Chapter 6 6.1 Eyes and Cameras: Everyday Light Sensors Telescopes: - PDF document

Chapter 6 6.1 Eyes and Cameras: Everyday Light Sensors Telescopes: Portals of Discovery • Our goals for learning • How does your eye form an image? • How do we record images? How does your eye form an Refraction image? • Refraction is the bending of light when it passes from one substance into another • Your eye uses refraction to focus light Example: Refraction at Sunset Focusing Light • Sun appears distorted at sunset because of how light • Refraction can cause parallel light rays to converge bends in Earth’s atmosphere to a focus 1

Image Formation How do we record images? • The focal plane is where light from different directions comes into focus • The image behind a single (convex) lens is actually upside-down! What have we learned? Focusing Light • How does your eye form an image? Digital cameras – It uses refraction to bend parallel light rays so detect light with that they form an image. charge-coupled – The image is in focus if the focal plane is at devices (CCDs) the retina. • How do we record images? – Cameras focus light like your eye and record the image with a detector. • A camera focuses light like an eye and captures the – The detectors (CCDs) in digital cameras are image with a detector like those used on modern telescopes • The CCD detectors in digital cameras are similar to those used in modern telescopes What are the two most important 6.2 Telescopes: Giant Eyes properties of a telescope? • Our goals for learning 1. Light-collecting area: Telescopes with a larger collecting area can gather a greater • What are the two most important properties amount of light in a shorter time. of a telescope? 2. Angular resolution: Telescopes that are • What are the two basic designs of larger are capable of taking images with telescopes? greater detail. • What do astronomers do with telescopes? 2

Light Collecting Area Bigger is better • A telescope’s diameter tells us its light- collecting area: Area = π (diameter/2) 2 • The largest telescopes currently in use have a diameter of about 10 meters Angular Resolution Angular Resolution • The minimum • Ultimate limit to angular separation resolution comes that the telescope from interference of light waves within a can distinguish. telescope. • Larger telescopes are capable of greater resolution because there’s less interference Angular Resolution Angular Resolution • Ultimate limit to • The rings in this resolution comes image of a star come from interference of from interference of light waves within a light wave. telescope. • Larger telescopes • This limit on are capable of angular resolution is greater resolution known as the because there’s less diffraction limit interference Close-up of a star from the Hubble Space Telescope 3

What are the two basic designs of Refracting Telescope telescopes? • Refracting • Refracting telescope: Focuses light with telescopes lenses need to be • Reflecting telescope: Focuses light with very long, mirrors with large, heavy lenses Reflecting Telescope Designs for Reflecting Telescopes • Reflecting telescopes can have much greater diameters • Most modern telescopes are reflectors What do astronomers do with Mirrors in Reflecting Telescopes telescopes? • Imaging: Taking pictures of the sky • Spectroscopy: Breaking light into spectra • Timing: Measuring how light output varies with time Twin Keck telescopes on Segmented 10-meter mirror Mauna Kea in Hawaii of a Keck telescope 4

Imaging Imaging • Astronomical • Astronomical detectors detectors can generally record forms of record only light our eyes one color of can’t see light at a time • Color is • Several images sometimes used must be to represent combined to different make full-color energies of pictures nonvisible light Spectroscopy Spectroscopy • A spectrograph • Graphing separates the relative different brightness of wavelengths of light at each Diffraction Light from light before they wavelength grating breaks only one star hit the detector shows the light into enters spectrum details in a spectrum Detector records spectrum Timing Want to buy your own telescope? • Buy binoculars first (e.g. 7x35) - you get much more for the same money. • Ignore magnification (sales pitch!) • Notice: aperture size, optical quality, portability. • Consumer research: Astronomy, Sky & Tel, Mercury. Astronomy clubs. • A light curve represents a series of brightness measurements made over a period of time 5

What have we learned? What have we learned? • What are the two most important properties of a • What do astronomers do with telescopes? telescope? – Imaging – Collecting area determines how much light a – Spectroscopy telescope can gather – Timing – Angular resolution is the minimum angular separation a telescope can distinguish • What are the two basic designs of telescopes? – Refracting telescopes focus light with lenses – Reflecting telescopes focus light with mirrors – The vast majority of professional telescopes are reflectors How does Earth’s atmosphere 6.3 Telescopes and the Atmosphere affect ground-based observations? • Our goals for learning • The best ground-based sites for astronomical observing are • How does Earth’s atmosphere affect ground-based observations? – Calm (not too windy) – High (less atmosphere to see through) • Why do we put telescopes into space? – Dark (far from city lights) – Dry (few cloudy nights) Light Pollution Twinkling and Turbulence Star viewed with ground- Same star viewed with Hubble Space Telescope based telescope • Scattering of human-made light in the atmosphere Turbulent air flow in Earth’s atmosphere distorts is a growing problem for astronomy our view, causing stars to appear to twinkle 6

Adaptive Optics Without adaptive optics With adaptive optics Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence Why do we put telescopes into Calm, High, Dark, Dry space? • The best observing sites are atop remote mountains Summit of Mauna Kea, Hawaii What have learned? Transmission in Atmosphere • How does Earth’s atmosphere affect ground- based observations? – Telescope sites are chosen to minimize the problems of light pollution, atmospheric turbulence, and bad weather. • Why do we put telescopes into space? – Forms of light other than radio and visible do not pass through Earth’s atmosphere. – Also, much sharper images are possible because there is no turbulence. • Only radio and visible light pass easily through Earth’s atmosphere • We need telescopes in space to observe other forms 7

How can we observe nonvisible 6.4 Eyes and Cameras: Everyday Light light? Sensors • A standard satellite dish • Our goals for learning is essentially • How can we observe nonvisible light? a telescope for observing • How can multiple telescopes work together? radio waves Radio Telescopes IR & UV Telescopes • A radio telescope is like a giant mirror that reflects radio waves to a focus SOFIA Spitzer • Infrared and ultraviolet-light telescopes operate like visible-light telescopes but need to be above atmosphere to see all IR and UV wavelengths X-Ray Telescopes X-Ray Telescopes • X-ray telescopes also need to be above the atmosphere • Focusing of X-rays requires special mirrors • Mirrors are arranged to focus X-ray photons through grazing bounces off the surface Chandra 8

How can multiple telescopes Gamma Ray Telescopes work together? • Gamma ray telescopes also need to be in space • Focusing gamma rays is extremely difficult Compton Observatory Interferometry Interferometry • Interferometery • Easiest to do is a technique with radio for linking two telescopes or more telescopes so • Now becoming that they have possible with the angular infrared and resolution of a visible-light single large one telescopes Very Large Array (VLA) Future of Astronomy in Space? • The Moon would be an ideal observing site 9