astro2 lecppt ch05

Chapter 5Lecture Outline: 

Chapter 5 Lecture Outline The Tools of the Astronomer

Telescopes: 

Telescopes Gather light for analysis. Different telescopes capture different wavelengths (optical, X-rays, etc.). Bigger telescopes gather more light. There are also “telescopes” for atomic particles (protons, neutrinos).

Optical Telescopes: 

Optical Telescopes Reflecting telescopes use mirrors. Refracting telescopes use lenses. The eye is a refracting telescope. Aperture: area over which light is collected. (larger aperture gathers more light). Focal length: distance between mirror (or lens) and the image. (Longer = larger image.)

The Eye: 

The Eye

A Reflecting Telescope: 

A Reflecting Telescope

Resolution: 

Resolution Resolution = smallest details that can be separated. Resolution is the angle between two objects that are just barely distinguishable. Diffraction sets the best possible resolution. Diffraction = bending of light around telescope optics.

The Diffraction Limit: 

The Diffraction Limit In arcseconds, the resolution is where  is the wavelength observed and D is the diameter of the telescope. The value of  is called the diffraction limit. For visible light and D = 3m,  = 0.04 arcseconds. For a particular diameter, the resolution is better at shorter wavelengths. For a particular wavelength, larger telescopes produce better resolution.

Seeing: 

Seeing Blurring in the atmosphere degrades the resolution. Seeing = size of image produced. Typically, seeing = 0.5 to 1 arcsecond. Space-based telescopes do not have this problem. Some ground-based telescopes have adaptive optics to improve the seeing.

Adaptive Optics: 

Adaptive Optics Keck Observatory

Optical Windows: 

Optical Windows The atmosphere does not transmit all wavelengths of light. Many important wavelengths (e.g., ultraviolet, X-ray) do not reach the ground. Satellites are needed for these wavelengths.

Blockage by the Atmosphere: 

Blockage by the Atmosphere Photos: (EM spectrum and telescopes, Kitt Peak) NOAO/AURA/NSF; (Compton) NASA; (Chandra) NASA/CXC/SAO; (Fuse) Graphic courtesy of Orbital Sciences Corp.; (SIRFT) NASA/JPL/Caltech; (JCMT) Joint Astronomy Center in Hilo, Hawaii; (VLA) NRAO VLA Image Gallery.

Capturing the Light: 

Capturing the Light Astronomers don’t look through telescopes. Electronic detectors record the light. CCDs = Charge-Coupled Devices (as in digital cameras for home use). Two modes: Direct imaging (pictures). Spectroscopy (light spread out by wavelength).

Direct Imaging: 

Direct Imaging (a) Courtesy of Ed Grafton, (b) Steve Larson / University of Arizona

Spectroscopy: 

Spectroscopy Howard Voss

Radio Telescopes: 

Radio Telescopes For observations with  > 10 cm. Telescopes can be very large, but still have low resolution because of large . Can increase resolution by combining many telescopes into an interferometric array. Resolution is like that of a telescope many km across.

The Very Large Array: 

The Very Large Array Image courtesy of NRAO / AUI

Other Detectors: 

Other Detectors In a few cases, astronomers capture things other than light: Neutrinos – light subatomic particles. Elementary particles – protons and electrons. Gravity waves – distortions in spacetime predicted by relativity.

Getting Above the Atmosphere: 

Getting Above the Atmosphere Balloons and aircraft: raise the telescope above clouds and water vapor (infrared astronomy). Satellites: Detect wavelengths that the atmosphere blocks (ultraviolet and X-rays). Can produce very sharp images (e.g., the Hubble Space Telescope).

Going There: 

Going There Spacecraft have visited most of the planets. Mission types: Flyby. Spacecraft passes by the planet one or more times. Orbiters. Spacecraft enters into orbit for extended observations. Landers / atmospheric probes. Direct samples of atmospheres or surfaces. Sample return. Material returned to Earth for analysis.

Planetary Explorers: 

Planetary Explorers NASA / JPL NASA / JPL

Other Tools: Physics Experiments: 

Other Tools: Physics Experiments Laboratory measurements find the behavior of matter under various conditions. Particle accelerators probe physics of protons, neutrons, neutrinos. These give insight to the conditions inside stars or during the Big Bang.

Other Tools: Computers: 

Other Tools: Computers Computers are essential for astronomy. Image processing and data analysis. Theoretical calculations (astrophysics). Publication of results in professional journals. Creation of large databases on the Internet. Greatest insight comes from the interaction of observation and theory.

A Numerical Simulation: 

A Numerical Simulation Frank Summers, Space Telescope Science Institute; Chris Mihos, Case Western Reserve University; Lars Hernquist, Harvard University

Concept Quiz – Resolution : 

Concept Quiz – Resolution A space-based telescope’s resolution is set by diffraction. At what wavelength would it produce the sharpest images? Optical Ultraviolet Infrared

Concept Quiz – Bigger Telescopes: 

Concept Quiz – Bigger Telescopes Why do astronomers want to build bigger telescopes? To eliminate the effects of seeing. To search for life on Mars. To observe fainter or more distant objects.

Concept Quiz – Telescopes : 

Concept Quiz – Telescopes Pick the incorrect answer: Why build telescopes on the ground when we can launch them into space? Ground-based telescopes are useful for studying visible and infrared light. Ground-based telescopes get much higher resolution. They are far cheaper to build and fix than space-based telescopes.

Geometric Optics and Lenses : 

Geometric Optics and Lenses Animation: Sections 5.1 Click to launch: ► PC version ► Mac Version