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First Telescope • 1608 - Hans Lippershey. Dutch lens grinder. • 1609 - Galileo. Built his own telescope, used it for scientific study. – Mountains and valleys on Moon – Moons of Jupiter – Phases of Venus – Saturn’s rings – Sunspots
Powers of a Telescope 1. Lightgathering power. This is the ability of a telescope to collect light.
Powers of a Telescope 2. Resolving Power – the ability of a telescope to reveal fine detail.
When light is focused into an image, a blurred fringe surrounds the image (diffraction fringe). We can never see any detail smaller than the fringe. Large diameter telescopes have small fringes and we can see smaller details. Therefore the larger the telescope, the better its resolving power. Optical quality and atmospheric conditions limit the detail we can see.
Powers of a Telescope 3. Magnifying power – the ability to make the image bigger
Magnification of a telescope can be changed by changing the eyepiece. We cannot alter the telescope’s light-gathering Or resolving power. Astronomers identify telescopes by diameter because that determines both light-gathering power and resolving power.
Optical Telescopes Refracting telescope uses a large lens to gather and focus light. Reflecting telescope uses a large mirror
Focal length – the distance from the lens or mirror to the image formed of a distant light source
Primary lens: the main lens in a refracting telescope. It is also called an objective lens.
Primary mirror: the main mirror in a reflecting telescope. It is also called an objective mirror.
Eyepiece: A small lens to magnify the image produced by the objective (primary) lens
Chromatic Aberration When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light.
Chromatic Aberration If we focus on the blue image, the red image is out of focus and we see a red blur around the image. This color separation is called chromatic aberration.
Achromatic Lens An achromatic lens is made of two components made of different kinds of glass and brings the two different wavelengths to the same focus. Other wavelengths are still out of focus.
Yerkes Refracting Telescope • Largest refracting telescope in the world is at Yerkes Observatory in Wisconsin • Lens is 1 m in diameter • ½ tonne • The glass sags under its own weight
Newton’s Reflecting Telescope • 1666 - Newton found that a prism breaks up white light into a rainbow of colours – Telescope lenses do the same • 1663 - James Gregory designed a telescope with a large concave primary mirror and a smaller concave secondary mirror • 1672 - Newton modified the design, and it won huge acclaim
Benefits of Reflecting Telescopes Less expensive. Only the front surface of the mirror must be ground. The glass doesn’t need to be perfectly Transparent The mirror can be supported over its back surface to reduce sagging. They do not suffer from chromatic aberration because the light is reflected toward the focus before it can enter the glass.
Four ways to look through reflecting telescopes
Hershel’s Telescope • Late 1770 s, William Hershel was making the best metallic mirrors and telescopes in the world. • 1781 - Discovered Uranus • 1789 - Built a giant telescope which he used with his sister Caroline – His telescope had a 125 cm mirror – 40 ft in length
Rosse’s Telescope • 1838 - Earl of Rosse, Ireland, taught himself mirror-making and built a 91 cm telescope • 1842 - attempted to build a 181 cm telescope but it broke when moved – built another one that couldn’t be moved
Observatories • 1874 - 91 cm telescope and observatory at University of California (James Lick) • 1880 - 76 cm telescope in France • 1897 - 102 cm telescope at Yerkes Observatory in Wisconsin • 1908 - 153 cm telescope on Mount Wilson California (George Ellery Hale) • 1917 - 254 cm telescope also built on Mount Wilson (John D. Hooker) • 1948 - 500 cm mirror. Hale Observatory, Mount Palomar, California.
Observatories are built on top of mountains because: 1) air is thin and more transparent 2) the sky is darker 3) stars are brighter 4) wind blows smoothly over some mountaintops 5) there is less pollution
New Generation Telescopes
Keck Telescope • 1993 – Keck telescope 1000 cm mirror, made of smaller segments • Photographic plates were more sensitive and permitted a permanent record of observations – Photographic plates have since been replaced by electronic imaging devices
A large mirror sags in the middle. To prevent this: 1. Mirrors can be made very thick but they are very heavy and very costly.
A large mirror sags in the middle. To prevent this: 2. Spincasting – an oven turns and molten glass flows outward in a mold to form a concave upper surface.
A large mirror sags in the middle. To prevent this: 3. A mirror can be made in segments.
A large mirror sags in the middle. To prevent this: 4. Thin mirrors (floppy mirrors) can have their shape controlled by a computer – called active optics. They cool quickly to adjust to surrounding temperatures.
Radio Telescopes • Objects in space emit light waves of many different wavelengths. • Radio Telescopes receive very long wavelengths (radio waves). • 1937 – first Radio telescope; picks up long wave radio emissions from deep space
Handicaps to Radio Telescopes 1. Poor resolution To improve resolution, two or more radio telescopes can be combined to improve the resolving power (called a radio interferometer). Resolving power equals the separation of the telescopes.
Handicaps to Radio Telescopes 2. Low intensity In order to get strong signals focused on the antenna, the radio astronomer must build large collecting dishes. The largest dish is the 300 m dish at Arecibo, Puerto Rico.
Handicaps to Radio Telescopes 3. Interference This occurs because of poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems.
Space Telescopes • • • Hubble Spitzer Kepler Webb Chandra Hershel Planck Fermi XMM-Newton