|
#1
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
over kill and confusing.
I thought it was usefull |
#5
|
|||
|
|||
Mook wrote in :
over kill and confusing. It was certainly better than YOUR answer. |
#6
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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
|
|||
|
|||
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 |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Archimedes Number - Erratum | Ralph Hertle | Misc | 0 | November 7th 04 02:26 AM |
PDF (Planetary Distance Formula) explains DW 2004 / Quaoar and Kuiper Belt | hermesnines | Astronomy Misc | 10 | February 27th 04 03:14 AM |
New Solar System Model that explains DW 2004 / Quaoar / Kuiper Belt and Pluto | hermesnines | Misc | 0 | February 24th 04 09:49 PM |
Shenzhou has landed | Rick DeNatale | History | 74 | October 25th 03 07:23 PM |
Electric Gravity&Instantaneous Light | ralph sansbury | Astronomy Misc | 8 | August 31st 03 02:53 AM |