Quick question about focal lengths...

[orby1]

Hello, I'm looking at different telescopes in a catalogue and on the net, and some are quoted as having a focal length of f/5 or f6, and some are something like f/1000, or f/1200.

Am I being a spaz??? For example, quoting this catalogue, this Skywatcher Explorer 200 I'm looking at here is apparently "a 200mm f/1000 parabolic reflector" with "focal length 1000mm (f/5)"

I've done a fair bit of photography and so know all about f-stops on cameras and how they work, so, I'm puzzled by how a telescope could have a focal length of f/1000.

It looks like the actual, physical length of the tube is 1000mm, but why would they say that makes it an f/1000 'scope?

[John]

I'm not suprised that you are confused !.

References like F/5 and F/8 refer to the focal ratio of the scope which is the same as the f-stops on cameras - it's the ratio between the aperture and the focal length.

The focal length of a particular scope lens or mirror is usually quoted in mm but I agree that it's confusing that it's also preceeded by "F/" in some cases.

Telescope focal ratios vary between F/4 and F/15 (or so) wheras telescope focal lengths vary between 400mm and 4000mm.

So the scope you were looking at had an aperture of 200mm (the diameter of the main mirror) and a focal length of 1000mm and therefore a focal ratio (focal length divided by aperture) of 5 - referred to as F/5.

[LukeSkywatcher]

I was puzzled by the same thing with my new scope. It is an f5 scope. Its length is 650mm yet the manual refers to is as a f650 scope.

f650?????...........damn i thought my f11 scope was slow. Imagine how slow an f650 scope would be. I could walk (wheel my way) to Alpha Centuri faster then the scope could see it.

[ollypenrice]

Paul, the manual is wrong, of course. For focal length the convention is to say f= and for focal ratio to drop the equals sign.

Without being sanctimonious, could we just have a care over terms of abuse, even used self deprecatingly?

Olly

[scotastro]

Not sure if you need thgis info but I'll throw it in anyway.

One issue often misunderstood is the effects of focal length and aperture on the object being viewed..

For viewing deep sky objects - galaxies nebulae, etc you need aperture not magnification. Most DSOs are big but faint So a short focal length scope f4.5 or f5 is fine. However to view planets you need high magnification (they are bright so smaller aperture is ok) a scope of f6 and above is much better - even up to f11 f12 for Maks and SCTs)

Mike

[orby1]

Thanks for all your help, people!!! I get it now, it's just a confusion between focal lengths and focal ratios! DOH!!!

In reply to Scotastro - it seems I'd have to decide whether I want a 'scope best used for DSO's, or one best used for planetery observing. I can barely afford one, let alone two!

So my question is.....

What would be the problem buying as wide a scope as possible for when I'm galaxy hunting, and getting some higher magnification EP's if I want to observe planets, all in the same 'scope???

Thanks again everyone.