Chapter 6 Astronomical Instruments PDF

 Refraction is the bending of light when it passes from one substance into another.  Your eye uses refraction to focus light.  Sun appears distorted at sunset because of how light bends in Earth’s atmosphere.  Refraction can cause parallel light rays t...

 Refraction is the bending of light when it passes from one substance into another.  Your eye uses refraction to focus light.  Sun appears distorted at sunset because of how light bends in Earth’s atmosphere.  Refraction can cause parallel light rays to converge to a focus.  The focal plane is where light from different directions comes into focus.  The image behind a single (convex) lens is actually upside-down!  Digital cameras detect light with charge-coupled devices (CCDs).  A camera focuses light like an eye and captures the image with a detector.  The CCD detectors in digital cameras are similar to those used in modern telescopes.   Charge-Coupled Devices (CCDs). (a) This CCD is a mere 300-micrometers thick (thinner than a human hair) yet holds more than 21 million pixels. (b) This matrix of 42 CCDs serves the Kepler telescope. (credit a: modification of work by US Department of Energy; credit b: modification of work by NASA and Ball Aerospace) 1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.  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. a) It’s 5 times greater. b) It’s 10 times greater. c) It’s 25 times greater. a) It’s 5 times greater. b) It’s 10 times greater. c) It’s 25 times greater.  The minimum angular separation that the telescope can distinguish  Ultimate limit to resolution comes from interference of light waves within a telescope. Larger telescopes are capable of greater resolution because there’s less interference.  The rings in this image of a star come from interference of light wave. This limit on angular resolution is known as the diffraction limit. Close-up of a star from the Hubble Space Telescope  Refracting telescope: focuses light with lenses  Reflecting telescope: focuses light with mirrors  Refracting telescopes need to be very long, with large, heavy lenses.  Reflecting telescopes can have much greater diameters.  Most modern telescopes are reflectors. Twin Keck telescopes on Segmented 10-meter mirror Mauna Kea in Hawaii of a Keck telescope  Imaging: taking pictures of the sky  Spectroscopy: breaking light into spectra  Timing: measuring how light output varies with time  Astronomical detectors generally record only one color of light at a time.  Several images must be combined to make full- color pictures.  Astronomical detectors can record forms of light our eyes can’t see.  Color is sometimes used to represent different energies of non- visible light.  A spectrograph separates the different wavelengths of light before they hit the detector.  Graphing relative brightness of light at each wavelength shows the details in a spectrum.  A light curve represents a series of brightness measurements made over a period of time.  Buy binoculars first (e.g., 7×35)—you get much more for the same money.  Ignore magnification (sales pitch!).  Notice: aperture size, optical quality, portability.  Consumer research: Astronomy, Sky & Telescope, Mercury, astronomy clubs  The best ground-based sites for astronomical observing are:  calm (not too windy)  high (less atmosphere to see through)  dark (far from city lights)  dry (few cloudy nights)  Scattering of human-made light in the atmosphere is a growing problem for astronomy. Bright star viewed with Same star viewed with ground-based telescope Hubble Space Telescope Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle.  Without adaptive optics With adaptive optics Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence.   The best observing sites are atop remote mountains. Summit of Mauna Kea, Hawaii  Only radio and visible light pass easily through Earth’s atmosphere.  We need telescopes in space to observe other forms.  A standard satellite dish is essentially a telescope for observing radio waves. Radio telescopes: Similar to optical reflecting telescopes Prime focus Less sensitive to imperfections (due to longer wavelength); can be made very large Radio Astronomy Largest radio telescope: 300-m dish at Arecibo  Radio Astronomy Longer wavelength means poor angular resolution Advantages of radio astronomy: Can observe 24 hours a day Clouds, rain, and snow don’t interfere Observations at an entirely different frequency; get totally different information Radio Astronomy Interferometry: Combine information from several widely- spread radio telescopes as if they came from a single dish Resolution will be that of dish whose diameter = largest separation between dishes Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes Radio Astronomy Can get radio images whose resolution is close to optical:  Very Long Baseline Array. This map shows the distribution of 10 antennas that constitute an array of radio telescopes stretching across the United States and its territories. Interferometry can also be done with other wavelengths, but much harder due to shorter wavelengths:  Atacama Large Millimeter/Submillimeter Array (ALMA). Located in the Atacama Desert of Northern Chile.(credit: ESO/S. Guisard)  Other Astronomies Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses  Other Astronomies Infrared telescopes can also be in space or flown on balloons:  Other Astronomies Ultraviolet images. (a)The Cygnus loop supernova remnant (b) M81  Other Astronomies X-rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques X-rays will reflect at a very shallow angle, and can therefore be focused:  Other Astronomies X-ray image of supernova remnant Cassiopeia A:  Other Astronomies Gamma rays cannot be focused at all; images are therefore coarse: Different Views of Our Universe We are familiar with the view of our surrounding universe in visible light Different Views of Our Universe RADIO WAVES INFRARED X-RAYS GAMMA RAYS  James Webb Space Telescope (JWST). This image shows some of the mirrors of the JWST as they underwent cryogenic testing. The mirrors were exposed to extreme temperatures in order to gather accurate measurements on changes in their shape as they heated and cooled. (credit: NASA/MSFC/David Higginbotham/Emmett Given)  Artist’s Conception of the European Extremely Large Telescope. The primary mirror in this telescope is 39.3 meters across. The telescope is under construction in the Atacama Desert in Northern Chile. (credit: ESO/L. Calçada)