I am using a convex lens with $f=5$ cm and $5$ cm aperture, a concavo-convex lens with $f=100$ cm and $5.5$ cm aperture. I am trying to make telescope and it should have magnification of 20 x. But the image coming of eyepiece is very small and cant be seen clearly. Please help with the problem?
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1$\begingroup$ 50mm aperture with 1000mm focal length is a f/20 ration - which is very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vigneting (that's what you see with the 'small image'). With those pre-condition you won't get a bright telescope with a wide FOV. $\endgroup$– planetmakerAug 27, 2020 at 9:59
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$\begingroup$ @planetmaker I think that answers the OP's question, perhaps with minimal editing it could be made into a proper answer. $\endgroup$– James KAug 27, 2020 at 19:57
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$\begingroup$ What the comments and answer suggest is that you need to read up on things like "exit aperture" and "Field of view" , as those parameters matter -- and will affect your choice of focal lengths for the two main elements in your design. You might consider going to a 3-or-4 element system if you have some ray-trace software available to assist. $\endgroup$– Carl WitthoftAug 28, 2020 at 13:54
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A 50mm aperture with 1000mm focal length is an f/20 ratio - which is a very very small aperture for the focal length and will result in a very faint image. The small opening angle will result in vignetting (that's what you see with the 'small image'). With those pre-conditions you won't get a bright image with a wide FOV.