Focal Ratio

[Benno (Ben)]

Hey guys 'n' girls,
just a quick one in regards to focal ratio. as i am still new to some of this, im getting confused with focal ratio. i know that a shorter focal ratio is better for deep sky and longer for planets/moon etc, but what i dont understand is which way it goes. for example if i had a telescope that was f/5 would that be the short focal ratio or would say one with f/15 be the shorter one? ive seen that Dobs are around the f/6 mark but the Maksutov-Cassegrain are about f/13. any help would be greatly appreciated.
cheers
Benno

[iceman (Mike)]

Hi Benno

Short is f/5. Sometimes it's also called a "fast" focal ratio or a "fast" scope.

Long is f/10 and above. Sometimes it's also called a "slow" focal ratio or a "slow" scope.

I wouldn't necessarily say fast is better for deep-sky and longer is better for planets/moon. It just depends what you want to do. I love fast scopes for the wider field of view and more clear contrasty images they provide.

Fast scopes have more problems with coma and is more demanding on collimation and eyepiece design. Slow scopes just don't let as much light in, have a narrower field of view and more magnification for a given eyepiece.

[Dennis]

For the same objective diameter, another feature of slow (F8, F10, etc), long focal length 'scopes is that focusing is not quite as critical. Using the analogy of camera lenses, you get a “better” depth of field, or “wider zone” of focus with a long focal length (F8 +) ‘scope. In a short focal length ‘scope (F4, F6), the zone of critical focus is quite small in comparison.

Cheers

Dennis

[Benno (Ben)]

oh ok thats cool then, so say i wanted to check out more deep space stuff with also planetary views which would be the best for an allrounder? its more of a case for beginner stuff but also for a scope that will do what i want and one that i will be able to keep for years and keep learning, know what i mean?
example if i went a Meade ETX-125PE with a focal ratio of f/12 would that be cool for short exposer shots as well as just the simple viewing and learning of the sky? or would a 10" Dob be the choice to learn and get shots of the deep space stuff, but not so much planetary? sorry if this all seems confusing, but im a bit stuck with a few things and id rather learn from lotsa questions then not asking anything and buying something that i didnt really research properly. plus you guys all know what your on about and i like picking your brains

[fraunhofer]

Perhaps my comments may assist you with regard to f/ratio. The f/number (focal ratio) of any telescope tells two things about the telescope and that is it's intended purpose and it's photographic performance. The brightness of a star depends on the telescope's aperture not the telescope's f/number. All telescopes of the same aperture at any magnification will show the same visual brightness. There are many that insist their long focal ratio telescope gets higher contrast, this is not correct. A refracting telescope does have more contrast than any other because of it's optical system not because of the f/ratio. You can see that when you are comparing very well made and very well corrected refractors, you will see there is no gain in contrast regardless of the f/ratio of each telescope. Even if a reflecting telescope is well made and has the same size secondary mirror obstruction as another, it will have the same contrast regardless of the f/number of each telescope. All of the confusion and there is much on this issue, is because of the photographic use of the f/number. A faster f/ratio does mean brighter images on film but not in a telescope. Aperture not f/ratio is the important factor with a telescope. Some photographers have a great deal of trouble with this concept. The f/number of any objective lens or a mirror of a telescope has nothing to do with the visual brightness of an image, the bigger the aperture the better. Happy skies and good luck.