Download Astronomical Imaging: Overview

SIMG-217
Fundamentals of
Astronomical Imaging
Instructor: Joel Kastner
Office: 17-3190
Phone: 475-7179
Email: [email protected]
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
•
3 mandatory experiments, selected from:
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•
Star Colors from Digital Images
Spectroscopic Imaging of Gases
Multiwavelength Imaging of the Sun
Multiwavelength Imaging of the Orion Nebula
Final Project; one of:
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–
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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
•
•
•
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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
• Create an “image” of a scene that may be
measured to calculate some parameter of
the scene
–
–
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–
Diagnostic X ray
Digital Photograph
“CAT” Scan (computed tomography)
“MRI” (magnetic resonance imaging)
Imaging Systems
“Chain” of stages
One possible (in fact, common) sequence:
1.
2.
3.
4.
5.
Object/Source
Collector (lens and/or mirror)
Sensor
Image Processing (computer or eye-brain)
Display
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
• 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
• 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
• 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
• Optical Instruments, (1610+)
– Refracting Telescope
• Galileo
• Lippershey
• Hevelius
– Reflecting Telescope
• Newton (ca 1671)
– Spectroscope
• Newton
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
n

Optical Dispersion
• “Refractive Index” n measures the velocity
of light in matter
c
n
v
c = velocity in vacuum  3 108 meters/second
v = velocity in medium measured in same units
n  1.0
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
Newton’s Reflector
• ca. 1671
• 1"-diameter mirror
• no chromatic
aberration from
mirror!
H.C. King, History of the Telescope
Reflection from Concave Mirror
f
All colors “focus” at same distance f
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
• Image Recording Systems
– Chemical-based Photography
• wet plates, 1850 +
• dry plates, 1880+
• Kodak plates, 1900+
– Physics-based Photography, 1970 +
• Electronic Sensors, CCDs
Electromagnetic Spectrum
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
• 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
• 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 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
• Airborne Observatories
–
–
–
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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
–
–
–
–
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“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
• 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
Visible
Near Infrared
2Mass
Far Infrared
ISO
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irregions.html
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
Image courtesy of NRAO/AUI
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
Very Large Array = VLA
• 27 telescopes
• each 25m diameter
• transportable via
rail
• separations up to 36
km (22 miles)
Image courtesy of NRAO/AUI
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?
• 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
• 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/Photo/StarF/Cygnus_50mm.html
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
• 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
Approximate view of Orion with unaided eye on a
clear winter night (except for the added outlines)
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
• 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
m  2.5  log10  
 F0 
F, F0: number of photons received per second from
object and from reference source, respectively.
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?
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
– What if your retina could store collected signal over much
longer times before reporting to the brain?
Signal Collection
(integration of signal)
Constant
signal
(light)
arriving
over
a0
time
t
a0·t
Total
signal
(light)
collected
over time
t
If your eye could integrate (collect) light longer,
you might see this when you look at Orion!
Note: Stars have different colors
Betelgeuse
(a red supergiant)
Rigel
(a blue supergiant)
“Twinkling”
• Obvious when viewing stars, e.g., Sirius
– “point source”
• Not apparent when viewing planets
– “finite-size source”
• One Rationale for Space Observatories
Twinkling
Atmospheric Effects
Distorts the Image
distortion varies with time
Remove the Atmosphere:
No Twinkling
Undistorted
Image
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
• 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
“Visible Light” spans only a TINY range
of available electromagnetic information
VLA
Differences Among Telescopes
• Mechanism of Light Collection
– Reflection
• Diameters of Light “Collectors”
• Length of Optical Train
• Sensors
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
• All-sky views at various
wavelengths
• Images are centered on the
Milky Way galaxy, which
dominates the views
Gamma Ray
X-ray
Visible
Stars are only one ingredient
in a galaxy!
Infrared
Radio Waves
Images from NASA
Orion Nebula (Messier # 42 = M42)
Cloud of dust and gas
Stellar “Nursery”
Telescopic Images
HST image in visible light
Ground-based photography
The Young Stars in Orion viewed at
different wavelengths
optical (HST)
X-Ray (Chandra)
infrared (2MASS)
infrared (2MASS)
Radio (VLA --image courtesy
of NRAO/AUI )
Other Issues in Astronomical
Imaging
• Resolution
• Motion
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
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
• 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
East
6 AM
Observer
Facing South
Nadir
Sun: from Northern Hemisphere
On Meridian
at 12 N
Zenith
East
Observer
Facing South
Nadir
Sun: from Northern Hemisphere
Zenith
East
West
Observer
Facing South
Nadir
6 PM
Sun: from Northern Hemisphere
On Meridian
at 12 N
Zenith
East
West
6 AM
Observer
Facing South
Nadir
6 PM
Earth’s Rotation, W to E
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
• One complete rotation in 1 day

360

 15 per hour
24 hours
• Sun’s location in sky moves 15º per hour
BUT!
• Earth also revolves in its orbit about Sun
Earth’s Orbit
January 15
January 1
n.b., Earth is closest
to Sun in January
(orbit is elliptical, not
circular)
Motion of Earth Around Sun
• 365.25 days between arrivals at same point
in orbit
– reason for “leap years”
365.25 days/year


0.986
/day

360 /year

0.986
minutes
0.986 at 15 / hour =
 60
 3.94 minutes

15
hour


3.94 minutes of time for sky to rotate 0.986º
Earth’s Orbit
distant
star
Earth’s location
Observer’s midnight on day 1
12 M
star is overhead AT midnight
Earth’s Rotation
Earth’s Orbit about Sun
Earth’s Orbit
Sun Rises
6 AM
Earth’s Rotation
Earth’s Orbit about Sun
distant
star
Earth’s Orbit
Sun Overhead
12 N
Earth’s Rotation
Earth’s Orbit about Sun
distant
star
Earth’s Orbit
6 PM
Sun Sets
Earth’s Rotation
Earth’s Orbit about Sun
distant
star
Earth’s Orbit
12 M
Earth’s location
Observer’s midnight on day 2
star is overhead BEFORE midnight
distant
star
Earth’s Rotation
Earth’s Orbit about Sun
Earth’s Orbit
12 M
Earth’s location
Observer’s midnight on day 2
star is overhead BEFORE midnight
distant
star
12 N
6 AM
Earth’s location
Observer’s midnight on day 1
12 M
star is overhead AT midnight
Earth’s Rotation
Earth’s Orbit about 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
• “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)
Telescope Tracking
Polaris
Axis of
Rotation
Telescope Tracking
Polaris
Axis of
Rotation
Telescope Tracking
Polaris
Axis of
Rotation
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!