Can someone explain to me what focal length is?

[Csuka]

If some one could explain it to me i would appreciate it, thanks =)

[JamesF]

This will tell you more than you ever wanted to know:

Focal length - Wikipedia, the free encyclopedia

James

[rowan46]

essentially it's the length required to bring light to a focus

[Csuka]

Yes but how would you find the focal length of your telescope?

[ollypenrice]

It should be indicated. Do you have the aperture and the focal ratio? If so, mulitply them for the focal length.

Olly

[Csuka]

It should be indicated. Do you have the aperture and the focal ratio? If so, mulitply them for the focal length.

Olly

I found an old telescope in my garage, i read that to find the magnification of a telescope you need to divide focal length of scope, by focal length of eye piece, but i have no idea how to find out the focal length of either lol.

[rowan46]

are there any identifying marks? look for the makers name then google images until you find one that looks the same

[Alma]

A very simplistic analogy is to think of the telescope as a long tunnel. If you're standing close to the tunnel entrance you will have a wider field of view than if you move further back into the tunnel. This is the difference between a short focal length telescope (standing close to the tunnel entrance) and a longer focal length (moving deeper into the tunnel).

The view of the outside through the tunnel portal is equally bright but the field of view is narrower the deeper you move into the tunnel - it's just the same with telescopes.

Example: Say the the tunnel portal is a perfectly round 5 metres diameter. If you were to stand 25 metres inside the tunnel from the portal it would be the equivalent of a telescope with a focal length of 25 metres and a focal ratio of f/5. Standing twice as far back into the tunnel and you would see the equivalent of a 50 metre focal length scope and an f/10 focal ratio.

The analogy holds true for refractors but in the case of Newtonian and catadioptric scopes there would be a mirror in the middle of the tunnel to bounce the image back towards the entrance (and back down the tunnel again in the case of the catadioptric), so physically you wouldn't be standing so far back in the tunnel but it would appear as though you were.

Hope this helps.