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?
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.