Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 25: Telescopes

Key Ideas:

Light Gathering Power

The standard measure of a telescope is its Light Gathering Power:: The total collecting area of the telescope; Bigger Area = more light gathered
Express telescope size by its Diameter: Light Gathering Power increases as the square of the diameter of the primary collecting optic

Refracting Telescopes

Use lenses to gather light: Large Objective Lens; Smaller secondary lenses for eyepieces, cameras, or spectrometers
Most common telescope design before the 1900s
Size limited by lens mass: Lenses >40 inches in diameter sag under their own weight; Long tubes tend to flex and must be very massive to support the heavy lenses
Largest ever built is the 40-inch Yerkes Refractor

Reflecting Telescopes

Invented by Isaac Newton
Use curved mirrors to gather and focus light: Large Primary Mirror gathers and focuses light; Secondary Mirrors direct the light to instruments mounted on the telescope
Can support the mirror from the back: Permits the construction of very large telescopes; Can mount large instruments easily
All modern research telescopes are reflecting telescopes

Mountain-Top Observatories

Telescopes need remote, special locations: Dark skies, far from large cities; Clear, dry weather (at least) most of the year; Good "seeing" (steady atmosphere to reduce twinkling and smearing of images)
Best sites are high, dry mountain peaks: Chilean Andes near the Atacama Desert; Mauna Kea on Hawaii; Arizona (Kitt Peak, Mt. Hopkins, Mt. Graham)

The Largest Telescopes

10-meter Keck 1 and II (Caltech, UC, NASA)
8.2-meter SUBARU (Japan)
8.1-meter GEMINI Telescopes: US, UK, Canada, South America, and Australia; One in Hawaii, one on Cerro Pachon, Chile
8-meter Very Large Telescopes (European): 4 telescopes on Cerro Paranal, Chile
6.5-meter Magellan Baade and Clay Telescopes: 2 telescopes at Las Campanas Observatory, Chile
Twin 8.4-meter Large Binocular Telescope: Ohio State, Arizona, Italy, Germany, and the Research Corporation

Radio Telescopes

Use antennas to detect cosmic radio waves: Radio emission-lines of Hydrogen and Molecules (e.g. CO) from cold interstellar gas.; Radio continuum emitted by hot electrons, or electrons accelerated by strong magnetic fields
Interferometry: Trick of connecting many smaller radio telescopes to synthesize a large single dish (aperture synethsis)

Space Telescopes

Only radio, visible, and some infrared light can penetrate the Earth's atmosphere
Need to go into space for:: Mid to far-infrared; Ultraviolet; X-rays and Gamma-rays
Get above the atmosphere and weather
VERY EXPENSIVE (billions of dollars) to build and operate

Tools of the Astronomer

Modern telescopes use sensitive digital instruments to analyze light: Digital cameras (CCD detectors); Infrared cameras (IR sensor arrays); Optical and infrared spectrographs
Ohio State is one of the world's leading builders of advanced telescope instruments: Cameras and spectrographs on 10 telescopes; Large Optical Spectrographs for the LBT

Large Binocular Telescope (LBT)

Twin 8.4-meter mirrors on a single mount: Equivalent to a single 11.8-meter telescope; Largest optical/IR telescope yet built
LBT Partners:: University of Arizona (1/4); Italian National Astrophysics Institute (1/4); German Astrophysical Institutes (1/4); The Ohio State University (1/6); Research Corportation (1/12)

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.