SIMG-217 Fundamentals of Astronomical Imaging

SIMG-217Fundamentals of

Astronomical Imaging

Instructor: Joel Kastner

Office: 17-3190

Phone: 475-7179

Email: kastner@cis.rit.edu

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:– 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

• 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

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 = 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!

H.C. King, History of the Telescope

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

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– 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

• 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

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

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

Image courtesy of NRAO/AUI

• 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

• 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

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 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)

t

Constantsignal (light)arrivingover time

t

Totalsignal(light)

collected over time

a0

a0·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

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

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

Nadir

ObserverFacing South

East

6 AM

Sun: from Northern Hemisphere

Nadir

Zenith

On Meridianat 12 N

ObserverFacing South

East

Sun: from Northern Hemisphere

Nadir

Zenith

ObserverFacing South

East West

6 PM

Sun: from Northern Hemisphere

On Meridianat 12 N

Nadir

Zenith

ObserverFacing South

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

Earth’s Orbit about Sun

Sun Rises

Earth’s Orbit

distantstar

12 N

Earth’s Rotation

Earth’s Orbit about Sun

Sun Overhead

Earth’s Orbit

distantstar

6 PM

Earth’s Rotation

Earth’s Orbit about Sun

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 about Sun

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 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)

Axis of Rotation

PolarisTelescope Tracking

Axis of Rotation

PolarisTelescope Tracking

Telescope Tracking

Axis of Rotation

Polaris

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!