Telescope focal length question

[mik]

Hi if i put a 80mm extension tube onto a 400mm fl scope do i end up with a 480mm scope sorry for the daft question.

Mick.

[steppenwolf]

Sadly not as the focus 'sweet spot' will not have changed as you have not made a change to the optical light path. You will still need to focus the system at the original focus point which will be at 400mm, not 480mm.

[mik]

Thought so just needed to ask.

mick

[Paxo]

It's not as simple as that (I think!) the extension tube changes the range of accessories that can be brought to focus using your system but does not alter the optics in any way, such as a focal reducer does. Your optics, a combination of physical length and mirror/lens 'strength' ( f ) still remains the same.

Why use the - well they tend to be used when you wish to remove a diagonal from the system for example when you want to bring a DSLR to focus.

I'm sure someone else will come up with a better explanation.

Steve

[Kaptain Klevtsov]

If you add an 80mm extension tube to the 'scope, you'll end up with a 480m long 'scope. If you want it to reach focus though, you'll need to buy an 80mm unextension tube. These are rare, but there is a way round it which I'll tell you about for a fiver.

Kaptain Klevtsov

[AstroTiger]

i think this underlies a confusion amongst beginners, not know what kind of telescope to go for.

Exactly what a barlow lense does.

3 inch refractor or 10 inch dobby.

Perhaps someone could write a little about this?

I would but i am just as flumuxed.

I am aware that refractors are better by and large for lunar and planetary work, they allow less light.

But perhaps something on why focal length is so important might help ppl to fully understand their instruments?

Just a thought

AT

[ringz]

AT,

Refractors are by and large better for planetary and lunar observing because they have no central obstruction and thus better contrast not because they allow less light.

--

Martyn

[Ajohn]

Many telescopes are made so that the focuser movement and range is correct for use with a diagonal. Remove the diagonal and they won't focus any more because the length of the light path has been reduced. Fit the extension and they will.

Try to fit a camera and the scope may or may not need an extension.

I am slowly writing something up to post but there isn't much point unless some one sticks a pin in it.

John