ASTR 1040 Accel Astro: Stars & Galaxies Topics for Today Basic principles of eyes, camera, telescopes Twinkle and absorption by our atmosphere What light gets through, what does not Next lecture: Telescopes in space -- and why Prof. Juri Toomre TA: Nicholas Nelson Lecture 4 Thur 20 Jan 2011 zeus.colorado.edu/astr1040-toomre toomre Read Chap 14 (Our Star.. the Sun) Sign-up for Observatory Night #1, this Mon 7/8/9pm Homework Set # 1 due today Respond to next discussion Q on CULearn: What are pros and cons of a lunar observatory? Clicker Q: Radio Waves D. You are listening to a radio station broadcasting at a FM frequency of 97 Mhz. Which is true? A. The radio waves from the station have a wavelength of 97 million meters. B. The radio waves received by your radio are not light waves, but rather a special kind of sound wave. C. The radio station broadcasts it signal with a power of 97 million watts. D. The radio waves are causing electrons in your radio s antenna to move up and down 97 million times per second. D. c Radios You are listening to a radio station broadcasting at a frequency of 97 Mhz. Which is true? D. The radio waves are causing electrons in your radio s antenna to move up and down 97 million times per second. Wavelength = Speed of light / frequency = 3.0 x 10 10 cm sec -1 / 9.6 10 7 sec -1 = 312 cm f slow fast How does Doppler shift tell us the ROTATION RATE of a star? Doppler shifts from different portions of star broaden the spectral line Discussion of how do ROTATING STARS yield Doppler-broadened spectral emission lines 1
Electromagnetic Spectrum 10-16 10 +4 Discussion of CELL PHONE frequencies and wavelengths and what is involved with them 850 MHz 1850 MHz gamma-rays x-rays x ultraviolet visible infrared radio Where are the cell phones? wavelength x frequency = speed of light Now On to Telescopes VLA Scicorro,, NM Imaging with our Eyes pupil allows light to enter the eye lens focuses light to create an image retina detects the light and generates signals sent to brain Bending of Light to Focus (Form an Image) Telescopes and cameras work much like our eyes 2
Reflection and refraction (bending of light) Your digital camera (CCD detector) slower speed of light in glass In what ways is an electron orbiting the nucleus of an atom different from a planet orbiting the Sun? E. A. The central force is electromagnetic (+ and - charges attract), not gravity B. Not all orbits are allowed only only certain sizes (they are quantized) C. Because atomic orbits behave differently from regular orbits we call them orbitals D. An electron can jump or make a transition from one orbital to another E. All of the above Optical Telescopes of Two Types REFRACTOR (LENS) REFLECTOR (MIRROR) Largest Refractor Modern 8 m Telescope ( Reflector ) Yerkes 40 GEMINI 8 m Mauna Kea, HI 3
Modern Reflectors Different FOCUS arrangements (to get image) 6.5 m MMT Mt. Hopkins, AZ 8.3 m SUBARU Mauna Kea, HI Keck 10 m Twins (Segmented Reflectors) Mauna Kea, HI Another view Twin Keck 10 m TELESCOPES Mauna Kea, HI Problems in Looking Through Our Atmosphere Many wavelengths are absorbed (just don t t make it through to surface) Turbulence in atmosphere distorts light: stars appear to twinkle angular resolution is degraded Man-made light is reflected by air particles, yielding bright night sky this is light pollution Light Pollution 90% of Earth s s population cannot see the Milky Way 4
How many light bulbs does it take to screw up an astronomer? An immediately curable pollution: simply turn the lights off! Stop uplight, glare: wastes billions of $$ in energy, use low pressure sodium Several famous observatories are now useless Quest for Good Weather and Seeing Mauna Kea, Big Island of Hawaii, 14,000 elevation, middle of the Pacific Dry, high, dark and isolated.. Best on the planet? LA Basin View from Mt. Wilson Observatory, 1908 and 1998. Sites in Arizona, Chile, Canary Islands, Europe. Concept of Angular Resolution Ability to separate two objects Angle between two objects decreases as your distance to them increases Smallest angle at which you can distinguish two objects is your angular resolution Two Properties of Any Telescope 1. Resolution smallest angle which can be seen: = 1.22 / 2 R 2. Light-Collecting Area think of telescope as a photon bucket its area: A = R 2 Adaptive Optics (AO) de-twinkle stars Wavefronts of star light are deformed by atmosphere Can distort shape of mirror (very fast) to correct for distortions by atmosphere hot new technology Two stars! ( is light is light wavelength,, R is mirror or lens radius ) Nick will discuss RESOLUTION in Monday recitation AO mirror OFF AO mirror ON 5
DISCUSSION TOPIC JUST WHY DO BIG OPTICAL TELESCOPES USE MIRRORS (REFLECTORS) AND NOT LENSES (REFRACTORS)? Why big aperture telescopes are reflectors Can support mirror from back,, not just at edges as with lenses (biggest: 1 m lens, 10 m diam mirror) Mirror needs only one good optical surface to be ground, not four as with achromatic (2 elem) ) lens Can recoat mirror surface easily with highly reflective aluminum (even silver) Lens has to be optically pure and uniform,, but mirror can be made of anything that holds its shape (fuzed( quartz, zero expan pyroceramics, even beryllium) How do you point a space telescope in orbit? 1. Squirt from jets to change direction (hydrazine) 2. Torque by electric currents in big coils while flying through Earth s s magnetic field 3. Torque by electric motors spinning up or down reaction wheels Next to Space Astronomy, then Radio Astronomy then Our Nearest Star (the Sun) Chap 14 ANGULAR MOMENTUM DEMONSTRATION 6