#1
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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
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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
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F number of f/ratio
That should have been "F number or f/ratio..." |
#4
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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
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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
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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
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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
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over kill and confusing.
I thought it was usefull |
#9
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Mook wrote in :
over kill and confusing. It was certainly better than YOUR answer. |
#10
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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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