Think about stopping down your telescope. Many scopes come with lens caps that have a circular hole cut in the center with a secondary cap on that.
If you operate your scope with the lens cap ON but the secondary cap OFF you have now stopped down your scope. Your f8 scope could now be, say, an f20 scope without any changes to the objective lens diameter. This really freaked me out since I started in telescopes before cameras and I had the exact same confusion you have.
Do you have an old 35mm film camera sitting around? Open up the back and look through the lens, essentially, your eye is now the film. Press the shutter. You'll see a brief flash of light through the mostly circular aperture. (Even better, set the shutter speed slow so the brief flash is less brief.) Now play with the aperture setting, compare, say f2.8 with f16. Notice how the size of the circular hole changes?
If you don't have an old film camera, try this with your DSLR, but looking in the front, look for something to change inside the lens, direct center, as you play with aperture.
Cameras gets stopped down a lot. You need to do this to both change the length of the exposure as well as control depth of field.
Telescopes are rarely stopped down. You probably only want to do it for solar or lunar observing. Why? You don't need the extra light but unless you have an APO refractor, stopping it down will decrease the chromatic aberration considerably. I had a chance to see Galileo telescope in Philadelphia. It was perhaps 1 to 1.5 inches in diameter but it was stopped down to something tiny, like 0.5" or so! This was done to reduce the aberrations in his primitive lenses.