. The main disadvantage of such a telescope over a 'Keplerian telescope' with a converging lens for its eyepiece is the small field of view for a given magnification. But if only a small magnification is needed, as when viewing the stage in a theatre, the field of view is adequate, and the telescope can be short, hence opera glasses are a pair of Galilean telescopes.
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$\begingroup$ The advantage of a Galilean telescope is that the image is the right way up which is beneficial for watching an opera. $\endgroup$– KDPCommented Aug 25 at 18:30
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$\begingroup$ Yes indeed, without the need for inverting prisms as in binoculars. $\endgroup$ Commented Aug 25 at 18:59
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$\begingroup$ Right way up, compact, lightweight. What's not to like? :-) $\endgroup$– KDPCommented Aug 25 at 19:04
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$\begingroup$ The upright image also lent credence that the image seen was of reality, not some imaginary scene; an advantage when trying to sell the telescope to various security forces (At least, back then) $\endgroup$– DJohnMCommented Aug 25 at 21:23
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$\begingroup$ Just one magnification(and this also low) will be produced then, unlike using convex lens where multiple and large magnifications can be produced though inverted. We want to see bigger images, right? As far as i understood $\endgroup$– LavanyaCommented Aug 26 at 5:17