Week 7: 10/5 & 10/7 Capturing that radiation Chapter 6 (Telescopes & Sensors) Optical to Radio Summary What are we sensing? Matter! Matter is made of atoms (nucleus w/ protons, neutrons & cloud of electrons Emits photons Phases of matter? Changing temperatures or pressure changes phases changes emissions changes light! 5.4 Learning from Light Recognizing that light is giving us detailed information about what is happening at an atomic, hence compositional, level Our goals for learning What are the three basic types of spectra? How does light tell us what things are made of? How does light tell us the temperatures of planets and stars? How do we interpret an actual spectrum? What are the three basic types of spectra? Emission Line Spectrum Continuous Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types Three Types of Spectra 1
Continuous Spectrum Emission Line Spectrum The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines Absorption Line Spectrum How does light tell us what things are made of? A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum (also applies to stellar atmospheres) Spectrum of the Sun Chemical Fingerprints Chemical Fingerprints Energy levels of Hydrogen Each type of atom has a unique set of energy levels Each transition corresponds to a unique photon energy, frequency, and wavelength Downward transitions produce a unique pattern of emission lines 2
Absorption Spectra and Emission Spectra simultaneously Chemical Fingerprints Every element has a unique spectral fingerprint Test Emission Spectra Chemical Fingerprints Observing the fingerprints in a spectrum tells us which kinds of atoms are present Selective absorption of radiation in the Atmosphere O 2 and O 3 absorb almost 100% of the UV radiation at a < 0.3 m. H 2 O and CO 2 are strong absorbers of IR radiation and poor absorbers of visible radiation. 0.1 0.3 0.5 0.7 1 5 10 15 20 0.1 0.3 0.5 0.7 1 5 10 15 20 0.1 0.3 0.5 0.7 1 5 10 15 20 3
CH 4 and N 2 O are also strong absorbers of IR radiation Quick Test: Which letter(s) labels absorption lines? Common in outer solar system A B C D E Quick Test: Which letter(s) labels the peak (greatest intensity) of infrared light? Thought Question Which letter(s) labels emission lines? A B C D E A B C D E How do we interpret an actual spectrum? What is this object? By carefully studying the features in a spectrum, we can learn a great deal about the object that created it. Reflected Sunlight: Continuous spectrum of visible light is like the Sun s except that some of the blue light has been absorbed - object must look red 4
What is this object? What is this object? Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K Must be pretty cold! Carbon Dioxide: Absorption lines are the fingerprint of CO 2 in the atmosphere What is this object? What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere Mars! Recap What are the three basic type of spectra? Continuous spectrum, emission line spectrum, absorption line spectrum How does light tell us what things are made of? Each atom has a unique fingerprint. We can determine which atoms something is made of by looking for their fingerprints in the spectrum. And How does light tell us the temperatures of planets and stars? All stars emit a continuous spectrum that depends on temperature. The spectrum of that thermal radiation tells us the object s temperature. How do we interpret an actual spectrum? By carefully studying the features in a spectrum, we can learn a great deal about the object that created it. 5
5.5 The Doppler Effect Our goals for learning How does light tell us the speed of a distant object? How does light tell us the rotation rate of an object? How does light tell us the speed of a distant object? The Doppler Effect The Doppler Effect Same for Light Measuring the Shift Stationary Moving Away Moving Away Faster Moving Toward The amount of blue or red shift tells us an object s speed toward or away from us: Moving Toward Faster Measuring the Doppler Effect from shifts in the wavelengths of emission lines 6
Doppler Shift tells us ONLY about the part of an object s motion toward or away from us: Quick Test: I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. 450nm=blue 700nm=red Lab Spectra Redshifted to here a) It is moving away from me. b) It is moving toward me. c) It has unusually long spectral lines. How does light tell us the rotation rate of an object? Spectrum of a Rotating Object Different Doppler shifts from different sides of a rotating ti object spread out its spectral lines Spectral lines are wider when an object rotates faster Recap Light can tell us: What something is made of What its temperature is If it is a solid or gas How does light tell us the speed of a distant object? The Doppler effect tells us how fast an object is moving toward or away from us. Blueshift: objects moving toward us Redshift: objects moving away from us How does light tell us the rotation rate of an object? The width of an object s spectral lines can tell us how fast it is rotating Chapter 6: Telescopes End of Chapter 5 slides 7
Telescope Instrument for gathering and focusing radiation from distant objects Typically X-ray, Visible, IR, Radio Functional Design Gather radiation Focus to a point (magnify) Create an image for analysis Electromagnetic Spectrum Electromagnetic Spectrum The Mark I Eyeball Biochemical Sensor Senses 16 gray scales 2,000 colors Resolution Spatial = 0.1 mm* Spectral = 0.15 nm Dynamic Range 300,000 steps * depends on distance to target. Roughly 200 meters from orbital altitudes How does your eye form an image? 8
Focusing Light Image Formation 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! How do we record images? Focusing Light 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 Observing Tools Camera and Film Recording Device Storage Medium Presentation Medium Comments Eyeball Brain Maps, Writings Most subjective BUT best long- term record. Camera Film Photograph Extremely Labor Intensive. Poor geometry. Digital Sensor Digital File Computer Visualization Cheap, reproducible, Easy to xmit & manipulate. Mechanochemical Senses 255 gray scales 10,000000 colors Resolution Spatial = 0.0001 mm* Spectral = 0.015 nm Dynamic Range 7 steps * depends on distance to target. As good as one centimeter from orbit. 9
Digital Sensors Electronic Senses 1,024 gray scales 16,400,000 000 colors Resolution Spatial = 0.0001 µm* Spectral = 1.0 nm Dynamic Range 1,024 steps Landsat 1 (1971) Digital Systems Mechanics and Products Spectral Analysis Spectral Analysis Multispectral Imaging 6.2 Telescopes: Light Buckets Our goals for learning What are the two most important properties of a telescope? What are the two basic designs of telescopes? What do astronomers do with telescopes? 10
M.A.M. 1. Mounting If you can t see it, nothing else matters 2. Aperture Diameter of the light-gathering portion (mirror, lens, or antenna) 3. Magnification How much bigger is it? Everything gets magnified (see #1 above) Handheld maximum typically 7-10X Useful Magnification ~ 40X per inch of aperture 2.4 inch telescope 100 power 6 inch telescope 240 power Atmosphere limits maximum power to ~ 300X The Magnification Myth Magnification 1X = 1 times the human eye, 10X = 10 times the human eye, etc. Magnification = How much closer, hence bigger Important properties of a telescope? 1. Light-collecting area: Telescopes with a larger collecting area (aperture) can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes with a larger aperture are capable of taking images with greater detail. Light Collecting Area Aperture A telescope s diameter tells us its light collecting area: Area = πr 2 The largest telescopes currently in use have a diameter of about 10 meters Galileo s telescope had an aperture of about 1 inch (6X) Bigger is better Angular Resolution The minimum angular separation that the telescope can distinguish. 11
The two basic telescope designs Refracting telescope: Focuses light with lenses Reflecting telescope: Focuses light with mirrors Refracting Telescope Refracting telescopes usually very long, with large, heavy lenses Reflecting Telescope Designs for Reflecting Telescopes Reflecting telescopes easier to build with much greater diameters Most modern telescopes are reflectors Mirrors in Reflecting Telescopes Imaging Astronomical detectors generally record only one color of light at a time Twin Keck telescopes on Mauna Kea in Hawaii Segmented 10-meter mirror of a Keck telescope Several images must be combined to make full-color pictures 12
Imaging Astronomical detectors can record forms of light our eyes can t see Color is sometimes used to represent different energies of nonvisible light Spectroscopy A spectrograph separates the different Light from Diffraction grating gbreaks wavelengths of light before they only one star enters light into spectrum hit the detector Detector records spectrum Spectroscopy Graphing relative brightness of light at each wavelength shows the details in a spectrum How does Earth s atmosphere affect ground-based observations? 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) Light Pollution Light Pollution Scattering of human-made light in the atmosphere is a growing problem for astronomy 13
Light Pollution Darkest Country on Earth Remediation A funny thing happened on the ground Twinkling and Turbulence Star viewed with groundbased telescope Same star viewed with Hubble Space Telescope 1990: The whole point of the Hubble was to get away from light pollution, dust, turbulence, and clouds. Turbulent air flow in Earth s atmosphere distorts our view, causing stars to appear to twinkle 14
Adaptive Optics 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 Adaptive Optics Increasing Signal to Noise ratio Over-sampling of hundreds (or thousands) of images 15
Why do we put telescopes into space? Transmission in Atmosphere Only radio and visible light pass easily through Earth s atmosphere We need telescopes in space to observe other forms Non-Optical Imaging Seeing things in a different light Magnetic Field Interactions Gamma IR Earth not to scale Vis X-ray X-ray X-Ray RGB composite Magnetospheres Radio Telescopes X-ray Size is a function of wavelength, so they re BIG Interferometry common Also on spacecraft Jupiter in X-ray and Visible light 16
Same Function Imaging Magnetic Fields Collect Focus Produce Image Interferometry Ground-based Astronomy in Space Signals from many small but separate telescopes are electronically combined to equal one very large telescope Farside of the Moon observing site No atmosphere Very long nights No light pollution Minimal radio noise Spacecraft Payloads Telescope are critical payloads Cassini Payload Typical manifest Imaging Radar UV/Visible instrument Near IR instrument Wide field (panoramic or mapping) camera Narrow field (telephoto) camera Star Camera (navigation) 17
Science Operations Deep Space Network Different telescopes for measuring: Composition Temperature Crystalline Structure Surface constitution Atmospherics Imaging Radar (microwave) Active imaging Instrument produces the energy and then measures return signal Sees through clouds Imaging Radar measures surface roughness & orientation Imaging Radar at Titan Imaging Radar products for NEAs Dark = smooth Bright = rough Likely seeing methane lakes 1999JM8 216 Kleopatra Toutatis 18
Ground-penetrating Radar Saturn s Mega-ring About 300 Sr out from the planet. The moon Phoebe occupies this orbit and is likely source. Water ice and dust Particles. Image Slices Discovered by the Spitzer IR telescope (66 million miles from earth) Term Exam 2 Similar in structure to Exam #1 Essay worth 20 points this time 2 @ 10 pts each (select 2 from 4) Objective worth 80 points 40 @ 2 pts each 20 @ 4 pts each??? Covering the parts of Newton (from Chapter 4 we discussed in class) + Chapters 5 and 6 19