Published on November 15, 2007
SIMG-217Fundamentals of Astronomical Imaging: SIMG-217 Fundamentals of Astronomical Imaging Instructor: Joel Kastner Office: 17-3190 Phone: 475-7179 Email: [email protected] Course Description: Course Description Familiarizes students with the goals and techniques of astronomical imaging. The broad nature of astronomical sources will be outlined in terms of requirements on astronomical imaging systems. These requirements are then investigated in the context of the astronomical imaging chain. Imaging chains in the optical, infrared, X-ray, and/or radio wavelength regimes will be studied in detail as time permits. (prerequisite: 1051-215 or permission of instructor) Laboratories: Laboratories 3 mandatory experiments, selected from: Star Colors from Digital Images Spectroscopic Imaging of Gases Multiwavelength Imaging of the Sun Multiwavelength Imaging of the Orion Nebula Final Project; one of: collect/process images taken at RIT observatory (weather, time permitting) detailed followup of one of the above lab experiences Student-proposed project/investigation in astronomical imaging Topics: Topics Review of Imaging Systems Issues in Astronomical Imaging Systems History of Astronomical Imaging Systems Contemporary Astronomical Imaging Systems What does the future hold for astronomical imaging? Goal of Imaging Systems : Goal of Imaging Systems Create an “image” of a scene that may be measured to calculate some parameter of the scene Diagnostic X ray Digital Photograph “CAT” Scan (computed tomography) “MRI” (magnetic resonance imaging) Imaging Systems : Imaging Systems “Chain” of stages One possible (in fact, common) sequence: Object/Source Collector (lens and/or mirror) Sensor Image Processing (computer or eye-brain) Display Issues in Astronomical Imaging: Issues in Astronomical Imaging Distances between objects and Earth Intrinsic “brightness” of object generally very faint large image collectors large range of brightness (dynamic range) Type of energy emitted/absorbed/reflected by the object wavelength regions Other considerations: motion of object brightness variations of object Astronomical Imaging: Overview: Astronomical Imaging: Overview When you think of a clear, dark night sky, what do you visualize? The human visual system is fine-tuned to focus, detect, and process (i.e., create an “image” of) the particular wavelengths where the Sun emits most of its energy evolutionary outcome we see best in the dominant available band of wavelengths As a result, when we look at the night sky, what we see is dominated by starlight (like the sun) We think of stars and planets when we think of astronomy History of Astronomical Imaging Systems: History of Astronomical Imaging Systems Oldest Instruments, circa 1000 CE – 1600 CE Used to measure angles and positions Included No Optics Astrolabe Octant, Sextant Tycho Brahe’s Mural Quadrant (1576) Star Catalog accurate to 1' (1 arcminute, limit of human resolution) Astronomical Observatories as part of European Cathedrals Mural Quadrant: Mural Quadrant Observations used by Johannes Kepler to derive the three laws of planetary motion Laws 1,2 published in 1609 Third Law in 1619 H.C. King, History of the Telescope History of Astronomical Imaging Systems: History of Astronomical Imaging Systems Optical Instruments, (1610+) Refracting Telescope Galileo Lippershey Hevelius Reflecting Telescope Newton (ca 1671) Spectroscope Newton Hevelius’ Refractor: Hevelius’ Refractor ca. 1650 Lenses with very long focal lengths – WHY? to minimize “induced color” (“chromatic aberration”) due to variation in refractive index with wavelength H.C. King, History of the Telescope Optical Dispersion: Optical Dispersion n Optical Dispersion: Optical Dispersion “Refractive Index” n measures the velocity of light in matter c = velocity in vacuum 3 108 meters/second v = velocity in medium measured in same units n 1.0 Optical Dispersion: Optical Dispersion Refractive index n of glass tends to DECREASE with increasing wavelength focal length f of lens tends to INCREASE with increasing wavelength Different colors “focus” at different distances “Chromatic Aberration” Chromatic Aberration: Chromatic Aberration Newton’s Reflector: Newton’s Reflector ca. 1671 1"-diameter mirror no chromatic aberration from mirror! H.C. King, History of the Telescope Reflection from Concave Mirror: Reflection from Concave Mirror All colors “focus” at same distance f f Larger Reflecting Telescopes: Larger Reflecting Telescopes Lord Rosse’s 1.8 m (6'-diameter) metal mirror, 1845 H.C. King, History of the Telescope History of Astronomical Imaging Systems: History of Astronomical Imaging Systems Image Recording Systems Chemical-based Photography wet plates, 1850 + dry plates, 1880+ Kodak plates, 1900+ Physics-based Photography, 1970 + Electronic Sensors, CCDs Electromagnetic Spectrum: Electromagnetic Spectrum History of Astronomical Imaging Systems: History of Astronomical Imaging Systems Infrared Wavelengths (IR) Longer waves than visible light conveys information about temperature images “heat” Absorbed by water vapor in atmosphere Courtesy of Inframetrics History of Astronomical Imaging Systems: History of Astronomical Imaging Systems Infrared Astronomy Wavelengths are longer than for visible light IR wavelengths range from ~1 micron to ~200 microns Over major portions of this range, IR is absorbed by water vapor in atmosphere Infrared Astronomy: Infrared Astronomy Because infrared light is generated by any “warm” objects, detector must be cooled to a lower temperature Uncooled detector is analogous to camera with an internal light source camera itself generates a signal Cooling is a BIG issue in Infrared Astronomy History of Astronomical Imaging Systems: History of Astronomical Imaging Systems History of Astronomical Infrared Imaging 1856: using thermocouples and telescopes (“one-pixel sensors”) 1900+: IR measurements of planets 1960s: IR survey of sky (Mt. Wilson, single pix detector) 1983: IRAS (Infrared Astronomical Satellite) 1989: COBE (Cosmic Background Explorer) History of Astronomical Imaging Systems: Infrared Astronomy: History of Astronomical Imaging Systems: Infrared Astronomy Airborne Observatories Galileo I (Convair 990), 1965 – 4/12/1973 (crashed) Frank Low, 12"–diameter telescope on NASA Learjet, 1968 Kuiper Airborne Observatory (KAO) (36"–diameter telescope) Stratospheric Observatory for IR Astronomy (under development: 2.4-meter diameter telescope on 747) Spaceborne Observatories “Orbiting Astronomical Observatory” (OAO), 1960s “Infrared Astronomical Satellite” (IRAS), 1980s Hubble Space Telescope (HST), 1990 (some IR astronomy) Infrared Satellite Observatory (ISO), 1995-1998 Spitzer Space Telescope (Aug. 2003-present) Kuiper Airborne Observatory: Kuiper Airborne Observatory Modified C-141 Starlifter 2/1974 – 10/1995 ceiling of 41,000' is above 99% of water vapor, which absorbs most infrared radiation Infrared Images: Infrared Images Visible Near Infrared Far Infrared 2Mass ISO http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irregions.html History of Astronomical Imaging Systems: Radio Astronomy: History of Astronomical Imaging Systems: Radio Astronomy Radio Waves Wavelengths are much longer than visible light millimeters (and longer) vs. hundreds of nanometers Selective History 1932: Karl Jansky (Bell Telephone Labs) investigated use of “short waves” for transatlantic telephone communication 1950s: Plans for 600-foot “Dish” in Sugar Grove, WV (for receiving Russian telemetry reflected from Moon) 1963: Penzias and Wilson (Bell Telephone Labs), “Cosmic Microwave Background” 1980: “Very Large Array” = VLA, New Mexico Jansky Radio Telescope: Jansky Radio Telescope Image courtesy of NRAO/AUI Large Radio Telescopes: Large Radio Telescopes http://www.naic.edu/about/ao/telefact.htm 305m at Arecibo, Puerto Rico 100m at Green Bank, WV Image courtesy of NRAO/AUI Very Large Array = VLA: Very Large Array = VLA Image courtesy of NRAO/AUI 27 telescopes each 25m diameter transportable via rail separations up to 36 km (22 miles) Issues in Astronomical Imaging: Issues in Astronomical Imaging Distances between objects and Earth Intrinsic “brightness” of object Type of energy emitted/absorbed/reflected by the object wavelength regions Motion of object What “Information” is Available from Astronomical Objects?: What “Information” is Available from Astronomical Objects? Emission of Matter Particles (protons, electrons, ions) “solar wind” solar “magnetic storm” aurorae (“northern lights”) Emission of Energy Light (in photon and/or wave model) visible light “invisible” light (ultraviolet, infrared, radio waves, X rays, ...) “Interaction” of matter and light Absorption/Reflection Matter can obscure light Example of Obscuration of Light by Matter: http://www.astro.univie.ac.at/~exgalak/koprolin/Photo/StarF/Cygnus_50mm.html Example of Obscuration of Light by Matter Dark Band in the Milky Way galaxy in “Cygnus” (the “northern cross” Light from stars “behind” the band is obscured The “Task” of Imaging: The “Task” of Imaging Collect the “information” from the object emitted light or particles absorbed light “Organize” it = “arrange” it View it Make judgments based upon observations Problems of Astronomical Imaging: Problems of Astronomical Imaging Objects are “Faint” little energy reaches Earth must expose for a “long” period of time to collect enough information (energy) Effects of Earth’s Atmosphere “twinkling”, disrupts images absorption of atmospheric molecules good and bad! reason for space-based observatories The Night Sky: Orion: The Night Sky: Orion Approximate view of Orion with unaided eye on a clear winter night (except for the added outlines) Star Brightness measured in “Magnitude” m: Star Brightness measured in “Magnitude” m Uses a “reversed” logarithmic scale Smaller Magnitudes Brighter Object (“golf score”) Sun: m -27 Full Moon: m -12 Venus (at maximum brilliancy): m -4.7 Sirius (brightest distant star): m -1.4 Faintest stars visible to unaided eye: m +5 to +6 Star Brightness measured in “Magnitude” m: Star Brightness measured in “Magnitude” m Decrease of 1 magnitude object brighter by factor of 2.5 decrease of 5 magnitudes from one star to another star increase in brightness by factor 100 decrease of 2.5 magnitudes from one star to another increase in brightness by factor 10 F, F0: number of photons received per second from object and from reference source, respectively. Magnitudes and Human Vision: Magnitudes and Human Vision Sensitivity of human vision is limited (in large part) by the length of time your brain can wait to receive and interpret the signals from the eye How long is that? How do you know? What if your retina could store collected signal over much longer times before reporting to the brain? Time between movie frames = 1/24 second Eye collects light for about 1/20 second before reporting to brain Time between video frames = 1/30 second Signal Collection(integration of signal): Signal Collection (integration of signal) t Constant signal (light) arriving over time t Total signal (light) collected over time a0 a0·t If your eye could integrate (collect) light longer,you might see this when you look at Orion!: If your eye could integrate (collect) light longer, you might see this when you look at Orion! Note: Stars have different colors: Note: Stars have different colors Betelgeuse (a red supergiant) Rigel (a blue supergiant) “Twinkling”: “Twinkling” Obvious when viewing stars, e.g., Sirius “point source” Not apparent when viewing planets “finite-size source” One Rationale for Space Observatories Twinkling: Twinkling Atmospheric Effects Distorts the Image distortion varies with time Remove the Atmosphere:No Twinkling: Remove the Atmosphere: No Twinkling Undistorted Image Stellar “Speckle”: Stellar “Speckle” Motivation for “Adaptive Optics” (AO) Detect and “undo” the distortions of the atmosphere on the images “Rubber-mirror” telescopes http://op.ph.ic.ac.uk/ao/overview.html Space Observatories: Space Observatories Located “above” the atmosphere No “twinkling” No absorption of wavelengths BUT: How to get the data down? LOTS of data EACH 4000 4000 RGB color image has 96 Megabytes of data (4000400023) Data transfer rate is important “Visible Light” spans only a TINY range of available electromagnetic information : “Visible Light” spans only a TINY range of available electromagnetic information VLA Differences Among Telescopes: Differences Among Telescopes Mechanism of Light Collection Reflection Diameters of Light “Collectors” Length of Optical Train Sensors NASA’s “Great Observatories”: NASA’s “Great Observatories” Chandra (July 1999) (formerly “AXAF” = Advanced X-ray Astrophysics Facility) HST = Hubble Space Telescope (1990) Spitzer Space Telescope (Aug. 2003) (formerly SIRTF = Space InfraRed Telescope Facility) Gone but not forgotten: Compton GRO = Gamma Ray Observatory Multiwavelength astronomy : Multiwavelength astronomy All-sky views at various wavelengths Images are centered on the Milky Way galaxy, which dominates the views Images from NASA Stars are only one ingredient in a galaxy! Orion Nebula (Messier # 42 = M42): Orion Nebula (Messier # 42 = M42) Cloud of dust and gas Stellar “Nursery” Telescopic Images: Telescopic Images HST image in visible light Ground-based photography Slide56: The Young Stars in Orion viewed at different wavelengths optical (HST) Slide57: infrared (2MASS) Radio (VLA --image courtesy of NRAO/AUI ) Other Issues in Astronomical Imaging: Other Issues in Astronomical Imaging Resolution Motion Resolution: Resolution Depends on wavelength l Longer waves “poorer” resolution for same size telescope Radio telescopes have HUGE collectors Motivation for “indirect” imaging algorithms “interferometry” increases resolution in a limited number of directions Proper Motion of Astronomical Objects : Proper Motion of Astronomical Objects movement of sky due to Earth’s rotation Earth rotates “counterclockwise” seen from above north pole, towards the east Sky appears to move from east to west Solar Day = 24h exactly Earth rotates 360.986º = 360º56'00" in 1 Solar Day 1 full revolution of sky = 360º in 23h 56'00“ 24 hours 15º per hour Proper Motion of Astronomical Objects : Proper Motion of Astronomical Objects movement of sky due to Earth’s revolution about Sun 360º in 365 days 1º per day 4 minutes of time per day Star positions change from night to night at same hour sets one hour earlier after about two weeks Sun: from Northern Hemisphere: Sun: from Northern Hemisphere Nadir Observer Facing South East 6 AM Sun: from Northern Hemisphere: Sun: from Northern Hemisphere Nadir Zenith On Meridian at 12 N Observer Facing South East Sun: from Northern Hemisphere: Sun: from Northern Hemisphere Nadir Zenith Observer Facing South East West 6 PM Sun: from Northern Hemisphere: Sun: from Northern Hemisphere Sun: from Northern Hemisphere: Sun: from Northern Hemisphere Nadir Zenith On Meridian at 12 N Observer Facing South East West 6 AM 6 PM Direction of Rotation of Earth: Direction of Rotation of Earth Sun Appears to: “Rise” in East “Set” in West (Actually, the Horizon) “Falls” in the East “Rises” in the West Earth rotates from West to East Speed of Rotation: Speed of Rotation One complete rotation in 1 day Sun’s location in sky moves 15º per hour BUT!: BUT! Earth also revolves in its orbit about Sun Earth’s Orbit: Earth’s Orbit January 1 January 15 n.b., Earth is closest to Sun in January (orbit is elliptical, not circular) Motion of Earth Around Sun: Motion of Earth Around Sun 365.25 days between arrivals at same point in orbit reason for “leap years” 3.94 minutes of time for sky to rotate 0.986º Earth’s Orbit: Earth’s Orbit Earth’s location Observer’s midnight on day 1 star is overhead AT midnight Earth’s Orbit: Earth’s Orbit Sun Rises Earth’s Orbit: Earth’s Orbit Sun Overhead Earth’s Orbit: Earth’s Orbit 6 PM Sun Sets Earth’s Orbit: Earth’s Orbit Earth’s location Observer’s midnight on day 2 star is overhead BEFORE midnight Earth’s Orbit: Earth’s Orbit Earth’s location Observer’s midnight on day 2 star is overhead BEFORE midnight Earth’s location Observer’s midnight on day 1 star is overhead AT midnight 12 N Earth’s Motion Around Sun: Earth’s Motion Around Sun Star “on the meridian” at 12:00M on December 1 will be “on the meridian” at about: 11:56 PM on December 2 11:52 PM on December 3 11:00 PM on December 15 10:00 PM on January 1 Time when star is at the same point in the sky (rising, on meridian, setting) get earlier by about 1 hour every 2 weeks Chief Impact of Earth-Sun Motion on Astronomical Imaging: Chief Impact of Earth-Sun Motion on Astronomical Imaging “diurnal” rotation of Earth requires compensating motion of the camera/telescope to keep the object in the field of view: camera/telescope moves from East to West axis of rotation points at celestial pole (at Polaris in northern hemisphere) Slide80: Axis of Rotation Polaris Telescope Tracking Slide81: Axis of Rotation Polaris Telescope Tracking Telescope Tracking: Telescope Tracking Axis of Rotation Polaris Proper Motion of Astronomical Objects : Proper Motion of Astronomical Objects “real” relative motion of object “proper motion” generally VERY small except for nearby objects Moon: 360º in 1 month 12º per day ½º per hour Moon moves its own diameter in the sky in about one hour Determines lengths of phases of eclipses Proper motions of Asteroids and Comets can be large must be “tracked” to take long-exposure images Apparent proper motions of planets are quite small Apparent proper motions of stars (even nearby stars) are very small – but still very measurable!