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Astronomical Tools. Essential Questions. What is Light? How do telescopes work, and how are they limited? What kind of instruments do astronomers use to record and analyze light? Why do astronomers use radio telescopes? Why must some telescopes go into orbit?. A particle and a wave….
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Essential Questions • What is Light? • How do telescopes work, and how are they limited? • What kind of instruments do astronomers use to record and analyze light? • Why do astronomers use radio telescopes? • Why must some telescopes go into orbit?
A particle and a wave… • Electromagnetic radiation comes in many types but all have the same speed. • Speed = frequency x wavelength (c = f x λ) • Particles of light are called photons.
Light • Light is the visible form of electromagnetic radiation. • It’s an electric and magnetic disturbance that transports energy at the speed of light. • The EM Spectrum includes: Radio, Microwave, Infrared, Visible, Ultraviolet, X-Rays, and Gamma Rays. • A photon, or particle of light, can be thought of as a bundle of waves that sometimes acts like a wave and sometimes like a particle.
Visible Light • The visible spectrum ranges from 700 nm red light to 400 nm violet light. • ROY G BIV • As wavelengths decrease, energy increases. • As frequency increases so does energy.
Try a few calculations… • Find the frequency of red light (λ = 700 x 10-9 m) • Find the frequency of violet light (λ = 400 x 10-9 m) • Find the wavelength of your favorite radio station… (remember M = mega or 106)
Telescopes • Earth based or space based, telescopes are designed to do one thing very well… • Gather and focus as much light as possible.
Bigger is Better • Telescopes allow astronomers to gather light, see fine detail, and magnify images. • The first two depend on the telescope’s diameter or aperture. • Consequently, astronomers always strive for larger scopes.
Refracting Telescopes • A refracting telescope uses a lens to gather and focus light. • Because of chromatic aberration, refractors cannot bring all colors to focus at the same spot—color fringes result. • An achromatic lens partially corrects for this, but such lenses are expensive. • Refracting lenses cannot be made larger than 1 meter—they warp if too heavy.
Yerkes Refractor • The world’s largest refracting telescope is located in Wisconsin. • The diameter of the lens is 40 inches. • Any more than this, and the lens will warp and change shape due to its own weight.
Reflecting Telescopes • Use a mirror to gather and focus light. • Less expensive than refractors of the same diameter. • Do not suffer from chromatic aberration. • Most recently built large telescopes are reflectors. • The mirror can be supported from below, so there is no limit to size (except for $).
The Keck Observatory • The world’s largest reflecting telescopes are the twin Keck reflectors in Hawaii. • Each scope has a 10 meter diameter mirror! • Actually a group of segmented mirrors put together.
Types of Reflectors • Prime focus—instruments/eyepiece on the light gathering end of scope. • Newtonian Focus—instruments/eyepiece on side of scope, a secondary mirror bounces light out the side. • Cassegrain Focus—instruments/eyepiece behind the primary mirror, light enters through a hole in the primary.
Telescopes High and Dry • Astronomers build observatories on mountains for two reasons: • Better “seeing” due to less turbulence in the atmosphere. • Air is thinner and dryer and is therefore more transparent especially to infrared. • After all, WIRO—Wyoming Infrared Observatory
Advances in Optical Telescopes • Sometimes telescopes are linked through a technique called interferometry. • Resolution is equal to the separation of the two scopes. • Adaptive optics have also been used to compensate for atmospheric distortion.
Modern Instruments • For many years astronomers used photographic plates to record images at the telescope. • Modern electronic systems such as CCDs (advanced digital camera chips) have replaced most photographic plates.
Spectrographs • Spectrographs using prisms or a grating spread starlight out according to wavelength. • This reveals a spectrum showing hundreds of spectral lines produced by atoms in the object being studied.
Radio Astronomy • Astronomers use radio telescopes for three reasons: • Can detect cool hydrogen • Can see through dust clouds • Can detect objects invisible at other wavelengths
Radio astronomy details • Radio telescopes can be used during the day • Consist of a dish reflector, an antenna, an amplifier and a data recorder. • The largest single, fully-steerable, radio telescope is in Green Bank, WV. (100 m) • The largest single radio telescope is located in Puerto Rico and at Arecibo. (300 m) Maybe you’ve seen it in a 007 movie…
Radio Interferometry • Radio telescopes have poor resolution due to the long wavelength of radio waves. • Often linked to get better resolution. • The VLA in New Mexico can be 30 km across!
Our troublesome atmosphere • Although our atmosphere is nice to breathe and shields us from many forms of nasty radiation and meteors… • It only allows us to use ground based telescopes in the visible, radio/microwave, and some infrared parts of the spectrum. • For the other wavelengths, we must go into space…
Space Based Telescopes • Earth’s atmosphere absorbs some radiation and distorts light that does get through. • Telescopes in orbit avoid this and are limited only by their size. • Some have proposed lunar observatories!
Multi-wavelength Crab The Crab Nebula in Radio
