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Stuff about telescopes, binos and spotting scopes: (Yes, you can expect test questions...) Magnification: The first number of a binocular or spotting scope specification indicates magnification. The "X" following this number is an abbreviation for magnification (power). (Example: 7 X 35, the "7" is the magnification). Similarly, a spotting scope that magnifies twenty times makes the object appear twenty times closer. The resolving ability of the spotting scope's optical system must be wqual to that of your eye for this rule to apply. An inferior optical system will not allow or produce optimal results. To calculate magnification, simply divide the telescope focal length (given on the front of the telescope of shown below) by the eyepiece focal length marked on the telescope eyepiece. Information you may need for calculating magnificaton on other telescopes: The Little Thompson Observatory telescope: - Aperature (F-Stop) = f/14 - Diameter of the primary mirror = 18 inches - Focal Length = F-Stop times diameter of the light collecting area or (14)*(18)*(25.4)mm = 6400.8 mm Telescopes at Stargazer: Telescope in the dome - Celestron Schmidt-Cassegrain with a 14 inch mirror - Focal length of 3910 mm - Aperature of F/11 The smaller, portable telescopes - The 6" telescope FL = 1200 mm (F/8 refractor) - The 8" telescope FL = 2000 mm - The 9.25" telescope FL = 2350 mm - The 11" telescope FL = 2800 mm Calculations: How Powerful is it? Telescope Magnification = The focal length of the telescope divided by the focal length of the eyepiece. For Example: A telescope with a 1,000 mm focal length using a 30 mm eyepiece would give a magnification factor of 33X ( 1000 / 30 = 33 ) A telescope with a 1,000 mm focal length using a 9 mm wide-angle eyepiece has a magnification factor of 111 X ( 1000 / 9 = 111 ) To calculate magnification from mirror size and focal ratio, multiply the focal ratio and multiply by a conversion factor (25.4) to change inches into millimeters. Then calculate the magnification as usual. Calculating the aperature or f-stop of the big telescope in the dome at Stargazer Observatory: Information on Celestron telescopes. Information on Meade telescopes. CALCULATING EXIT PUPIL: The diameter of the objective lens ( in mm ) divided by the power of the eyepiece. e.g. In a 11 x 80 binocular: 80 mm / 11 = 7.27 mm * The EXIT PUPIL of a telescope is the circular beam of light that leaves the eyepiece being used. It is measured in millimeters. * An 8 inch aperture telescope (203 mm) used with a 20 mm eyepiece is working at 102 power and has an exit pupil of 2 mm (203/102) = 2 mm * You can also calculate the exit pupil by dividing the focal length of the eyepiece (in mm) by the focal ratio of the telescope... Objective Lens Diameter (Aperture): The second number (7 X 35) tells you the diameter (in millimeters) of the front lens or lenses. Large objectives generally give you brighter images than smaller apertures at any given power in low light viewing. A general rule for a binocular is 5mm of objective diameter for each power, and for spotting scopes, 2mm of objective diameter or aperture. This will produce an exit pupil of 5mm for the 7 X 35mm binocular for ease of use and twilight viewing. Exit Pupil: The human eye pupil dilates from about 1mm in bright light to about 5mm in near-darkness. Exit pupil in a binocular or spotting scoep is the effective diameter of light (in mm) exiting the unit. It is calculated by dividing the objective lens diameter by the magnification. (e.g. 7 X 35 binocular should have a 5mm exit pupil 35mm divided by 7 = 5mm) A spotting scope is most effective when the exit pupil is 2mm or larger. Not all brands of binocular utilize the full objective lens diameter, therefore will not produce the calculated exit pupil value. Twilight Factor: The term is a calculation to determine low light viewing efficiency. It relates to the exit pupil size which should be no smaller than 5mm for optimum performance in low light. Comparison here is a must as numerous other factors contribute to light transmission. Optical design, optical glass, anti-reflection coatings and construction all play an important part. Eye Relief: The distance between the rear (ocular) lens and your eye necessary to see the full image is eye relief. Proper optical design will provide sufficient eye relief to comfortably use with eyeglasses. Eyecups, which will fold down or twist down for eyeglass wearers, aid in holding the binocular or spotting scope at the proper eye relief. Center Focus: This mechanism will focus both sides of the binocular simultaneously. Due to convenience and speed, most modern binoculars feature center focusing. This is accomplished by turning the focusing wheel between the two binocular barrels. Focus Lock: A focus lock will incorporate a feature which allows the user to lock in the adjustment that is perfect for his or her viewing situation and eyesight. This is a much better situation than focus-free brands of binoculars. Prisms: Binoculars and spotting scopes use prisms to magnify and transmit light to the eye. Two types of prisms are used, porror prism and roof prism. Roof prisms allow the design to be more lightweight and compact. Porro prisms result in a longer light path and spaces the objectives further apart than your eyes to provide more three-dimensional images. The actual glass material used to make either type prism is vry important to light transmissions. Collimation: The optics in both binocular barrels must be precisely aligned internally and externally to produce one image. Proper alignment reduces eye fatigue and eliminates headaches caused by your eyes straining to accommodate incorrect collimation.
How Powerful is Practical? The maximum practical magnification of a telescope is 50 to 60 times the aperture of the telescope under ideal conditions... In other words, if you own a telescope with a 4 inch diameter front lens,( The Aperture ) the maximum practical magnification is 200 to 240X In actual use, 20 to 35X may be the most useful magnification... FOCAL LENGTH: The distance in an optical system from the lens to the point where the unit is in focus. The longer the Focal Length of the telescope, th larger the image. APERTURE: The Diameter of the Objective Lens. OBJECTIVE LENS: The front element or lens of the telescope. EYEPIECE LENS: The lens you look through... LIGHT GATHERING ABILITY: The major function of a telescope is to Collect Light. FIELD OF VIEW: The size ( in degrees ) of the area you can see when looking through the lens of a telescope or optical instrument. e.g., the width in feet of the viewing area you would see ( linear ) with binoculars. * The larger the field of view, the larger the area you see in the eyepiece. * The greater the magnification, the smaller the field of view... TWILIGHT FACTOR: Measures viewing efficiency and image detail in Twilight conditions. It is obtained by multiplying the square root of the power by the instrument's objective in mm. The larger the TWILIGHT FACTOR, the more efficient the instrument's dim light performance. ANGULAR FIELD OF VIEW: The apparent field of view of the eyepiece divided by the magnification being used at the time... (To calculate the linear field, multiply the angular field of view by 52.5 ) FOCAL RATIO: (F / STOP) The Focal Length of the telescope divided by the diameter of the objective lens ( in mm ) * The Smaller the F/Stop, the less light gathered by the instrument. * The LARGER the F/Stop, the greater amount gathered by the instrument... * e.g., F/4 lets in twice the amount of light pass through the lens as F/5.6 does, and FOUR TIMES the amount of light as F/8 does...
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