Telescopes Telescopes Key Concepts 1) A refracting telescope uses a lens to gather light; a reflecting telescope uses a mirror. ) The main purposes of a telescope are to gather light and resolve fine detail. 3) Bigger is better. (For telescopes, at least.) We learn about the universe by gathering light from distant objects. Our eyes are good, but not perfect, at detecting light. Some objects emit light (stars, Sun); others reflect light (planets, Moon). Our vision is blurred on small scales. We can t see faint sources. We can t see ultraviolet, infrared, etc. Telescopes ( far lookers ) remedy some of our eyes problems. A refracting telescope uses a lens to gather light. Light is bent (or refracted ) when going from air to glass (or vice versa). glass Galileo s telescope (early 17 th century) revolutionized astronomy & cosmology. 1
Blue light is bent through a larger angle than Red light. A convex lens (thick in the middle) focuses light to a point: This is why prisms spread light into a spectrum. from star Light from a large area is funneled into a small area. Galileo made a refracting telescope, with an added lens to act as an eyepiece. A reflecting telescope uses a mirror to gather light. from star to eye When light reflects from a mirror, the angle of incidence equals angle of reflection. A mirror shaped like a parabola focuses light to a point: Isaac Newton s Reflecting Telescope from star focus Light from a large area is funneled into a small area.
Lenses and mirrors, if shaped correctly, produce an accurate image of an object. The main purposes of a telescope are to gather light and resolve detail. Telescope = light bucket. Bigger bucket = more light. The amount of light collected per second is proportional to the area of the lens (or mirror). Area D 4 D = diameter of lens/mirror The Pleiades = a cluster of stars. Without a telescope, most people can see six stars in the Pleiades. With large modern telescopes, over a thousand stars are seen in the Pleiades. With his small telescope, Galileo saw more than thirty. 3
A bigger lens or mirror is able to resolve finer detail. Smallest angle resolved is proportional to 1/D. low resolution high resolution Two stars are resolved if they are seen as separate points. Magnification is not as important: Big, blurry image is less useful than small, sharp image. The Andromeda Galaxy, as seen by unaided eyes, is a faint oval smudge. Andromeda Galaxy. Andromeda Galaxy. Star atlas of Al Sufi, AD 964. With large modern telescopes, the Andromeda Galaxy looks like this. BIGGER IS BETTER! A larger lens or mirror means more light, & higher resolution. 4
The world s biggest telescopes are reflectors, not refractors. What s wrong with lenses? Lenses absorb light. Lenses sag. Lenses have chromatic aberration: colors don t focus at the same point. World s largest refracting telescope: Yerkes Observatory, D = 1 meter, completed 1897. A modern reflecting telescope: 8.4-meter LBT Mirror 1 Largest single mirror yet built Large Binocular Telescope: two mirrors, each with D = 8.4 meters. Mountain-Top Observatories The United States at Night Telescopes need remote, special locations: Dark Skies, far from large cities Clear, Dry Weather most of the year Good seeing (steady atmosphere to reduce twinkling & 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) 5
Summit of Mauna Kea, Hawaii Your eye s pupil is 5 millimeters in diameter. The Keck Telescope, in Hawai i, has a mirror with D = 10 meters. How much more light can be gathered by the Keck Telescope, in a given time, than by your eye? a) 00 times more b) 000 times more c) 40,000 times more d) 4,000,000 times more 10,000 mm 000 5 mm 4,000,000 Radio and microwave telescopes use a reflecting dish to focus waves. Radio telescopes can be huge, because they don t have to be fantastically smooth. Parabolic dish of a radio telescope acts as a mirror, reflecting radio waves to the focus. Mirrors must be smooth compared to the wavelength of light observed. microwaves: λ > 1 mm, radio: λ > 100 mm Very Large Array Socorro New Mexico 7 5-meter Radio Telescopes mounted on rails Turbulence in air makes stars twinkle and limits resolution. City lights drown out faint stars. Good idea: Put a telescope in orbit! 6
Space Telescopes Only radio, visible, & 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 & weather. Very expensive ($Billions) to build & operate. Ultraviolet, X ray, gamma ray, and most infrared light can be seen from orbit. Earth s atmosphere is opaque at many wavelengths. Infrared light is absorbed by water vapor in the Earth s atmosphere. Orion Spitzer Space Telescope X rays are absorbed by most molecules in the Earth s atmosphere. X-ray sky Chandra X-ray Observatory Infrared light is produced by cool stars and by interstellar dust. X rays are produced by very hot gas. Gamma rays are absorbed by just about everything. Gamma ray sky Gamma rays are produced in extremely energetic events. 7