Telescopes: Portals of Discovery Chapter 6 PDF

6.1 Eyes and Cameras: Everyday Light Sensors The Eye Uses a lens to bend the light entering the eye to focus on the retina (light sensitive region) Muscles can adjust the shape of the lens, and adjust the size of the opening (Iris) to let in more or less light. Inv...

6.1 Eyes and Cameras: Everyday Light Sensors The Eye Uses a lens to bend the light entering the eye to focus on the retina (light sensitive region) Muscles can adjust the shape of the lens, and adjust the size of the opening (Iris) to let in more or less light. Inverted Images Images through a lens get flipped over. Brain actually sees image upside down then flips it over. Camera Works the same way as your eye. Uses a lens to focus the image, which flips it over. Light sensitive chemicals on the film (detector) respond to the light and save the image via a chemical reaction. A shutter opens and closes allow to enter. The longer it is open, the more light that can reach the film (exposure time) Image Processing Today, many images are stitched together electronically to give us the pretty pictures we see. Can remove effects from the atmosphere and temperature variations. Can add false color to bring out details. 6.2 Telescopes: Giant Eyes Light Gathering Power Need to collect as much light as possible Bigger the telescope, the more light it can collect. Proportional to the Area of the opening D = Diameter of the opening Resolving Power Ability of a telescope to separate two closely spaced objects Angular Resolution Angular Resolution - the angle where you can still distinguish two objects apart. AR = physical separation x (360/2 pi x distance apart) AR = 206,265 “ x (physical separation / distance) d = physical separation D = Distance from Earth Problem A binary star system is 20 light years away and its two stars are separated by 200 million kilometers. Can the Hubble telescope resolve the two stars if its angular resolution is 0.05 arc seconds? (1 ly = 9.46 x 1012 km) A book How far away can you place a book and still read it with the hubble telescope (AR = 0.05 arcseconds). Assume letter separation of 0.2 mm. The Diffraction Limit Depends on the wavelength of light and the size of the telescope mirror/lens. Waves of light overlap and interfere with each other blurring the image Diffraction Limit = 2.5 x 105 (arc seconds) x wavelength of light / diameter of the telescope Problem What is the diffraction limit of the Hubble telescope with a wavelength of 500 nm if opening of telescope is 2.4 meters? How large would a telescope have to be to have a diffraction limit of 0.001 arcseconds for a wavelength of 500 nm? How big would it need to be at a wavelength of 3 m (FM radio waves) ? Concentrate the Light A lens or a mirror is used to bend the light into an image. Point where light rays from far away meet is called the focal point or focus Focal length depends on the curvature of the lens or mirror Detector The image needs to then be processed ○ Can use your eye Film CCD camera Spectrometer Film / Glass Plates Can be exposed for hours in a row to maximizes trapped light.  Clyde Tombaugh noticed the dot (arrowed on each plate) had moved in the 6 days between Jan. 23 & Jan. 29, 1930. CCD (Charge Coupled Device) Like a solar panel, produces a charge when struck by light More photons hit the CCD, More charge is stored Spectrograph Light sent through a prism Accurately measure the wavelength of each color Lens versus Mirror Refracting telescope uses a lens to bend light to a focus ○ Light must pass through the lens Reflecting telescope uses a curved mirror. Refracting Issues Light must pass through the lens Large lens warps under its own weight Issue with chromatic aberration - different colors focus at different points Reflecting Benefits Light does not pass through so it can be supported from the back of the mirror Can make very large No chromatic aberration Time Monitoring Many objects vary with time With a telescope and a lot of time, we can watch these events how they change over time. Image shows light from a planet and star 6.3 Telescopes and the Atmosphere Light Pollution City lights can drown out the light of stars We move telescopes high up mountains and far from city lights. Adaptive Optics Atmosphere is constantly stirred by wind causing turbulence in the air. Blurs images. AO uses infrared pulses to measure blurring of atmosphere Allows sharper images Telescopes usually placed on mountaintops to reduce turbulence Where is the best place for a telescope? Minimize light pollution Minimize blurring from the atmosphere and need for adaptive optics Use all year long no matter the weather or time of day 6.4 Telescopes Across the Spectrum Radio Telescopes Have a metal curved dish and a receiver. Long wavelengths require very large telescope to get good angular resolution. Detector placed at the focus of the dish Infrared Telescopes Infrared blocked by the atmosphere. Must be placed in space. Must also be shielded from heat from the Earth and Sun. SOFIA Stratospheric Observatory For Infrared Astronomy Placed inside a Boeing 747 Spitzer Space Telescope Launched 2003, ended 2020 Cooled with Liquid Helium Can see the cold dust clouds around stars and in galaxies James Webb Telescope Launched December 25, 2021 at 4:20 AM Visible and Infrared Ultraviolet Telescopes Also need to be in space Hubble Telescope able to see in the UV band, launched in 1986 X-ray Telescopes Also have to be put in space Hard to focus X rays Used to see very high temperature objects Chandra Observatory launched 1999, still working Gamma Ray Telescopes Unable to focus gamma rays Can detect photons and determine their direction Compton Gamma Ray Observatory launched in 1991 to 2000 Neutrinos Tiny particles lighter than electrons that are emitted in radioactive decays and fusion reactions inside stars Travel near the speed of light Very hard to detect as the do not interact very well with normal matter. Strong signals produced as stars explode Detectors buried deep underground Gravitational Waves Very massive objects colliding release gravitational waves Only recently have been successfully tested proving Einstein's Theory of general Relativity Send a ripple of gravity across space like ripples on a pond LIGO Uses interferometry to detect gravity waves A laser is sent down the two shafts that are exactly the same length. A small gravitational shift would cause one beam to have a slightly different arrival time at the detector. Multiple telescopes Systems can use multiple telescopes and combine their signals to give high angular resolution VLA (Very Large Array) Consists of 27 radio telescope dishes that can move along railroad tracks to give a huge radio telescope. Can be 600 meters wide up to 21 km. World Wide Arrays Radio telescopes around the world can be used to give a telescope effectively the size of the planet when the signals are linked together. Plans are in place to place satellites in long orbits around the Earth to give even higher angular resolutions