magnification of a lens depends on the size of 'f' focal lenght?

Question

what if i am able to get a lens of such focal lenght which is exteremely larger than a football stadium, would it be a powerfull telescope than the one NASA use?



its mean, if focal lenght increases, magnification increases
f directly proportional to Mg

Answer 1

You can get a lens with a focal length as long as you please; one with a very long focal length is almost flat. But the magnification is not the only thing you need to worry about: there is the f-number, the ratio of focal length to lens diameter. (This is the same f-number that you see on your camera lens.) Unless the diameter is large, the f-number will be so large that little light will be collected. There are also severe mechanical problems supporting a large lens; vibration will do you serious dirt if you are trying to take a long exposure photograph.

Answer 2

You are correct that the size of the image is proportional to the focal length of the lens. However, image size isn't important to the functioning of the telescope. It's the aperture (diameter) of the lens that's important - more aperture means a brighter and sharper image. You can magnify the image from a short focal-length telescope to see all the detail there is to see, so making the lens longer just makes it harder to handle.

You'll notice that when they talk about building more powerful telescopes, it's the diameter of the main mirror they talk about, not its focal length.