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Topic: Focal ratio

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	[No title] (Site not responding. Last check: 2007-10-14)
		The focal ratio is found by dividing the focal length by the diameter of teh mirror.
		The focal ratio of the Hale telescope is f/3.3.
		focal length (1) focal ratio = --------------- diameter 154 in = -------- 14 in = 11 ************************ * * For part (a) * the focal ratio = 11 * * * ********************** focal length focal ratio = --------------- diameter substitute 3.3 for focal ratio** and 200 in for diameter.
	www.physicstutor.com /misc/4.html (242 words)

	F Box Experiments
		Focal ratio is the mathematical description of the relationship between focal length and aperture.
		Focal Plane is the image plane where all the light rays from all the aspects of the original distant source come together after passing through a lens or being reflected by a curved mirror to form an image.
		Focal Length is the distance between the center of the lens or curved mirror and the focal point (where the light rays come together.) Focal length is important in telescopes for several reasons.
	hou.lbl.gov /~vhoette/Explorations/OpticalPowers/4-fbox.html (1341 words)

	Celestron -- Telescope Basics
		Most manufacturers specify the focal length of their various instruments; but, if it is unknown and you know the focal ratio you can use the following formula to calculate it: focal length is the aperture (in mm) times the focal ratio.
		This is the ratio of the focal length of the telescope to its aperture.
		The faster the focal ratio, the more coma that will be seen near the edge although the center of the field (approximately a circle, which in mm is the square of the focal ratio) will still be coma-free in well-designed and manufactured instruments.
	www.celestron.com /tb-trms.htm (2665 words)

	Focal Ratio
		The focal length is the length of the light path from the main mirror or lens of a telescope to prime focus (the location of the eyepiece, or a camera for astrophotos).
		Additionally, the focal length of the telescope divided by focal length of the eyepiece you are using results in the magnification.
		Larger focal ratio telescopes are often considered better for planets and double stars, because it is easier to obtain the high powers necessary to view these objects with the "slow" scope.
	www.geocities.com /CapeCanaveral/Lab/3230/focalratio.html (642 words)

	Diffraction Effects (Site not responding. Last check: 2007-10-14)
		Focal ratio describes the relationship between the aperture of a telescope and the focal length.
		Focal ratios of f/8 and larger are described as slow.
		The longer (slower) the focal ratio, the less sharply the light rays must be bent.
	www.newastro.com /newastro/book/C2/focal_ratio.htm (266 words)

	F-number - Wikipedia, the free encyclopedia
		In photography the focal ratio varies the focal-plane illuminance (or optical power per unit area in the image) and is used to control variables such as depth of field.
		The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD.
		In every lens there is, corresponding to a given apertal ratio (that is, the ratio of the diameter of the stop to the focal length), a certain distance of a near object from it, between which and infinity all objects are in equally good focus.
	en.wikipedia.org /wiki/Focal_ratio (2991 words)

	Useful Formulas for Amateur Astronomers
		The focal ratio of a telescope is mostly used when considering exposure time for astrophotography, but it is also a general characteristic of the telescope that can be useful in other discussions.
		focal ratio = focal length of telescope / aperture of telescope
		Example: using a 25mm eyepiece in a telescope with a focal ratio of f/10 results in an exit pupil of 2.5mm (25 / 10 = 2.5).
	www.nexstarsite.com /_RAC/articles/Formulas.htm (930 words)

	An Analysis of the Impact of Focal Ratio on Off-axis Images (Site not responding. Last check: 2007-10-14)
		Also, the shorter ratios place great demands on the ability the user to properly collimate such a mirror so that its true optical center resides in the center of the field of view.
		Inasmuch as we're dealing with coma as the primary aberration it is interesting to note that the impact of coma increases linearly with respect to distance from the center of the field and cubically with respect to focal ratio.
		For any given f ratio and area of the sky to be covered, the actual physical diameter of the diffraction limited area (defined area of aberration) at the focal plane remains the same, even as the telescope grows larger.
	www.rfroyce.com /mirror_performance.htm (2105 words)

	Telescope Numbers (Site not responding. Last check: 2007-10-14)
		Each eyepiece has a focal length (a number followed by "mm" is usually stamped on the eyepiece.) Likewise, each telescope has a focal length, as well as an aperture and a focal ratio.
		Magnification is the focal length of the telescope divided by the focal length of the eyepiece (use the same units, if the eyepiece is specified in mm, then make sure the telescope focal length is also expressed in mm).
		Or, the exit pupil is also equal to the focal length of the eyepiece (in mm) divided by the focal ratio of the telescope (the f number).
	www.bicomnet.com /ritchieobs/pages/numbers.htm (1086 words)

	BirdForum - Optical Question, FOV/Focal Ratio
		Focal length determines image scale, and the shorter the focal length the smaller the object is in the image plane.
		Often the focal length of the telescope is not specified, nor is the diameter of the field stop given.
		The ultimate limit on any particular focal length telescope is the size of the field stop in the eyepiece, and this is limited by the size eyepiece barrel the telescope uses.
	www.birdforum.net /showthread.php?t=26042 (1022 words)

	Telescopes dimensions focal length ratio magnifications
		The distance between the center of the mirror and the point of it being focused by the eyepiece is the focal length.
		A refracting telescope's lens is similar except the light travels through the lens where it is bent by its curvature to the focal length and then magnified by the eyepiece.
		Eyepieces also have focal lengths and these are written in millimetres on the side.
	www.telescopes-astronomy.com.au /telescopes017.htm (187 words)

	How to Choose: Introduction
		The focal length is the distance the light transverses to reach the eyepiece; the focal length is often (not always, like many reflectors, such as cassegrains) the length of the telescope itself—how far the light must travel after entering the lens at the front before it hits the eyepiece.
		The focal ratio is the quotient obtained by dividing the focal length by the diameter of the objective.
		Like focal ratio, aperture size influences binoculars’ ability; however, the influence is on a much smaller scale than a telescope and not used much to categorize binoculars.
	mason.gmu.edu /~jrahm/505/astronomy/howtochooseintro.htm (1090 words)

	ASTRONZ
		Focal Length This is the distance from the front surface of the mirror (or from the plane of the lens for a refractor) at which an imaginary parallel beam of light would be brought to a focus.
		The focal ratio is calculated by dividing the focal length (F) by the aperture (D).
		The alternative long focal ratio telescope brings the light gradually to a focus over a distance which is many times the aperture of the main mirror.
	www.astronomy.co.nz /pub/TelescopeFundamentals.asp (1509 words)

	Focal Reducer Placement (Site not responding. Last check: 2007-10-14)
		The action of the focal reducer is to cause the incoming light rays to converge at a greater angle than they were before entering the reducer.
		To avoid needing to move the telescope’s secondary mirror to achieve focus, the focal reducer must be placed at a position such that its light cone intersects with the native light cone of the telescope.
		The focal reducer must be placed a distance Y in front of the native focal point of the telescope.
	www.rc-astro.com /resources/reducer.html (1124 words)

	Section 2: Telescope making (Site not responding. Last check: 2007-10-14)
		To determine the magnification for your scope, divide its focal length by the focal length of your eyepiece.
		The shorter the focal length, the deeper the curve in the mirror.
		Remember, magnification is the focal length of your scope divided by the focal length of your eyepiece.
	www.starastronomy.org /TelescopeMaking/faq/atmsect2.html (3555 words)

	Telescope Basics
		focal length is the distance that the light in the telescope travels from the objective lens or mirror to the point where that light reaches focus (the focal plane).
		If the focal length of your telescope is not printed on the telescope, you can determine it by multiplying the focal ratio by the aperture.
		If the focal ratio of your telescope is not printed on the telescope, you can determine it by dividing the focal length by the aperture.
	www.synapticsystems.com /sky/scopes/telbasic.html (2191 words)

	Telescope / CCD Equipment (Site not responding. Last check: 2007-10-14)
		For deep sky photography a focal reducer is used to reduce the focal ratio to f6.3.
		The ratio of the focal length to the aperture of the telescope.
		The focal ratio is calculated by dividing the focal length by the aperture.
	www.universebeyond.com /astronomy/astropics/ccd/Hardware.html (1192 words)

	Planetarium.Net Telescope Buying Guide: Focal Ratio
		A telescope’s focal ratio is its focal length divided by the aperture.
		Where the focal length is the distance from the main lens (or mirror) to where the light converges to a focus.
		For example, if a telescope has a focal length of 60 inches and a six-inch aperture, then its focal ration is f/10.
	www.planetarium.net /edcenter/scope/Fratio.htm (270 words)

	The 2.5-meter Telescope (Site not responding. Last check: 2007-10-14)
		delta is the total longitudinal focal deviation from a plane at the indicated angle, ht the height in the focal surface, lindev the deviation from a best-fit straight line relating the input angle to the height, and lin ht that linear relation.
		At each of seven field angles from the center to the extreme edge, the focal properties are given on a surface which represents the average focal surface over the spectrograph wavelength range of 3900 Å to 9200 Å (the spectrograph is described fully in Chapter 7).
		The angular accuracy required for the rotator is reduced from that needed for the axes by the ratio of the telescope focal length to the field radius, a factor of 34.
	www.astro.princeton.edu /PBOOK/telescop/telescop.htm (8813 words)

	Telescope Science: All About Focal Length and Focal Ratio (via CobWeb/3.1 planetlab2.cs.virginia.edu) (Site not responding. Last check: 2007-10-14)
		The optical length of a telescope can be expressed in two ways: As the focal length - the length of the light path to the focal plane in millimeters or as the Focal Ratio - ratio of the focal length divide by the aperture in millimeters.
		The focal length of a telescope is important because that is used to calculate the power or magnification that a given eyepiece will have with a given telescope.
		Frequently, there is a false impression that the surface brightness of an astronomical object is determined by the focal ratio of the telescope.
	telescopescience.com.cob-web.org:8888 /focal_length.html (284 words)

	Telescopes focal ratio dimensions, magnifications aperture
		Having the focal length and the aperture in millimters, a different equation can be done.
		The focal ratio describes how long and wide the telescope is. It could be short and fat (short focal ratio) or long and skinny (long focal ratio) or anywhere in between.
		Focal ratios can range from F/3 to F/20 and beyond these ranges in some extreme cases.
	www.telescopes-astronomy.com.au /telescopes018.htm (107 words)

	Focal ratio - A New Focal Ratio for the 4-m (1Sep92) (via CobWeb/3.1 planetlab2.cs.virginia.edu) (Site not responding. Last check: 2007-10-14)
		Where the focal length is the distance from the main lens (or mirror) to where the
		The aperture is normally expressed as a focal ratio or f stop.
		Focal ratio is the ratio of the focal length of the telescope to its aperture For example, a 6" telescope with a 48" focal length has a focal ratio of 8
	schachtler.okeysearch.com.cob-web.org:8888 /?q=schachtler-focal-ratio (247 words)

	How does "F" ratio affect imaging - IceInSpace Forums
		If the focal reducer can decrease exposure time how does this provide more light to the chip.????..the light path distance is still the same as F10 but now has to futher travel through another lense such as a Focal reducer.
		A focal reducer that halves the f ratio of the telescope is actually doing the opposite of a 2x barlow.
		Ultimately the focal ratio would have to be one of the most important aspects when imaging.
	www.iceinspace.com.au /forum/showthread.php?t=14179 (2078 words)

	focal ratio
		The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number, f/# = F/D.
		Borrowing the language of photography, small focal ratios, below about f/6, are said to be fast and result in a brighter image for a given aperture.
		Large focal ratios, equal to or above about f/8, are said to be slow.
	www.daviddarling.info /encyclopedia/F/focal_ratio.html (133 words)

	Focal Ratio (Site not responding. Last check: 2007-10-14)
		The focal ratio of an optical system is defined as the focal length divided by the aperture.
		So, a telescope with a focal length of 600mm and a 60mm aperture has an f-ratio or f-number, of f/10.
		High focal ratios of f/11 and up are ideal for planetary observations and splitting double stars, but yield lower image brightness for a given focal length of eyepiece and a narrower field.
	www.twcac.org /Tutorials/focal_ratio.htm (131 words)

	BirdForum - Foacl ratio of binoculars (Site not responding. Last check: 2007-10-14)
		A scope or the binoculars which have the objective and the eyepiece (ocular) do not have a focal length - they are afocal - they only have magnification.
		The focal ratio is also slightly different from normal photography.
		You actually get the focal ratio or f-number directly from the camera if the exit pupil of the binoculars exceeds the "aperture" of the camera lens.
	www.birdforum.net /printthread.php?t=33513 (256 words)

	"Purchasing A Telescope" Project by Josh Baxley - The Telescope of Choice Page (Site not responding. Last check: 2007-10-14)
		This telescope has a 4" aperture, length of 32", and a f/9 focal ratio.
		I need to know both the focal length of both the telescope and the eyepeice.
		I chose this telescope mainly for its aperture size, becuase this is the root of the light gathering power.
	www.isomedia.com /homes/joshb/scope/scopes.html (314 words)

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