SIMG-217 Fundamentals of Astronomical Imaging

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SIMG-217Fundamentals of Astronomical Imaging

• Instructor Joel Kastner
• Office 17-3190
• Phone [phone]
• Email kastner_at_cis.rit.edu

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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)
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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

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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?

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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)

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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

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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

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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

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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 Brahes Mural Quadrant (1576)
• Star Catalog accurate to 1' (1 arcminute, limit
  of human resolution)
• Astronomical Observatories as part of European
  Cathedrals

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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
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History of Astronomical Imaging Systems

• Optical Instruments, (1610)
• Refracting Telescope
• Galileo
• Lippershey
• Hevelius
• Reflecting Telescope
• Newton (ca 1671)
• Spectroscope
• Newton

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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
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Optical Dispersion
n
?
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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

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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

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Chromatic Aberration
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Newtons Reflector

• ca. 1671
• 1"-diameter mirror
• no chromatic aberration from mirror!

H.C. King, History of the Telescope
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Reflection from Concave Mirror
f
All colors focus at same distance f
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Larger Reflecting Telescopes

• Lord Rosses 1.8 m (6'-diameter) metal mirror,
  1845

H.C. King, History of the Telescope
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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

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Electromagnetic Spectrum
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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
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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

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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

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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)

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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)

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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

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Infrared Images
Visible Near
Infrared Far Infrared
2Mass
ISO
http//coolcosmos.ipac.caltech.edu/cosmic_classroo
m/ir_tutorial/irregions.html
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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

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Jansky Radio Telescope
Image courtesy of NRAO/AUI
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Large Radio Telescopes
100m at Green Bank, WV
305m at Arecibo, Puerto Rico
Image courtesy of NRAO/AUI
http//www.naic.edu/about/ao/telefact.htm
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Very Large Array VLA

• 27 telescopes
• each 25m diameter
• transportable via rail
• separations up to 36 km (22 miles)

Image courtesy of NRAO/AUI
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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

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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

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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

http//www.astro.univie.ac.at/exgalak/koprolin/Ph
oto/StarF/Cygnus_50mm.html
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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

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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 Earths Atmosphere
• twinkling, disrupts images
• absorption of atmospheric molecules
• good and bad!
• reason for space-based observatories

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

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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.
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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
Time between video frames 1/30 second

• ? Eye collects light for about 1/20 second
  before reporting to brain

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Signal Collection(integration of signal)
Constant signal (light) arriving over time
a0
t
a0t
Total signal (light) collected over time
t
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If your eye could integrate (collect) light
longer,you might see this when you look at Orion!
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Note Stars have different colors
Betelgeuse (a red supergiant)
Rigel (a blue supergiant)
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Twinkling

• Obvious when viewing stars, e.g., Sirius
• point source
• Not apparent when viewing planets
• finite-size source
• One Rationale for Space Observatories

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Twinkling
Atmospheric Effects Distorts the Image
distortion varies with time
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Remove the AtmosphereNo Twinkling
Undistorted Image
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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

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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

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Visible Light spans only a TINY range of
available electromagnetic information
VLA
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Differences Among Telescopes

• Mechanism of Light Collection
• Reflection
• Diameters of Light Collectors
• Length of Optical Train
• Sensors

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NASAs 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

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Multiwavelength astronomy

• All-sky views at various wavelengths
• Images are centered on the Milky Way galaxy,
  which dominates the views

Stars are only one ingredient in a galaxy!
Images from NASA
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Orion Nebula (Messier 42 M42)
Cloud of dust and gas Stellar Nursery
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Telescopic Images
HST image in visible light
Ground-based photography
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The Young Stars in Orion viewed at different
wavelengths
optical (HST)
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infrared (2MASS)
Radio (VLA --image courtesy of NRAO/AUI )
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Other Issues in Astronomical Imaging

• Resolution
• Motion

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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

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Proper Motion of Astronomical Objects

• movement of sky due to Earths 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

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Proper Motion of Astronomical Objects

• movement of sky due to Earths 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

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Sun from Northern Hemisphere
East
6 AM
Observer Facing South
Nadir
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Sun from Northern Hemisphere
On Meridian at 12 N
Zenith
East
Observer Facing South
Nadir
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Sun from Northern Hemisphere
Zenith
East
West
6 PM
Observer Facing South
Nadir
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Sun from Northern Hemisphere
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Sun from Northern Hemisphere
On Meridian at 12 N
Zenith
East
West
6 AM
6 PM
Observer Facing South
Nadir
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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

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Speed of Rotation

• One complete rotation in 1 day
• Suns location in sky moves 15º per hour

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BUT!

• Earth also revolves in its orbit about Sun

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Earths Orbit
January 15
January 1
n.b., Earth is closest to Sun in January (orbit
is elliptical, not circular)
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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
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Earths Orbit
Earths location Observers midnight on day
1 star is overhead AT midnight
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Earths Orbit
Sun Rises
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Earths Orbit
Sun Overhead
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Earths Orbit
6 PM
Sun Sets
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Earths Orbit
Earths location Observers midnight on day
2 star is overhead BEFORE midnight
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Earths Orbit
Earths location Observers midnight on day
2 star is overhead BEFORE midnight
12 N
Earths location Observers midnight on day
1 star is overhead AT midnight
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Earths Motion Around Sun

• Star on the meridian at 1200M on December 1
  will be on the meridian at about
• 1156 PM on December 2
• 1152 PM on December 3
• 1100 PM on December 15
• 1000 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

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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)

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Telescope Tracking
Polaris
Axis of Rotation
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Telescope Tracking
Polaris
Axis of Rotation
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Telescope Tracking
Polaris
Axis of Rotation
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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!