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F number



 
 
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  #1  
Old November 20th 04, 02:14 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

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  #2  
Old November 20th 04, 02:34 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, 03:22 AM
Alan French
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F number of f/ratio

That should have been "F number or f/ratio..."

  #4  
Old November 20th 04, 05:08 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.
  #5  
Old November 20th 04, 05:18 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
  #6  
Old November 20th 04, 07:29 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) "


  #7  
Old November 20th 04, 01:25 PM
Alan French
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"eric" wrote in message
...
Hi Eric,

Low F is more bright, and Hi F give more contrast. [SNIP]


The image brightness depends on aperture, not f/ratio. F/ratio has no
relation to contrast, although an obstructed instrument of high or "slow:
f/ratio is likely to have a smaller obstruction, resulting in a slight
increase in contrast.

Clear skies, Alan

  #8  
Old November 20th 04, 01:57 PM
HAVRILIAK
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over kill and confusing.


I thought it was usefull
  #9  
Old November 20th 04, 02:03 PM
Paul Lawler
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Mook wrote in :

over kill and confusing.


It was certainly better than YOUR answer.
  #10  
Old November 20th 04, 02:04 PM
HAVRILIAK
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As a long time photographer, I had a hard time with this. In my min

I think in the preceeding discussion there has been a lot of confusion
between point objects such as stars and extended objects such as nebulae.
 




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