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.

Thought there would be more replies to this post, since you are keeping them buys elsewhere with this posting, hehe

That is correct 100%

The focal ratio is the ratio of the focal length of the telescope to its aperture. It’s calculated by dividing the focal length by the aperture (both must be in the same units). For example, a telescope with a 2032mm focal length and an aperture of 8" (203.2mm) has a focal ratio of 10 (2032/203.2 = 10) or f/10.

It’s variously abbreviated as f-stop, f/stop f-ratio, f/ratio, f-number, f/number, f/no., etc.

Smaller f-numbers will give brighter photographic images and the option to use shorter exposures. An f/4 system requires only ¼ the exposure time of an f/8 system. Thus, small focal ratio lenses or scopes are called “fast” and larger f/numbers are called “slow”. Fast focal ratios of telescopes are f/3.5 to f/6, medium are f/7 to f/11, and slow are f/12 and longer.

Whether a telescope is used visually or photographically, the brightness of stars (point sources) is a function only of telescope aperture - the larger the aperture, the brighter the images. Extended objects will always appear brighter at lower magnifications. The main advantage of having a fast focal ratio with a visual telescope is that it will deliver a wider field of view than slower f-numbers.

You are pushing it Ned.

Well here is what I say, here goes with a quick go at the mystical numbers on your telescopes and how to decode them.

I'll start off with some definitions in case some of these are non-obvious, the whys and wherefores of the f/ number will have to wait until I can condense the f/ concept better as I've just tried and messed up.

Aperture
This is the diameter of the hole that the light enters the telescope through. A 100mm refractor will have a front lens of roughly 100mm. A reflector will have a tube slightly larger than the aperture as the actual aperture is the primary mirror (the tube is merely a way of holding all the bits together).

Focal length
This is the distance from the primary (front lens in a refractor or the big mirror at the bottom of the tube in a Newtonian) to the focal point. The focal point is where you put the eyepiece or camera.

f/ ratio
This is the relationship between the aperture and the focal length.

So what does all this malarky mean?

The significance of these three numbers is, in simple terms:-
Aperture
More aperture means that you are collecting more light and more light makes the image appear brighter. Bright things are easier to see and to photograph. More is better.

Focal length
This has a direct bearing on the magnification. No other number is involved as far as the telescope is concerned. The magnification is calculated by dividing this number by the focal length of the eyepiece. Using the above example and a 10mm eyepice gives a magnification of 1000/10 = 100. If you use a 20mm eyepiece the magnification goes down to 1000/20 = 50.
For imagers that doesn't help much so there's another way of looking at it. If you could look through a hole the size of your camera's sensor you would be able, by moving your eye from side to side, to see a section of the sky. Bigger sensor, more sky. To work it out more exactly you need to imagine that all the light comes through a tiny section in the centre of the lens or primary mirror so you can use the focal length of the mirror and the sensor size to work out how much sky you can "see" using that sensor. If you sketch an isosceles triangle with a base dmension equal to your sensor width and a height equal to the focal length of the 'scope, the angle at the pointy end is the angle which also covers the piece of sky that you can image widthwise. Do the same for the height and you get the angle corresponding to the amount of sky you can image in the other direction.

"It all depends on what you want to observe and how much you are willing to carry and to spend."


In general, fast ratios have a wider fov (for a given aperture), but make a steeper light cone, which requires more expensive eyepieces (for all apertures). But, really,
field of view is more a question of overall focal length than focal ratio. For reflectors, faster means a larger central obstruction, which detracts from contrast (though this is often exaggerated) and increased coma. For refractors, longer, slower focal ratios make for better color correction, but narrower fov. Large Newtonians get very long, and tall, at slow or even medium f ratios, and very heavy in any event. Just to cover a few issues...

No way around it - you have to decide what you want to look at, what you're willing to carry and how much you are willing to pay.

Nevertheless, there are some designs and apertures and focal ratios that are more common as 'general purpose' telescopes - 4" refractors, f/6 to maybe f/9
- 6" to 8" reflectors, f5 to f/8 for the 6", f/5 or f/6 for
the 8".
- The 8" SCT, f/10, with the reputation of being "jack of all trades, master of none", but it's 8" of functional aperture in a small, light weight package.

So it is, that most of us own more than one telescope, sometimes many, many more that one! But one thing that's sure, if you put in your time to become an experienced observer, you will see more, whatever aperture you use, and whatever design.