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