Will someone PLEASE explain focal ratios to me!?!?

[petermarknimmo]

As above, I see focal ratios mentioned in the descriptions of telescopes and being discussed on this forum, but haven't actually got a clue what differences they give. Could someone explain it to me?

[Cornelius Varley]

Basically the focal ratio of a telescope is the focal length of the telescope divided by the size of the primary optic (lens or mirror)

Peter

[brantuk]

Yup - a 1500mm focal length dob scope with an aperture of 300mm has a focal ratio of 1500/300 = f-5 focal ratio (fast). A 2meter focal length Sct with a 200mm aperture = f-10 (slow). The speed relates to imaging and a fast focal ratio will gather light quicker than a slow scope - meaning that less time to take an exposure. Bit like fast and slow film in a camera. Generally fast scopes benefit from better quality eyepieces whereas slow scopes are more forgiving with lesser quality eyepieces.

[LukeSkywatcher]

Basically the focal ratio of a telescope is the focal length of the telescope divided by the size of the primary optic (lens or mirror)

Peter

To add to this:

"Fast" scopes are scopes that allow the light gathered to get to your eye quicker than "slow" scopes. The light in slow scopes takes longer to get to your eye.

The light in "fast" scopes is more concentraterd and produces sharper/brighter images whereas the image in slower scopes is not as sharp or as bright.

It really depends on the make of scope (Refractor,reflector etc).

A fast scope has a focal ratio of about f5...............whereas a slow scope has a ratio of f11.

basically it is all about how long the light takes to arrive at your eyeball and how much that light gets diffused in the scope before it arrives at your eyeball.

My F11 (90mm) Refractor is brilliant for observing planets...........................while my F5 (130mm) Heritage reflector is great for observing DSO.

[LukeSkywatcher]

Yup - a 1500mm focal length dob scope with an aperture of 300mm has a focal ratio of 1500/300 = f-5 focal ratio (fast). A 2meter focal length Sct with a 200mm aperture = f-10 (slow). The speed relates to imaging and a fast focal ratio will gather light quicker than a slow scope - meaning that less time to take an exposure. Bit like fast and slow film in a camera. Generally fast scopes benefit from better quality eyepieces whereas slow scopes are more forgiving with lesser quality eyepieces.

My 8SE (SCT) has a focal length of 2032mm. I have used it so little since i bought it that i really dont know if it is best for observing planets or DSO.

[brantuk]

My Sct f-10 and refractor f-7 have quite long focal lengths and make planets/moon nice and crisp and sharp to view. The dob at f-5 however is much better on dso's (though shows planets well too). Much like you Luke - I'm quoting form memory lol - the weather just hasn't been much up to it this year

[LukeSkywatcher]

The dob at f-5 however is much better on dso's (though shows planets well too). Much like you Luke - I'm quoting form memory lol - the weather just hasn't been much up to it this year

Same here. My Heritage (f5) shows planets well and the moon, but it really picks up those DSO's.

2012 is a year i really cant wait to get out of astronomically. Its been RUBBISH. Maybe the winter will be good?

[JamesF]

As has already been said, fundamentally the focal ratio is the ratio of the focal length to the aperture. When you see something with a focal ratio given as, say, f/7 that means that the aperture is the focal length ("f" in the "f/7") divided by 7.

In terms of "what it means in the real world" you can think about it many ways. "Fast" scopes (that is, ones where the divisor number is small) have to "bend" the light path of incoming light far more than "slow" ones (where divisor is large). That means they're much more demanding in terms of optical quality and why average quality eyepieces, for instance, are not suited to fast scopes. If you think of all the light reaching the primary optics being funnelled into a cone with the eyepiece at the tip, fast scopes have a short, sharply angled-cone, whereas slow scopes have a longer less steeply-angled cone. The terms "fast" and "slow" come, as far as I'm aware, from the photography world where they refer to the required exposure times for lenses as a means of comparing them.

Another way to look at it is that if we choose two different telescopes with the same focal length, the fast scope will have a larger aperture than a slow one meaning that it can funnel far more light into the eyepiece at once, and give a wider field of view. If you half the focal ratio, going from, say, an f/10 telescope to an f/5, then you actually quadruple the amount of light that can be collected in a given time. There's always a catch however, which is that as magnification is the focal length of the telescope divided by the focal length of the eyepiece, it's much easier to get high magnifications (such as you might want for planetary viewing) with a long focal length telescope and these will typically be slow scopes because the larger optics become the harder and more expensive they are to make.

Fast refractors typically show lots of colour aberration because it's much harder to bend the incoming light path by large angles and get it right for all possible frequencies of light. To fix this takes expensive glasses and coatings, which is why well-corrected fast refractors are so expensive compared to normal achromatic ones.

James