Telescopes - Telescopes Lowell 24-inch Telescope Keck 10-m Telescope Manipulate Light In order for a telescope to be advantageous it must

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Unformatted text preview: Telescopes Lowell 24-inch Telescope Keck 10-m Telescope Manipulate Light In order for a telescope to be advantageous, it must manipulate light in a way that is beneficial to the observer. Optimal designs give the observer (1) a magnified view (so that it appears closer), (2) a brighter view (of the dim object), and (3) a high-resolution view (so fine details can be seen). There are two ways to manipulate light to our advantage: Refraction and Reflection Refraction Uses a lens to manipulate light. p g Focal Length is the distance from the lens to the focus. focus Refraction Focal Length is the same for either side of the lens. Refracting Telescope 1. Magnification Magnification occurs because the angular size of the image on the eye is much larger than the angular size of the object. Note that the image is upside down. Magnification = Focal Length of Objective / Focal Length of Eyepiece M = F / f PRS Question 1. Calculate the magnification for this telescope: Objective lens: diameter = 1 m; focal length = 2 m. Eyepiece: diameter = 5 cm; focal length = 20 mm. a. 10 X b. 20 X c. 100 X d. 200 X Reflection Uses a mirror to manipulate light. light Angle of Incidence = Angle of Reflection But flat mirrors do not have a focal length. Reflection Must use a curved (spherical) mirror to bring the light to a focus. The focal length is half the radius of curvature. Where d Wh do you put the eyepiece so t th i that the object is not obscured by your head? Reflecting Telescopes Reflecting Telescopes 1. Magnification Magnification = Focal Length of Primary Mirror / Focal Length of Eyepiece M = F / f (Same equation as for Refractors) 2. Light Gathering Ability Proportional to the Area (The Bigger, the Better) Refractors: Objective Lens Reflectors: Primary Mirror y Area = R2 LGA R2 LGA D2 PRS Question 2. Compare the light gathering capability of a 50-inch to a 10-inch telescope. a. 0.2 X b. 5 X c. 25 X d. 2500 X 3. Resolving an Image Resolution Resolution is a how small an angle the telescope can detect. It is a function of the wavelength observed and the diameter of the telescope. = (1.22) (206265) / D = 2.5 x 105 / D is in arcseconds Resolution = 2.5 x 105 / D Example: Georgia Tech 16-inch telescope 16 inch = 2.5 x 105 (500 x 10-9 m) / (0.4 m) = 0.31 arcsec The atmosphere limits all telescopes to a resolution of ~1.0 arcseconds. Formulae Magnification M=F/f Light Gathering Ability LGA R2 (or D2 ) Resolution = 2.5 x 105 / D 25 Chromatic Aberration Different Wavelengths are brought to a Different Focal Point. The solution is to add different types of material and different shapes together. (Only affects Refracting Telescopes.) Spherical Aberration Different Reflections are brought to a Different Focal Point. The solution is to use a Parabolic Mirror. (Only affects Reflecting Telescopes.) Comparison Lens Mirror Two sides to grind and polish One side to grind and polish g p Lens Mirror Light must travel through the glass Light only interacts with the surface g y Lens Mirror Only supported around the edge Supported on back and sizes; therefore, larger Formulae Magnification M=F/f Light Gathering Ability LGA R2 (or D2 ) Resolution = 2.5 x 105 / D 25 ...
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This note was uploaded on 03/04/2012 for the course PHYS 2022 taught by Professor Jarrio during the Spring '12 term at Central GA Tech.

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