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Astronomy 114 Lecture 26: Telescopes Martin D. Weinberg [email protected] UMass/Astronomy Department A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—1/17 Announcements Quiz #2: we’re aiming for this coming Friday . . . A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—2/17 Announcements Quiz #2: we’re aiming for this coming Friday . . . Today: Optics and Telescopes Optics and Telescopes, Chap. 6 Tomorrow: Galaxies Galaxies, Chap. 26 A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—2/17 Telescopes What is a telescope? Collects light Focuses (concentrates) the photons onto a dectector Goals for telescopes Make sensitive observations ⇒ distant objects Make BIG telescopes Resolve small details on the sky ⇒ distant or nearby objects Angular resolution A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—3/17 Types of telescopes Refracting Focuses light through a lens. Examples: Camera lens Magnifying glass or eye glasses Reflecting Focuses light by reflecting light and changing its path in a coordinated way Examples: shaving or make-up mirror A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—4/17 Refraction (1/2) A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—5/17 Refraction (1/2) Demo A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—5/17 Refraction (1/2) A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—5/17 Refraction (2/2) Parallel light rays from a distant point come together at a focus point. Parallel rays from another distant point come together at a different focus point in the same plane ("focal plane"). A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—6/17 Refraction (2/2) Parallel light rays from a distant point come together at a focus point. Parallel rays from another distant point come together at a different focus point in the same plane ("focal plane"). A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—6/17 Refraction (2/2) A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—6/17 Refraction (2/2) Magnification = A114: Lecture 26—17 Apr 2007 Focal length of objective Focal length of eyepice Read: Ch. 6,26 Astronomy 114—7/17 Refraction (2/2) Magnification = Focal length of objective Focal length of eyepice Focal length of objective Diameter of objective Smaller f-ratio → more light reaches focal plane Focal ratio = A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—7/17 Refracting telescope Goal of objective lens is to collect as much light as possible Simple design, still used for small amateur telescopes Limitations: Bending depends on wavelength, focal point depends on color! Lens must be supported at edges. Heavy glass lens sags under its own weight, distorts optics. A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—8/17 Reflection Law of reflection A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—9/17 Reflection Use reflection to focus light. Invented by Newton. A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—9/17 Reflecting telescope A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—10/17 Reflecting telescope Largest refracting telescopes: 1-m primary lens Largest reflecting telescopes: 8-10 m primary mirror Mirror usually polished glass with thin layer of aluminum Can support mirror from behind to prevent sagging Nearly all modern telescopes are reflectors A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—10/17 Angular resolution Angular resolution–ability to see structure Limited by the wavelike properties of light Image of a point of light is spread by the edge of the objective mirror or lens A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—11/17 Angular resolution Angular resolution–ability to see structure Limited by the wavelike properties of light Image of a point of light is spread by the edge of the objective mirror or lens A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—11/17 Angular resolution Angular resolution–ability to see structure Limited by the wavelike properties of light Image of a point of light is spread by the edge of the objective mirror or lens A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—11/17 Angular resolution Angular resolution–ability to see structure Limited by the wavelike properties of light Image of a point of light is spread by the edge of the objective mirror or lens Radius of disk: θ = 1.22λ/D radians Points closer in angle than this are merged Examples: Human eye (0.2 cm pupil): about 1 arc minute 10 cm (4-inch) telescope: about 1 arc second 2.4 m (HST) telescope: about 0.05 arc second 10 m (Keck) telescope: about 0.01 arc second A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—11/17 Atmospheric seeing Blurring by air currents in atmosphere usually smears images over several arc-seconds ⇒ seeing Best sites: 0.5-1 arc second Atmospheric seeing limits the angular resolution of ground-based optical telescopes Solutions: 1. Put telescope in Earth orbit (hard, $$$) 2. Adaptively bend mirror to compensate for atmospheric motions (hard, $$$) A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—12/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Made of heavy metals Reflect the rays just a few degrees Mirrors are rotated parabolas and hyperbolas A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Gamma-ray telescopes give up on focusing entirely Use coded aperture masks The pattern of shadows the mask creates can be reconstructed to form an image A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 X-ray and gamma-ray telescopes High-energies photons interact with most materials X-ray telescopes use ring-shaped "glancing" mirrors Gamma-ray telescopes give up on focusing entirely Use coded aperture masks The pattern of shadows the mask creates can be reconstructed to form an image Satellites and balloons . . . why? A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—13/17 Transmittance of the Earth’s atmosphere A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—14/17 Detectors (1/3) First detector: human eye Use secondary lens ("eyepiece") to make converging rays parallel again Limitations: Short exposure time (1/30 second) Have to rely on observer’s description or drawing A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—15/17 Detectors (2/3) Better: film or photographic plates Put in focal plane, so extended image forms on plate Incoming photons cause permanent chemical change Long exposure ⇒ increased sensitivity Permanent record Inefficient: best emulsions miss 99% of incoming photons A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—16/17 Detectors (3/3) State of the art: electronic detectors (CCDs) Incoming photons knock electrons out of silicon Electrons counted electronically Excellent efficiency: up to 80% of incoming photons counted Digital data, easily analyzed by computers CCDs have revolutionized optical astronomy (since c. 1980) In digital cameras and camcorders A114: Lecture 26—17 Apr 2007 Read: Ch. 6,26 Astronomy 114—17/17