Ay 20 - Fall 2004 Lecture 2 - Caltech Astronomygeorge/ay20/Ay20-Lec2x.pdf(FIRST, NVSS, etc.), X-ray...
Transcript of Ay 20 - Fall 2004 Lecture 2 - Caltech Astronomygeorge/ay20/Ay20-Lec2x.pdf(FIRST, NVSS, etc.), X-ray...
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Ay 20 - Fall 2004Lecture 2
• Telescopes and basicoptics
• Atmospheric turbulenceand adaptive optics (AO)
• Radio telescopes andinterferometry
• Space observatories,high-energy astronomy
• Surveys, archives, data-rich astronomy, andVirtual Observatory (VO)
Note:This printout is missingmany pictures shown in
the class, in order tokeep the file sizereasonably small
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Basic Optics: Refraction
Index of refraction:n (l) = c / v (l)
Snell’s law: n1 sin q1 = n2 sin q2
If sin q2 = 1, then we have a total internal reflection forq1 > sin-1 (n2/n1) ; e.g., in optical fibers
e.g., nair ≈ 1.0003,nwater ≈ 1.33,nglass ~ 1.5, etc.
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Index of Refraction of the AirCauchy’s approximate formula:nair = 1.000287566 + (1.158102 œ 10 -9 m / l) 2 + O(l) 4
! ~ 5 œ 10 -6 in visible lightThus, Dl/l ~ 3 œ 10 -4 in visible light ~ 1 - 3 Å
Beware of the air vs. vacuum wavelengths in spectroscopy!Traditionally, wavelengths ≥ 3000 (2800?) Å are given as airvalues, and lower than that as vacuum values. Sigh.
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Lenses and Refractive OpticsNo longer used for professional telescopes,but still widely used within instruments
Focal length
Focal plane
Inverted images
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Lensmaker’s Formula
Using the Snell’s law, it canbe shown that
1/f = (n-1) (1/R1 + 1/R2)(aka the “lens power”)
where: f = focal lengthR1, R2 = curvature radii of thetwo lens surfaces
Note that for a sphericalmirror, f = R/2
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Magnification and Image Scale
M = F / f
y = f tan q ≈ f qscale: dq/dy = 1 / f
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ChromaticAberration
Cured by lensmultiplets …Or by reflectiveoptics!
Because the speedof light in anymedium is f(l)
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Reflecting Telescopes
Mirror figures: always conic sections, mostly paraboloid,sometimes hyperboloid (Cassegrain secondary, Ritchie-Chretenboth primary and sec.), rarely sphere (Schmidt, Maksutov).
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Gregorian vs. Cassegrain
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Palomar Hale 200-inch Telescope
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Keck
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Schmidt Telescopes:offering a large FOV(popular for sky surveys)
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Aberrations ChromaticAchromatic
Image Deterioration(spherical aberation,coma, astigmatism)
Image Distortion(Petzval field curvature,pincushion, barreldistortion)
Sphericalaberration:
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Spherical Aberration: The HST Saga
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Coma and Astigmatism
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Pincushion and Barrel Distortion
Petzval Field Curvature
Magnification varies asa function of off-axisdistance
Focal “plane” isactually spherical
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Modern Telescope Mirror Designs• Lightweight honeycomb structures• Thin meniscus (+ active optics)• Segmented (all segments parts of the same conic
surface); e.g., the Kecks, CELT/TMT• Multiple (each mirror/segment a separate
telescope, sharing the focus); e.g., HET, SALT• Liquid, spinningThe critical issues:
– Surface errors (should be < l/10)– Active figure support (weight, thermal)– Thermal equilibrium (figure, seeing)
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The History ofTelescopes
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Largetelescopeprojects1950-2020
Hale
Keck1
Keck2MMTHETGemini (x2)VLT (x4)Magellan….others
LBT (x2)GTC
CELT
HST
SIRTF
NGST
1949
1990
1995
2000
2005
2010
2015
2020
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Telescope Site Selection• Site selection is critically important
– Number of good nights and atmospheric quality determine theamount and the quality of the science done
• Site selection issues and problems– Atmospheric (seeing, transparency, AO issues, wind …)– Logistical (ease and cost of construction and operation)– Political/sociological (availability, security, staffing, etc.)– Geological (earthquakes, volcanos)
Historically, site selection was dominated by the seeing limitedvisible, conveniece (e.g., within a driving distance), and small orsubjective measurements. Nowadays the action is in the IR andAO, and the whole world is a stage.The Best Known Sites: Mauna Kea, Canarias, Northern Chile,Southern California + Baja, Namibia, Antarctica, + a few …
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Seeing measurementtelescopes at CerroTololo (CTIO) Ÿ
° Typical seeingdistribution
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And its successor:
James Webb SpaceTelescope (JWST)
Telescopes in Space
Hubble SpaceTelescope: only 2.4-m,but location, location,location!
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Diffraction-Limited Imaging(an ideal telescope)
The Airy function~ a Fourier transformof the actual opentelescope aperture
In reality, it tends to be morecomplex, due to the mirrorgeometry, etc.
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Diffraction-Limited ImagingWith no turbulence,
FWHM is diffractionlimit of telescope:
q [radians] ≈ l / DExample: l/D = 0.02 arc sec for l =
500 nm, D = 10 m
FWHM ~l/D
in units of l/D
1.22 l/D
With turbulence, image size (“seeing”) gets much larger,typically ~ 0.5 - 2 arcsec. In order to restore the intrinsicangular resolution, we need Adaptive Optics (AO)
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Turbulence arises in several placesstratosphere
tropopause
Heat sources within dome
boundary layer~ 1 km
wind flow over dome
10-12 km
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Images of a bright starLick Observatory, 1 m telescope
Longexposure
image
Shortexposure
image
Image withadaptiveoptics,(nearly)
diffractionlimited
J ~ 1 arc secJ ~ 0.1 arcsec ~ l / D
Speckles (each is atdiffraction limit of
telescope)
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Schematic of adaptive optics system
Feedbackloop: next
cyclecorrects
the (small)errors of
the lastcycle
Atmosphericturbulence
But you needa bright star
very close toyour target (afew arcsec) in
order tocompute the
correction
Deformable mirror
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If there’s no close-by bright star,create one with a laser!
Use a laser beam to create anartificial “star” at altitude of ~ 100km (Na layer, Na D doublet)
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Keck AO System Performance
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Single Dish (the bigger the better) …The Green Bank Telescope (GBT), D = 100 mArecibo, D = 300 m
… and Interferometers
They achieve the angular resolution corresponding to thelargest baseline between the elements (dishes), but thecollecting area is just the sum …
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How a Radio Telescope Works
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Problems With Single Dishes1. Poor resolution!2. Sidelobes pick up scattered radiation, interference
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How Interferometer WorksSignals from independent, separated receivers arecoherently combined (correlated). What is measured isthe amplitude of correlated signal as a function of aspatial baseline, i.e.,angular frequencyon the sky. This isa Fourier transformof the actual intensityimage on the sky.
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… how interferometer works …
Signals from individualelements are delayedelectronically, in orderto simulate a flatwavefront, for slightlydifferent arrivaldirections - thusmapping a field ofview.
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Very Long Baseline Interferometry (VLBI)• Antennas very far apart (~ Earth size)
H Resolution very high: milli-arcsec• Record signals on tape, correlate later• Now VLBA(rray)
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The Future of Radio Astronomy
Square KilometerArray (SKA)
ALMA
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X-Ray telescopes:Grazing incidence mirrors
Why? So that theprojected interatomicseparations are << l
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Detecting Ultra-High Energy Cosmic Rays
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CGRO/COMPTEL
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Sky Surveys, Archives, andVirtual Observatory
Astronomy is facing amajor data avalanche:it has become animmensely data-richscience
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… And also datacomplexity and quality,driven by the exponentialgrowth in detector andcomputing technology
19701975
19801985
19901995
2000
0.1
1
10
100
1000
CCDs Glass
The Exponential Growth of DataVolume in Astronomy
doubling t ≈ 1.5 yrs
• Large digital sky surveys are becoming the dominantsource of data in astronomy: ~ 10-100 TB/survey (soonPB), ~ 106 - 109 sources/survey, many wavelengths…
• Data sets many orders of magnitude larger, morecomplex, and more homogeneous than in the past
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1 microSky (DPOSS)
1 nanoSky (HDF-S)
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A Bit of History …
Modern sky surveyswere effectivelyinvented at Palomar:Zwicky, POSS-I,POSS-II (DPOSS), now Palomar-Quest…
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The Changing Face of Observational Astronomy
• Large digital sky surveys are becoming the dominantsource of data in astronomy: currently > 200 TB, andgrowing rapidly
• Spanning a range of wavelengths: visible (SDSS,DPOSS, etc.), IR (2MASS, COBE, IRAS, etc.), radio(FIRST, NVSS, etc.), X-ray (RASS, HEAO, etc.) …
• Also: digital libraries, electronic journals, space missionand observatory archives, microlensing experiments,searches for Solar system objects …
• Data sets orders of magnitude larger, more complex,and more homogeneous than in the past
• Roughly 1 TB/Sky/band/epoch– NB: Human Genome is < 1 GB, Library of Congress ~ 20 TB
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The Changing Style of ObservationalAstronomy
VirtualObservatory
Archives ofpointed
observations (~ afew TB)
Small samples ofobjects (~ 101 - 103)
Multiple, federatedsky surveys andarchives (~ PB)
Large,homogeneous skysurveys (multi-TB,~ 106 - 109 sources
Pointed,heterogeneousobservations(~ MB - GB)
Future:Now:The Old Way:
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It will be a complete, distributed, web-basedresearch environment for astronomy with massiveand complex data sets:• Federate major data archives, and provide toolsfor the data exploration• A framework to harness developments ininformation technology for the benefit of astronomy
In the US: the National Virtual Observatory (NVO)(see http://us-vo.org)Globally: International V.O. Alliance (IVOA)
So, What is a Virtual Observatory?
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VO: Conceptual Architecture
Data ArchivesData Archives
Analysis toolsAnalysis tools
Discovery toolsDiscovery toolsUser
Gateway
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This quantitative change in the informationvolume and complexity will enable the
Science of a Qualitatively Different Nature:• Statistical astronomy done right
– Precision cosmology, Galactic structure, stellar astrophysics …– Discovery of significant patterns and multivariate correlations– Poissonian errors unimportant
• Systematic exploration of the observableparameter spaces (NB: Energy content = Information content)
– Searches for rare or unknown types of objects and phenomena– Low surface brightness universe, the time domain …
• Confronting massive numerical simulationswith massive data sets
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Information Technology Ÿ New Science• The information volume grows exponentially
Most data will never be seen by humans! The need for data storage, network, database-related
technologies, standards, etc.• Information complexity is also increasing greatly
Most data (and data constructs) cannot becomprehended by humans directly!
The need for data mining and data understandingtechnologies, hyperdimensional visualization,AI/Machine-assisted discovery …
• These challenges are common to most sciences (andalso commerce, industry, security …) - what we developmay find some broad applications (remember WWW!)
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• Information technology is revolutionizing all. sciences, including astronomy. VO is the. framework for this change, the astronomy of the. 21st century• We are expecting a new era of systematic. exploration of the universe, with many new. discoveries and surprizes• The key issues are methodological: we have to. learn to ask new kinds of questions, enabled by. the massive data sets and technology
In Summary …