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  #1  
Old November 20th 04, 03:14 AM
Eric
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Default F number

I'm trying to get my head around how the f number affects things

Can someone fill this in for me?

Given all else remains the same...

low F ---------- vs ----------- hi F

Thanks
Eric

  #2  
Old November 20th 04, 03:34 AM
Sam Wormley
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Eric wrote:
I'm trying to get my head around how the f number affects things



Background References
http://scienceworld.wolfram.com/physics/f-Stop.html
http://hyperphysics.phy-astr.gsu.edu...lenseq.html#c1

The effect of atmospheric seeing on telescopes of differing focal ratios
http://www.fpi-protostar.com/bgreer/...ges/seeing.htm

How Telescopes Work
http://science.howstuffworks.com/telescope19.htm

Focal Ratio (f/number)

"Focal ratio or f/number relates to the brightness of the image and the
width of the field of view. The focal ratio is the focal length of the
objective lens or primary mirror divided by the aperture. The focal
ratio concept comes from the camera world, where a small focal ratio
means a short exposure time for the film, and was said to be "fast."
Although the same is true for a telescope, if a "fast" and a "slow"
telescope are compared at the same magnification for visual rather than
photographic viewing, then both telescopes will have the same quality
image. Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars (high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "

  #3  
Old November 20th 04, 08:29 AM
Mook
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over kill and confusing.




Sam Wormley wrote:

Eric wrote:
I'm trying to get my head around how the f number affects things


Background References
http://scienceworld.wolfram.com/physics/f-Stop.html
http://hyperphysics.phy-astr.gsu.edu...lenseq.html#c1

The effect of atmospheric seeing on telescopes of differing focal ratios
http://www.fpi-protostar.com/bgreer/...ges/seeing.htm

How Telescopes Work
http://science.howstuffworks.com/telescope19.htm

Focal Ratio (f/number)

"Focal ratio or f/number relates to the brightness of the image and the
width of the field of view. The focal ratio is the focal length of the
objective lens or primary mirror divided by the aperture. The focal
ratio concept comes from the camera world, where a small focal ratio
means a short exposure time for the film, and was said to be "fast."
Although the same is true for a telescope, if a "fast" and a "slow"
telescope are compared at the same magnification for visual rather than
photographic viewing, then both telescopes will have the same quality
image. Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars (high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "


  #4  
Old November 20th 04, 02:57 PM
HAVRILIAK
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over kill and confusing.


I thought it was usefull
  #5  
Old November 20th 04, 03:03 PM
Paul Lawler
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Mook wrote in :

over kill and confusing.


It was certainly better than YOUR answer.
  #6  
Old November 20th 04, 03:46 PM
Bill Ferris
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Unfortunately, Sam Wormley quoted the following from website:
Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars
(high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "


It's unfortunate because the above statements tend to perpetuate some
pernicious myths of visual observing. One, is that long f/ratio telescopes
aren't well-suited to deep-sky observing. Another, is that short f/ratio
scopes aren't suited to planetary observing. And a third, is that deep-sky
observing is best done at low power. None of the above could be less true.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond

  #7  
Old November 20th 04, 04:46 PM
Sam Wormley
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Bill Ferris wrote:
Unfortunately, Sam Wormley quoted the following from website:

Generally, the following information about focal ratios can be
helpful:

* f/10 or higher - good for observing the moon, planets and double stars
(high power)
* f/8 - good for all-around viewing
* f/6 or lower - good for viewing deep-sky objects (low power) "



It's unfortunate because the above statements tend to perpetuate some
pernicious myths of visual observing. One, is that long f/ratio telescopes
aren't well-suited to deep-sky observing. Another, is that short f/ratio
scopes aren't suited to planetary observing. And a third, is that deep-sky
observing is best done at low power. None of the above could be less true.

Regards,

Bill Ferris
"Cosmic Voyage: The Online Resource for Amateur Astronomers"
URL: http://www.cosmic-voyage.net
=============
Email: Remove "ic" from .comic above to respond


That's a fair criticism, Bill. The page I quoted strayed from a mathematical
description to one based on historical experience with instruments *not* of
superior quality. High quality optics should perform well independent of the
focal length to aperture ratio.

  #8  
Old November 20th 04, 06:08 AM
Mark Smith
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As a long time photographer, I had a hard time with this. In my mind,
lower F numbers meant a faster lens (you get more light to the film
plane). Unfortunately, that only kind of works with telescopes.

If you are going to use your telescope as a camera lens, you CAN use
the F number the same way as you do for cameras. However, that
doesn't really tell the whole number.

The real key for telescopes is aperature. Aperature is what allows
you to suck in light and, generally speaking, more aperauter = dimmer
objects seen.

For two telescope with equal aperatures:

A Higher F number means longer Focal Length.

A Higher F number means narrower FOV.

A Higher F number means a less radically curved primary.

A lower F number (especially in reflectors) MAY mean better eyepieces
are required to combat coma problems near the edge of the FOV.

A Higher F number will mean more radical jumps in magnification per mm
reduction in eyepiece focal length. This is NOT necessarily good. It
allows you to get insane magnifications if you desire, but you often
do not desire large magnification unless you are looking at planets or
the moon.

Hope that helps a little.
  #9  
Old November 20th 04, 06:18 AM
SaberScorpX
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The real key for telescopes is aperature. Aperature is what allows
you to suck in light and, generally speaking, more aperauter = dimmer
objects seen.


Aperture.
Aperature I can forgive. But not aperauter

SSX
  #10  
Old November 20th 04, 03:15 PM
Jon Isaacs
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My thinking:

F ratio is an important factor in astronomical scopes. But it can be confusing
and just about every rule of thumb has plenty of contradictions.

One rule of thumb that is often wrong:

A slow focal ratio scope has a less curved primary mirror. True for standard
Newtonians but SCTs and MAKs use fast primaries (F2 or so) and a magnifying
secondary.

But in general, focal ratio does have some real meaning. It is an indication
of potential aberrations, the difference between an F4 and an F6 Newtonian or a
F6 and an F9 achromat is big...

But the most important factors are that it is an indication of maximum possible
FOV and more importantly maximum exit pupil.

For example, the limitations of the ETX-125 are easily described by pointing
out the 1.25 inch focusr and F15 focal ratio. This means max FOV will be
around 0.8 degees with a 2 mm exit pupil...

Jon


 




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