1. SIMG-217 Fundamentals of Astronomical Imaging Systems Joel Kastner 76-2100 475-7179 kastner@cis.rit.edu

2. 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 require- ments are then investigated in the context of the astronomical imaging chain. Imaging chains in the optical, X-ray, and/or radio wavelength regimes will be studied in detail as time permits. (1051-215 or permission of instructor) Class 3, Lab 1,Credit 4 (W, S)

3. Laboratory 4 mandatory experiments, most likely: –Star Colors from Digital Images –Spectroscopic Imaging of Gases –Multiwavelength Imaging of the Sun –Multiwavelength Imaging of the Orion Nebula possibly: 1 optional experiment/Project –collect/process images taken at RIT observatory

4. 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?

5. 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)

6. Imaging Systems “Chain” of stages One possible (in fact, common) sequence: 1. Object/Source 2. Collector (lens and/or mirror) 3. Sensor 4. Image Processing (computer or eye-brain) 5. Display

7. 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

8. 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

9. 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

10. 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

11. History of Astronomical Imaging Systems Optical Instruments, (1610+) –Refracting Telescope Galileo Lippershey Hevelius –Reflecting Telescope Newton (ca 1671) –Spectroscope Newton

12. 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

13. Optical Dispersion n

14. Optical Dispersion “Refractive Index” n measures the velocity of light in matter c = velocity in vacuum  3  10 8 meters/second v = velocity in medium measured in same units n  1.0

15. 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”

16. Chromatic Aberration

17. Newton’s Reflector ca. 1671 1"-diameter mirror no chromatic aberration from mirror! H.C. King, History of the Telescope

18. Reflection from Concave Mirror All colors “focus” at same distance f f

19. Larger Reflecting Telescopes Lord Rosse’s 1.8 m (6'-diameter) metal mirror, 1845 H.C. King, History of the Telescope

20. 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

21. Electromagnetic Spectrum

22. 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

23. 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

24. 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

25. 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)

26. History of Astronomical Imaging Systems Airborne Observatories –Infrared Astronomy 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) 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

27. 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

28. Infrared Images Visible Near InfraredFar Infrared 2Mass ISO http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irregions.html

29. History of Astronomical Imaging Systems Radio Waves –Wavelengths are much longer than visible light millimeters (and longer) vs. hundreds of nanometers 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

30. Jansky Radio Telescope Image courtesy of NRAO/AUI

31. 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

32. Very Large Array = VLA Image courtesy of NRAO/AUI 27 telescopes each 25m diameter transportable via rail separations up to 36 km (22 miles)

33. 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

34. 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

35. 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

36. 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

37. 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

38. The Night Sky: Orion Approximate view of Orion with unaided eye on a clear winter night (except for the added outlines)

39. 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

40. Star Brightness measured in “Magnitude” m Increase of 1 magnitude  object fainter by factor of 2.5 –increase of 5 magnitudes  decrease in brightness by 0.01 –increase of 2.5 magnitudes  decrease in brightness by 0.1 F, F 0 : number of photons received per second from object and from reference source, respectively.

41. 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 before reporting to the brain (i.e., “integrate” the signal over time) Time between movie frames = 1/24 second  Eye “integrates” light for about 1/20 second Time between video frames = 1/30 second

42. Signal Integration t Signal t Integrated Signal a0a0 a0·ta0·t

43. If your eye could integrate longer, you might see this when you look at Orion!

44. n.b., Stars have different colors Betelgeuse (a red supergiant) Rigel (a blue supergiant)

45. “Twinkling” Obvious when viewing stars, e.g., Sirius –“point source” Not apparent when viewing planets –“finite-size source” One Rationale for Space Observatories

46. Twinkling Atmospheric Effects Distorts the Image distortion varies with time

47. Remove the Atmosphere: No Twinkling Undistorted Image

48. 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

49. 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 (4000  4000  2  3) –Data transfer rate is important

50. “Visible Light” spans only a TINY range of available electromagnetic information VLA

51. Differences Among Telescopes Mechanism of Light Collection –Reflection Diameters of Light “Collectors” Length of Optical Train Sensors

52. 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

53. Gamma Ray Multiwavelength astronomy X-ray Visible Infrared Radio Waves 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!

54. Orion Nebula (Messier # 42 = M42) Cloud of dust and gas Stellar “Nursery”

55. Telescopic Images HST image in visible light Ground-based photography

56. The Young Stars in Orion viewed at different wavelengths infrared (2MASS) optical (HST) X-Ray (Chandra)

57. infrared (2MASS) Radio (VLA --image courtesy of NRAO/AUI )

58. Other Issues in Astronomical Imaging Resolution Motion

59. Resolution Depends on wavelength –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

60. 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

61. 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

62. Sun: from Northern Hemisphere Nadir Observer Facing South East 6 AM

63. Sun: from Northern Hemisphere Nadir Zenith On Meridian at 12 N Observer Facing South East

64. Sun: from Northern Hemisphere Nadir Zenith Observer Facing South East West 6 PM

65. Sun: from Northern Hemisphere On Meridian at 12 N Nadir Zenith Observer Facing South East West 6 AM 6 PM Earth’s Rotation, W to E

66. Sun: from Northern Hemisphere Nadir Zenith On Meridian at 12 N Observer Facing South East West 6 AM 6 PM Earth’s Rotation, W to E

67. 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

68. Speed of Rotation One complete rotation in 1 day Sun’s location in sky moves 15º per hour

69. BUT! Earth also revolves in its orbit about Sun

70. Earth’s Orbit January 1 January 15 n.b., Earth is closest to Sun in January (orbit is elliptical, not circular)

71. 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

72. Earth’s Orbit distant star 12 M Earth’s location Observer’s midnight on day 1 star is overhead AT midnight Earth’s Rotation Earth’s Orbit about Sun

73. Earth’s Orbit distant star 6 AM Earth’s Rotation Earth’s Orbit about Sun Sun Rises

74. Earth’s Orbit distant star 12 N Earth’s Rotation Earth’s Orbit about Sun Sun Overhead

75. Earth’s Orbit distant star 6 PM Earth’s Rotation Earth’s Orbit about Sun Sun Sets

76. Earth’s Orbit distant star 12 M Earth’s location Observer’s midnight on day 2 star is overhead BEFORE midnight Earth’s Rotation Earth’s Orbit about Sun

77. Earth’s Orbit distant star 12 M 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 6 AM 12 N Earth’s Rotation Earth’s Orbit about Sun

78. 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

79. 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)

80. Axis of Rotation Polaris Telescope Tracking

81. Axis of Rotation Polaris Telescope Tracking

82. Axis of Rotation Polaris

83. 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 make long exposures Apparent proper motions of planets are quite small Apparent proper motions of stars are infinitesmally small!