some F/number questions??

Ok, Im trying to get a handle on this:
What exactly is the F number? How critical is it? Why are short F numbers good for some things and not for others? Is there advantages to short V long F numbers? How is it calculated?
Sorry for the amount of questions...

They're calculated by dividing the focal length of the scope by the aperture. So a 1000mm scope with an aperture of 200mm is F/5.

As for the rest I'm going to monitor this thread to find out myself

Here's something I posted a while back on the subject;

When people talk about 'fast' or 'slow' they are referring to the focal ratio of the scope. Smaller 'f' numbers like f5 mean the scope is 'fast'.

The f number is tied into the eyepiece magnification calculations. To be precise, it's the focal length / aperture. Here's a more complete description;

www.northern-stars.com/tel_magnification.htm
f/Ratio: The f/ratio of a telescope is determined by dividing the focal length of the primary lens or mirror by the aperture (the diameter of that same lens or mirror). This number tells us several important things about a telescope. For example, the lower the f/ratio the faster the telescope is said to be; that basically means that it provides a brighter image than a similar sized telescope with a higher f/ratio. Yet, remember that there is always a tradeoff, by getting brighter images you loose some magnifying power. Telescopes with low f/ratios give wide-field images with bright star fields, they are good for viewing star clusters and faint nebulae. Telescopes with high f/ratios aren't quite as bright, but yield higher magnifications with narrower fields of view. They are ideal for planet viewing and splitting binary stars.

f/6 and lower would be considered fast, ideal for viewing faint nebulosity and wide field objects.

f7 to f/10 would be midrange. The compromise range, telescopes with f/ratios in this range do fairly good at both ends.

f/11 and up would be considered slow, but ideal for planetary work or studying binaries.

Again, when calculating f/ratios, remember to use the same units of measure. Don't divide centimeters by inches!

So, a focal length of 400mm with an aperture of 80mm would give you f5 (400/80), which is a fast scope. If you put a 10mm eyepiece into this scope, you'll get a magnification of (focal length / eyepiece) = 400/10 = 40X.

A focal length of 800mm with an aperture of 80mm would give you f10 (800/80) - good for planets. If you put a 10mm eyepiece into this scope, you'll get a magnification of 800/10 = 80X.

There's another thing to bear in mind, called 'exit pupil' - But I'll let you digest this piece first!

Oh, and of course, there are the optics themselves. You can't expect a 4mm eyepiece in an f10 scope to give you the same nice easy sharp image that a 32mm eyepiece would give you.

Stephen,

On a related note, a "fast" scope (ex. f/5) will give you a wider field than a "slow" scope (f/10) which impacts what you observe/photograph. For example, if you want to observe the Orion Nebula (M42) in all its glory, it is best to use a "fast" scope since you need the widefield view to pull in the complete nebula.

In contrast, if you want a high-power view of a lunar crater, a planet or a sunspot, for example, you would go with a slower scope and its (much) narrower field of view.

Anthony.

So my soon to arrive 12" dob, which is f5 (1500/305) should be grand for my fave dso observing? And with my eps of 4, 6, 9, 12, 15, 25, 32mm - I should get magnifications of 375x, 250x, 166x, 125x, 100x, 60x and 47x.
What has the focal ratio got to do with how critical collimation is? Which is better for photography, fast or slow? Thanks for the replies. Its all becoming a little clearer. :roll:

Stephen - I guess this is one of those instances where what you are asking doesn't really have one single answer. The folks here have given you a ton of info - I hope its making sense to you. I'dd just add a bit more

If you compare two reflecting telescopes of the same aperture, the one with the lower f-number will give you a brighter image but will give you a smaller image of the object you are looking at.

The lower the magnification (for the same aperture) gives a wider field-of-view. As Anthony says, with a low magnification eyepiece on a fast f-ratio scope, you should be able to cram in loads of sky. Use a high magnification on a slow telescope and you'll be 'zooming in' to a much smaller piece of sky.

The general premise that a faster scope gives a wider field of view is correct - if you are comparing a fast scope and a slow scope that both have the same aperture. But if you start comparing different apertures, this premise doesn't hold up. A fast 20" reflector will not give a wider field of view than a slow 8" reflector.

As far as importance goes, I think 'fast' and 'slow' is not a major consideration for visual use (in terms of brightness) - but it can become very important if you want to start imaging (since the more light you can collect in a given period of time, the better).

Of course, fast and slow for visual use IS important when you want to figure out what magnification you will get with a particular eyepiece. This would be particularly relevent if you already own a set of eyepieces and you are upgrading to another OTA and want to figure out if they'll provide you with the same or different magnifications you are used to.

Are these posts helping you at all?

Cheers

Dave McD