Astronomy Chapter 3

Astronomy Chapter 3 - Introduction to Astronomy PHYS 101Y...

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Introduction to Astronomy PHYS 101Y Instructor: Prof. Igor Shovkovy Lecture notes. Part 3.
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Telescopes Telescopes aid our eyes , enabling us to see to much greater distances that are accessible to the naked eye alone The Gemini South Telescope in Chile (top, left) and the Gemini North Telescope in Hawaii (right) Telescopes also allow us to perceive cosmic objects that emit radiation at wavelengths to which we are blind
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Refracting Optical Telescopes Refracting lens : All light rays arriving parallel to the lens axis are refracted to pass through a single point, called focus
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Reflecting Optical Telescopes Images can be formed through reflection or refraction Reflecting mirror : All light rays arriving parallel to the mirror axis are reflected to pass through a single point, called again focus (Focus of the primary mirror is called the prime focus )
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How images are formed? Image formation: Light rays from slightly different directions are focused to slightly different points
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Comparison of Optical Telescopes Reflecting and refracting telescopes :
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Refraction of Light Different colors of light are distinguished by their frequency and wavelength . This is responsible for the chromatic aberration (i.e., focusing different colors differently), the deficiency of all refracting telescopes Note: 1 nm = 10 -9 m
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Absorption of Light Even best quality glass absorbs some light… The absorption is a relatively minor problem for visible radiation, but it can be severe for infrared and ultraviolet observations. This problem obviously does not affect mirrors
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Advantages of Reflecting Telescopes 1. Light traveling through lens is refracted differently depending on wavelength (aka chromatic aberration ) 2. Some light traveling through lens is absorbed 3. Large lens can be very heavy , and can only be supported at edge 4. Lens needs two optically acceptable surfaces, mirror only needs one Modern telescopes are all reflectors because of the following problems with the refractors:
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Telescope Designs Types of reflecting telescopes : Nasmyth focus
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Example of a Modern Telescope Details of the twin 10-m-
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Astronomy Chapter 3 - Introduction to Astronomy PHYS 101Y...

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