SIMG-217 Fundamentals of Astronomical Imaging Systems Joel Kastner 76-2100 475-7179...

SIMG-217Fundamentals of Astronomical

Imaging Systems

Joel Kastner

76-2100

475-7179

[email protected]

Course DescriptionFamiliarizes students with the goals and techniques of astronomicalimaging. The broad nature of astronomical sources will be outlined interms of requirements on astronomical imaging systems. These require-ments are then investigated in the context of the astronomicalimaging chain. Imaging chains in the optical, X-ray, and/or radiowavelength regimes will be studied in detail as time permits.

(1051-215 or permission of instructor) Class 3, Lab 1,Credit 4 (W, S)

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

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

• 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

“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

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


• 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


Optical Dispersion

n

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

v

cn

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!


Reflection from Concave Mirror

All colors “focus” at same distance f

f

Larger Reflecting Telescopes

• Lord Rosse’s 1.8 m (6'-diameter) metal mirror, 1845



• Image Recording Systems– Chemical-based Photography

• wet plates, 1850 +

• dry plates, 1880+

• Kodak plates, 1900+

– Physics-based Photography, 1970 +• Electronic Sensors, CCDs

Electromagnetic Spectrum


• Infrared Wavelengths (IR)– Longer waves than visible light

– conveys information about temperature• images “heat”

– Absorbed by water vapor in atmosphere

Courtesy of Inframetrics


• 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 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)


• 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

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 Far Infrared

2Mass ISO

http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/irregions.html


• 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

Jansky Radio Telescope

Image courtesy of NRAO/AUI

Large Radio Telescopes

http://www.naic.edu/about/ao/telefact.htm

305m at Arecibo, Puerto Rico

100m at Green Bank, WV


Very Large Array = VLA


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

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

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

• 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

• 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

100

2.5 logF

mF

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?

– 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

Signal Integration

t

Signal

t

IntegratedSignal

a0

a0·t

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

n.b., 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

TwinklingAtmospheric EffectsDistorts the Image

distortion varies with time

Remove the Atmosphere:No Twinkling

UndistortedImage

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 (4000400023)

– 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

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!

Orion Nebula (Messier # 42 = M42)

Cloud of dust and gasStellar “Nursery”

Telescopic Images

HST image in visible light

Ground-based photography

The Young Stars in Orion viewed at different wavelengths

infrared (2MASS)

optical (HST)

X-Ray (Chandra)

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


• 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

Nadir

ObserverFacing South

East

6 AM


Nadir

Zenith

On Meridianat 12 N


East


Nadir

Zenith


East West

6 PM


On Meridianat 12 N

Nadir

Zenith


East West

6 AM6 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

• Sun’s location in sky moves 15º per hour

36015 per hour

24hours

BUT!

• Earth also revolves in its orbit about Sun

Earth’s Orbit

January 1

January 15

n.b., Earth is closest to Sun in January(orbit is elliptical, notcircular)

Motion of Earth Around Sun

• 365.25 days between arrivals at same point in orbit– reason for “leap years”

365.25 days/year0.986 /day

360 /year

0.986 minutes0.986 at 15 / hour = 60 3.94minutes

15 hour

3.94 minutes of time for sky to rotate 0.986º

Earth’s Orbit

distantstar

12 M

Earth’s locationObserver’s midnight on day 1star is overhead AT midnight

Earth’s Rotation

Earth’s Orbit about Sun

Earth’s Orbit

distantstar

6 AM

Earth’s Rotation


Sun Rises

Earth’s Orbit

distantstar

12 N

Earth’s Rotation


Sun Overhead

Earth’s Orbit

distantstar

6 PM

Earth’s Rotation


Sun Sets

Earth’s Orbit

distantstar

12 M

Earth’s locationObserver’s midnight on day 2star is overhead BEFORE midnight

Earth’s Rotation


Earth’s Orbit

distantstar

12 M

12 M

Earth’s locationObserver’s midnight on day 2star is overhead BEFORE midnight

Earth’s locationObserver’s midnight on day 1star is overhead AT midnight

6 AM12 N

Earth’s Rotation


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)

Axis of Rotation

PolarisTelescope Tracking

Telescope Tracking

Axis of Rotation

Polaris


• “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!

SIMG-217 Fundamentals of Astronomical Imaging Systems Joel Kastner 76-2100 475-7179...

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Transcript of SIMG-217 Fundamentals of Astronomical Imaging Systems Joel Kastner 76-2100 475-7179...