Question for the group

I'm a newbie here, and a newbie to telescope making, so if I make a
massive faux pas or ask a stupid question, I apologize.
What I'm curious about is what the "f" stands for in relation to the
mirror size. I've seen it used in relation to cameras as well.
Thanks!-

V.

Hey Vince,
There are no stupid questions! We were all newbies at one time or another.
The "F" stands for focal length or focal ratio. As an example, if a 10"
mirror focuses at a focal length of 50" then the focal ratio is F5 (10"
mirror divided by 50" focal length). The same applies to refractors or
SCT's. Divide the aperture by the focal length. Hope this helps, and clear
skies to ya.

Duff Couillard
Calgary
"Vince Z." wrote in message
om...
I'm a newbie here, and a newbie to telescope making, so if I make a
massive faux pas or ask a stupid question, I apologize.
What I'm curious about is what the "f" stands for in relation to the
mirror size. I've seen it used in relation to cameras as well.
Thanks!-

V.

"Vince Z." wrote in message
om...
I'm a newbie here, and a newbie to telescope making, so if I make a
massive faux pas or ask a stupid question, I apologize.
What I'm curious about is what the "f" stands for in relation to the
mirror size. I've seen it used in relation to cameras as well.
Thanks!-

"F" stands for focal ratio. It is simply the telescopes focal length
divided by the aperture. So a 6" f/8 mirror would be a mirror of 6"
diameter with a focal length of 48". In a given aperture, a smaller or
"faster" f/ratio has a shorter focal length and, in general, allows a
somewhat wider view. A larger or "slower" f/ratio has a longer focal length
and provides a somewhat more restricted view.

The two important qualities of a telescope are aperture and focal length.
Aperture determines light grasp and resolution, and focal length determines
image scale and, when combined with the size eyepiece being used, the
maxiumm field of view.

There is one important difference between how photographers talk about
f/ratio and how astronomers talks about f/ratio. When a photographer talks
about a "faster" lens the focal length remains fixed and the aperture
changes - so faster photographic lenses actually collect more light than
their slower cousins. When amateur astronomers talk about "faster"
telescope the aperture remains fixed and the focal length changes. A faster
telescope of 8" aperture does not collect more light and the view is not
inherently brighter than a slower 8" telescope. At the same magnification
the views are the same brightness. An astrophotographer does get a shorter
exposure, but only because image scale is sacrificed and the photons are
concentrated on a smaller area of the film.

Clear skies, Alan

On 27 Nov 2004 10:41:26 -0800, (Vince Z.) wrote:

I'm a newbie here, and a newbie to telescope making, so if I make a
massive faux pas or ask a stupid question, I apologize.
What I'm curious about is what the "f" stands for in relation to the
mirror size. I've seen it used in relation to cameras as well.
Thanks!-

Usually, "f" stands for focal ratio, the ratio of focal length to diameter. "F"
(capitalized) usually is used for the focal length. These conventions are
regularly broken, however. So if you see "f" (or "F") followed by a small
number, say 4-20, it's the focal ratio. If you see it followed by a big number,
several hundred to several thousand, it is focal length.

Camera lenses are generally variable aperture, so the focal ratio is a useful
value. Telescopes usually have a fixed aperture and focal length, so the focal
ratio isn't a very useful thing- but it is still used (and misused) an awful
lot.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Hey Chris,
I did not know about the upper and lower case distinction. Thanks for that
bit of information
Duff Couillard

"Chris L Peterson" wrote in message
...
On 27 Nov 2004 10:41:26 -0800, (Vince Z.)
wrote:


Usually, "f" stands for focal ratio, the ratio of focal length to
diameter. "F"
(capitalized) usually is used for the focal length. These conventions are
regularly broken, however. So if you see "f" (or "F") followed by a small
number, say 4-20, it's the focal ratio. If you see it followed by a big
number,
several hundred to several thousand, it is focal length.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Sat, 27 Nov 2004 22:55:37 GMT, "Duff Couillard" wrote:

Hey Chris,
I did not know about the upper and lower case distinction. Thanks for that
bit of information
Duff Couillard

You're welcome. But like I said, don't expect everyone to follow that
convention. Context is usually your best bet.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

(Vince Z.) wrote in message . com...
I'm a newbie here, and a newbie to telescope making, so if I make a
massive faux pas or ask a stupid question, I apologize.
What I'm curious about is what the "f" stands for in relation to the
mirror size. I've seen it used in relation to cameras as well.
Thanks!-

V.

Thanks for all the information. I had an idea it was SOMETHING like
that, but, I wasn't entirely sure. All of my scopes up to this point
have been store bought, so, I've never really had to do any hard
research. Thanks again!

some people use "fl" for focal length because its un-ambiguous.
as in fl = 1000mm d = f =



Duff Couillard wrote:

Hey Chris,
I did not know about the upper and lower case distinction. Thanks for that
bit of information
Duff Couillard

"Chris L Peterson" wrote in message
...
On 27 Nov 2004 10:41:26 -0800, (Vince Z.)
wrote:


Usually, "f" stands for focal ratio, the ratio of focal length to
diameter. "F"
(capitalized) usually is used for the focal length. These conventions are
regularly broken, however. So if you see "f" (or "F") followed by a small
number, say 4-20, it's the focal ratio. If you see it followed by a big
number,
several hundred to several thousand, it is focal length.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com