Astrometry: Revealing the Other Astrometry: Revealing the Other Two - - PowerPoint PPT Presentation

SLIDE 1

Astrometry: Revealing the Other Astrometry: Revealing the Other Two Dimensions of Velocity Two Dimensions of Velocity Space Space

H.A. McAlister H.A. McAlister Center for High Angular Resolution Astronomy Center for High Angular Resolution Astronomy Georgia State University Georgia State University

SLIDE 2

Astrometry: What is It?

• Astrometry deals with the measurement of the positions and

motions of astronomical objects on the celestial sphere. Two main subfields are:

• Spherical Astrometry – Determines an inertial reference

frame, traditionally using meridian circles, within which stellar proper motions and parallaxes can be measured. Also known as fundamental or global astrometry.

• Plane Astrometry – Operates over a restricted field of view,

using, through much of the 20th century, long-focus refractors at plate scales of 15 to 20 arcsec/mm, with the goal of accurately measuring stellar parallax. Also known as long-focus or small-angle astrometry.

• Astrometry relies on specialized instrumentation and
• bservational and analysis techniques. It is fundamental to all
• ther fields of astronomy.

SLIDE 3

Astrometry: What are its tools?

• Spherical Astrometry:
• Meridian circles & transits
• Astrolabes
• Space (eg. Hipparcos)
• Phased optical interferometry
• Very long baseline radio interferometry
• Plane Astrometry:
• Photography (until relatively recently)
• Visual Micrometry (until relatively recently)
• CCD imaging
• Scanning photometry
• Speckle interferometry (O/IR)
• Michelson interferometry (O/IR)
• Radio interferometry
• Space (imaging & interferometry)

SLIDE 4

Some 19th & 20th C. Astrometric Instruments

Greenwich Meridian Circle House

Photo from National Maritime Museum, Greenwich

USNO 6-in Transit Circle 1898-1995

U.S. Naval Observatory Photo

Worley’s recording filar micrometer at USNO

• W. Finsen’s Eyepiece

Interferometer

• c. 1960

U.S. Naval Observatory Photo

Hipparcos Test 1988

ESA Photo

SLIDE 5

Spherical Astrometry

What does it measure? Spherical Astrometry attempts to measure stellar & planetary

positions and motions in an inertial reference frame. This is very challenging and must account for effects of:

• Proper motion
• Parallax
• Galactic rotation
• Precession
• Nutation
• Aberration
• Polar wandering
• Refraction
• Solar motion
• Earth dynamics (including time)

Oh, yeah, and whatever the heck systematics are in your instrumental and reduction methodology! Sound difficult? It is! A truly inertial reference frame has been achieved through the tie-in to extragalactic radio sources using VLBI.

SLIDE 6

Plane Astrometry

What does it measure? Plane Astrometry attempts to measure small angle effects

relative to a background of one or more reference stars. These effects include:

• Proper motion
• Parallax (relative and absolute)
• Secular acceleration
• Sub-motions arising from unseen companions
• Binary star relative separation and orientation
• Stellar diameters

And, here again, whatever the heck systematics are in your instrumental and reduction methodology! Also sound difficult? It is!

SLIDE 7

Astrometry

What does it give us? Spherical Astrometry gives us:

• Galactic kinematics and dynamics
• Solar system dynamics
• Time
• Periodic and secular changes in Earth orientation
• Navigation (mostly historical except during WWIII)

And, yet again, the systematics in your instrumental and reduction methodology must be understood! Sound useful? It sure is!

Plane Astrometry gives us, when combined with other

techniques, stellar:

• Luminosities
• Masses
• Diameters (and shapes)
• Temperatures
• Multiplicity frequencies
• Space velocities
• Distribution in solar

neighborhood

• Unseen companions
• Calibration of cosmic

distance scale

“Astrometry is the metrological basis of astronomy”

from J. Kovalevsky, Modern Astrometry, p.8.

SLIDE 8

Astrometry for Dummies

This says it all

(Circulated c. 1974 by Ron Probst at UVa)

SLIDE 9

Okay, here’s a basic: Trigonometric Parallax

A star is observed against a background of distant stars at 6-month intervals Earth’s orbit Trig Parallax =

• t1

t2 t2 t1 2 x The nearby star appears to shift back and forth with respect to the background stars Nearby Star Very Distant Background Stars

SLIDE 10

A star is observed against a background of distant stars at 6-month intervals Earth’s orbit Trig Parallax =

• t1

t2 t2 t1 2 x A somewhat more distant star presents a smaller back and forth motion with respect to the background stars More Distant Star Very Distant Background Stars

Okay, here’s a basic: Trigonometric Parallax

SLIDE 11

The Challenge of Astrometry

• Astrometry constantly pushes the limits imposed by the

current status of its instrumentation and reduction & analysis techniques.

• Historically, breakthroughs in astrometry are almost always

the result of discrete advances in instrumentation followed by refinements made by clever people.

• Ultimately, astrometry is all about understanding and

probing the distinction between precision and accuracy.

SLIDE 12

Gain in Accuracy

Adapted from Parallax by A. W. Hirshfield

Observer Technique Date Limiting Gain Accuracy

Hipparchus visual sextant 150 BC 5 arcmin 1 Tycho visual quadrant 1600 1 arcmin 5 Flamsteed mural quadrant 1700 10 arcsec 30 Bradley improved quadrant 1750 0.5 arcsec 600 Bessel visual heliometer 1835 0.1 arcsec 3x103 Schlesinger et al. early photography 1920 0.05 arcsec 6x103 USNO et al. later photography 1970 5 mas 6x104 Various speckle interferometry 1990 3 mas 1x105 USNO et al. CCD imagine 2000 1 mas 3x105 HIPPARCOS space craft 1990 1 mas 3x105 HST FGS 2000 0.5 mas 6x105 Various long-baseline interfer. 2000 100 µas 3x106 GAIA (ESA) space craft 2010 10 µas 3x107 OBSS (USNO/NASA) space craft ? 10 µas 3x107 SIM (JPL/NASA) space interferometry 2009 1 µas 3x108 Telescope Photography

SLIDE 13

3

13 13

Jesse Ramsden Jesse Ramsden

Dividing machines Dividing machines Lathes to make round Lathes to make round mechanical parts, screws mechanical parts, screws Alt Alt-

• azimuth circle 1789 for

azimuth circle 1789 for Piazzi’s Palermo Observatory Piazzi’s Palermo Observatory

14 14

Some results from astrometry 1800 Some results from astrometry 1800-

• 1900

1900

1801 : 1801 : First First asteroid asteroid, Ceres, , Ceres, discov

• discov. by

. by Guiseppe Guiseppe Piazzi Piazzi 1838 : 1838 : Parallax Parallax of stars by

• f stars by Bessel

Bessel, Henderson, Struve , Henderson, Struve 1850 : 20 1850 : 20 parallaxes parallaxes in a in a catalogue catalogue by Peters by Peters 1837 : From 390 proper motions 1837 : From 390 proper motions Argelander Argelander : solar : solar apex apex 1846 : 1846 : Neptune Neptune predicted predicted and and discovered discovered Celestial Celestial mechanics mechanics flourishes flourishes 1860 1860 -

• :

: Bonner Bonner + Cordoba + Cordoba Surveys Surveys 1,000,000 stars 1,000,000 stars 1890 1890 -

• :

: Photography Photography : : parallaxes parallaxes and sky and sky surveys surveys

15 15

Astrometry of small angles 1/2 Astrometry of small angles 1/2

i.e. within the telescope field of view i.e. within the telescope field of view

1611 1611 – – 1660 : 1660 : Estimation: Estimation: diameters, relative positions, diameters, relative positions, Galilei et al. Galilei et al. 1659 1659 – – 1990 : 1990 : Wire micrometer Wire micrometer by Huygens by Huygens Gascoigne 1640, published later Gascoigne 1640, published later From 1750 also From 1750 also heliometer = heliometer = divided divided-

• lens micrometer

lens micrometer for the same purposes for the same purposes Science : Science :

Diameters of planets Diameters of planets Relative positions Relative positions Double stars : stellar masses Double stars : stellar masses Relative parallaxes 1838 Relative parallaxes 1838-1900 1900

Principle of the wire micrometer

. .

A

.

A B C

. . .

16 16

Astrometry of small angles 2/2 Astrometry of small angles 2/2

i.e. within the telescope field of view i.e. within the telescope field of view

1890 1890 -

• 1990 :

1990 : Photography : Photography : same science as micrometer : same science as micrometer :

Diameters of planets Diameters of planets Relative positions of stars and solar system objects Relative positions of stars and solar system objects Double stars : stellar masses Double stars : stellar masses Relative parallaxes Relative parallaxes 1990 1990 -

• : 1,000,000,000 stars, e.g. US Naval Observatory Catalogues

: 1,000,000,000 stars, e.g. US Naval Observatory Catalogues

1920 1920 -

• :

: Interferometry : Interferometry : stellar diameters, double stars stellar diameters, double stars 1990 1990 -

• :

: Space astrometry : Space astrometry : Hipparcos, Hubble Hipparcos, Hubble 1980 1980 -

• :

: CCD astrometry : CCD astrometry :

• n ground : 100,000,000
• n ground : 100,000,000 stars to 17th mag + solar system

stars to 17th mag + solar system from space : from space : Hubble relative parallaxes 0.001” Hubble relative parallaxes 0.001”

17 17

Some results from astrometry 1900 Some results from astrometry 1900-2000 2000

By 1900 : 539 stars 0.01”/a motions Decl. > By 1900 : 539 stars 0.01”/a motions Decl. > -

• 10 deg

10 deg

1905 : Hertzsprung discovers dwarfs/giants using motions for distances 1905 : Hertzsprung discovers dwarfs/giants using motions for distances

100 stars 100 stars 0.04” 0.04” relative parallaxes relative parallaxes By 1950 : 33,342 stars 0.01”/a motions, By 1950 : 33,342 stars 0.01”/a motions, 5822 stars 5822 stars 0.01” 0.01” relative parallaxes relative parallaxes 500 stars with <10% 500 stars with <10% error on distances error on distances

! ! 1970

1970 -

• :

: Radio astrometry : Radio astrometry : accurate absolute positions, accurate absolute positions, reference system by quasars, Earth rotation reference system by quasars, Earth rotation 1996 : 1996 : Hipparcos satellite : Hipparcos satellite : accurate large accurate large & small angles small angles 120,000 stars 0.001”/a motions (N 120,000 stars 0.001”/a motions (N & S) S) 120,000 stars 120,000 stars 0.001” 0.001” absolute parallaxes absolute parallaxes 22,000 stars with <10% 22,000 stars with <10% error on distances error on distances 2000 : 2000 : Tycho Tycho-

• 2 :

2 : 2.500,000 stars 0.002”/a motions 2.500,000 stars 0.002”/a motions USNO : USNO : 1,000,000,000 stars to 20th mag 1,000,000,000 stars to 20th mag

18 18

Copenhagen meridian Copenhagen meridian circle circle Photoelectric Photoelectric astrometry astrometry begins begins in 1925 in 1925

Bengt Bengt Strömgren Strömgren 1925 and 1933 1925 and 1933 Experiments Experiments with with photoelectric photoelectric recording recording of transits

• f transits

Courtesy: Steno Museum, Aarhus Courtesy: Steno Museum, Aarhus

SLIDE 14

4

19 19

y x ~ time x ~ time star star t1 t2

Ideas 1960 Ideas 1960

• P. Lacroute 1967: Go to space!
• P. Lacroute 1967: Go to space!
• E. Høg 1975: Design of Hipparcos
• E. Høg 1975: Design of Hipparcos

+ switching mirror + switching mirror

B.

• B. Strömgren

Strömgren 1933: 1933: slits slits + + switching switching mirror mirror Atomic Atomic bombs bombs 1957 : 1957 : Counting Counting techniques techniques

• E. Høg 1960 :
• E. Høg 1960 : Slits

Slits + + counting counting >>> >>> implementation implementation on

• n meridian

meridian circles circles Light intensity Light intensity = Photons = Photons per second per second Slits + Photon counting vs. Time Slits + Photon counting vs. Time => Astrometry + Photometry => Astrometry + Photometry y x1 x2 x x1 x2 x t1 t2 time t1 t2 time

20 20

Carlsberg automatic meridian circle Carlsberg automatic meridian circle

• n La Palma from 1984
• n La Palma from 1984

Photoelectric Photoelectric during during 14 14 years years Then Then from 1998 from 1998 CCD CCD micrometer micrometer : 20,000,000 star 20,000,000 star

• bservations
• bservations

per per year year 0.1” per obs. 0.1” per obs.

21 21

Hipparcos and Tycho 1975 Hipparcos and Tycho 1975-

• 2000

2000

Focal Focal plane of plane of Hipparcos Hipparcos – – Tycho Tycho Mission Mission concept concept 1975 1975 Mission Mission approval approval Feb. 1980

• Feb. 1980

Tycho Tycho proposal proposal April 1981 April 1981 Observing Observing 1989 1989 -

• 93

93 Catalogues Catalogues 1996 1996 Tycho Tycho-

• 2

2 Catalogue Catalogue in 2000 in 2000 2.500,000 stars 2.500,000 stars 500 citations 500 citations until until 2008 2008 Modulating grid Modulating grid Star mapper Star mapper grid grid

22 22

Telescope Telescope of

• f Hipparcos

Hipparcos

Schmidt type Schmidt type system system

D = 0 D = 0.29 m 29 m F = 1.4 m F = 1.4 m

Two fields on the sky Two fields on the sky

Observing 1989 Observing 1989 -

• 93

93

23 23

Telescope Telescope and and payload payload of Gaia

• f Gaia

Launch Launch 2012 2012

Two SiC primary mirrors Two SiC primary mirrors

1.45 1.45 × × 0.50 m 0.50 m2 at 106.5

at 106.5° ° Basic angle Basic angle monitoring system monitoring system Combined Combined focal plane focal plane (CCDs) (CCDs) F = 35 m F = 35 m Rotation axis (6 h) Rotation axis (6 h)

Figure courtesy EADS Figure courtesy EADS-

• Astrium

Astrium

Superposition of Superposition of two Fields of View two Fields of View SiC toroidal SiC toroidal structure structure (optical bench) (optical bench)

Two anastigmatic off Two anastigmatic off-axis telescopes axis telescopes

Astrometric Astrometric Astrometric Astrometric Astrometric Astrometric Astrometric Astrometric Accuracy Accuracy Accuracy Accuracy Accuracy Accuracy Accuracy Accuracy versus Time versus Time versus Time versus Time versus Time versus Time versus Time versus Time

Hipparchus/Ptolemy Hipparchus/Ptolemy Hipparchus/Ptolemy Hipparchus/Ptolemy -

• 1000 stars

1000 stars 1000 stars 1000 stars

1000 1000 1000 1000 100 100 100 100 10 10 10 10 arcsec 1 arcsec 1 arcsec 1 arcsec 1 0.1 0.1 0.1 0.1 0.01 0.01 0.01 0.01 0.001 0.001 0.001 0.001 0.0001 0.0001 0.0001 0.0001 0.00001 0.00001 0.00001 0.00001

Erik Høg 1995/2008 150 BC … 1600 1800 2000 Year 150 BC … 1600 1800 2000 Year 150 BC … 1600 1800 2000 Year 150 BC … 1600 1800 2000 Year The Landgrave of Hesse The Landgrave of Hesse The Landgrave of Hesse The Landgrave of Hesse -

• 1000 stars

1000 stars 1000 stars 1000 stars FK5 FK5 FK5 FK5 -

• 1500

1500 1500 1500

Positions Positions Positions Positions

Parallaxes Parallaxes Parallaxes Parallaxes = = = = Small angles Small angles Small angles Small angles

All parameters All parameters All parameters All parameters

Tycho Tycho Tycho Tycho-

• 2

2 2 2

• 2.5 million

2.5 million 2.5 million 2.5 million Hipparcos Hipparcos Hipparcos Hipparcos -

• 120,000

120,000 120,000 120,000 Bessel Bessel Bessel Bessel -

• 1 star

1 star 1 star 1 star Jenkins Jenkins Jenkins Jenkins -

• 6000

6000 6000 6000 Gaia 23 million Gaia 23 million Gaia 23 million Gaia 23 million SIM SIM SIM SIM -

• 10,000

10,000 10,000 10,000 USNO USNO USNO USNO -

• 360

360 360 360 ROEMER ROEMER ROEMER ROEMER -

• 45 million

45 million 45 million 45 million Proposal 1992 Proposal 1992 Proposal 1992 Proposal 1992 Flamsteed Flamsteed Flamsteed Flamsteed -

• 3000

3000 3000 3000 Tycho Brahe Tycho Brahe Tycho Brahe Tycho Brahe -

• 1000

1000 1000 1000 Argelander Argelander Argelander Argelander -

• 34,000

34,000 34,000 34,000 Bradley Bradley Bradley Bradley -

• aberration

aberration aberration aberration PPM PPM PPM PPM -

• 379,000

379,000 379,000 379,000 Gaia Gaia Gaia Gaia

• 1200 million

1200 million 1200 million 1200 million Lalande Lalande Lalande Lalande -

• 50,000

50,000 50,000 50,000

SLIDE 15

Astrometry Through the Ages VII.

The Race for Stellar Parallax I.

Friedrich Wilhelm Bessel (1784-1846)

• Quit school at 14 to become an accountant and spent his “leisure” time

studying navigation and astronomy.

• Calculated orbit of Comet Halley in 1804.
• At Konigsberg Observatory, Bessel undertook measuring positions of

50,000 stars and determined parallax of 61 Cygni in 1838.

• Discovered the submotion of Sirius.

Thomas Henderson (1798-1844)

• A Scottish lawyer by trade, his amateur astronomy successes earned

him the first directorship at the Cape Observatory in S. Africa.

• He spent 13 months there during 1832-33, a period of time he loathed,

and obtained many stellar position measurements.

• Calculated the parallax of Centauri in 1838 during his retirement,

but Bessel and Struve had already beaten him in the literature. Friedrich Georg Wilhelm von Struve (1793-1864)

• From 1820-1839 he was director of the Dorpat Observatory in

Tartu, Estonia, where he measured the parallax of Vega in 1839.

• Founded Pulkovo Observatory near St. Petersburg in 1839.
• First “modern” observer of double stars and published his great

Stellarum Duplicium et multiplicium Mensurae Micrometricae in 1837.

SLIDE 16

Astrometry Through the Ages VIII.

The Race for Stellar Parallax II.

Bessel (61 Cyg) HIPPARCOS (61 Cyg) % error 0.314 0.292 7.5 Struve (Vega) HIPPARCOS (Vega) % error 0.261 0.129 102 Henderson ( Cen) HIPPARCOS (Vega) % error 1.0 0.742 35

Who did the best job? So, Bessel wins not only in timing but in quality! “It is the greatest and most glorious accomplishment which practical astronomy has ever witnessed.”

Sir John Herschel upon Bessel’s award of the Gold Medal of the Royal Astronomical Society

By 1904, parallaxes of only 72 stars were listed in Newcomb’s The Stars, with vastly discordant results. The field had hit a wall and required a new approach.

SLIDE 17

Astrometry in Transition

19th and Early 20th Century Parallax Technique

Bessel used the 6-in Koenigsburg heliometer (built by Fraunhofer) to visually measure the offset of 61 Cyg and a reference star. Schlesinger used the 30-in Thaw refractor at Allegheny Observatory to embark on a revolutionary program of photographic astrometry.

SLIDE 18

The Modern Era I.

Photography Enters the Fray

Frank Schlesinger (1871-1943)

• Started career as a surveyor then entered Columbia to study

astronomy, earning his PhD in 1898 after experimenting with “plate constant solutions” to measuring stellar positions on photograph plates.

• Hired by George Ellery Hale to work with the Yerkes refractor

in 1902, then became director of Allegheny Observatory in 1905, and finally director at Yale University Observatory in 1920.

• Introduced the “method of dependences” and invented the

rotating sector.

• Established precepts that dominated long-focus astrometry

until the advent of the CCD.

• Founded the Yale southern observatory in S. Africa.
• Produced two parallax catalogs as well as The Bright Star

Catalogue.

“Measures of stellar distances presented difficulties so great that even today we possess reliable knowledge on the approximate distances of of not over a hundred stars. At no point in astronomical science is fuller knowledge more desirable, more pressingly urgent, than in the subject of stellar distances.”

W.W. Campbell 1910

SLIDE 19

The Modern Era II.

Schlesinger’s Technique

Standard Coordinates and “Dependences”:

• Compensate for plate-to-plate differences in origin, tilt, and scale using measurements of

reference star positions on a series of photographic plates. For example, the simplest such equations of condition in the “standard coordinates” xs and ys are: axxs + bxys + cx = xs – xobs and ayxs + byys + cy = ys – yobs for which a, b and c are the plate constants and easily determined by least squares.

• Additional terms can be added to compensate for coma, magnitude, color, etc. effects

resulting from telescope aberrations and the non-linear photographic detection process.

• Schlesinger introduced a mathematical simplification to reduce computation effort

(remember this was when “computers” were humans with adding machines and tables of logarithms!) incorporating “dependences” that also led to weights for the reference stars.

Left: Gaertner single-axis measuring machine, designed by Schlesinger, purchased for McCormick Observatory by S.A. Mitchell in 1916 for $650. Its precision was ±1µm. Right: McCormick advanced its computing capability in 1936 with the purchase of a Marchant Calculating Engine.

From the “McCormick Museum” at www.astro.virginia.edu

SLIDE 20

The Modern Era III.

The Original Parallax Club (~1900-1920)

Observatory Telescope PI

Allegheny 30-in refractor

• F. Schlesinger

Dearborn 18.5-in refractor

• P. Fox

Greenwich 26-in refractor

• F. Dyson

McCormick 26-in refractor S.A. Mitchell

• Mt. Wilson

60-in reflector

• A. van Maanen

Sproul 24-in refractor J.A. Miller

In 1924, Schlesinger’s first General Catalogue of Trigonometric Parallaxes included results for 1,870 stars. That number climbed to 4,260 stars. The 1995 “Fourth Catalogue” by van Altena, Lee & Hoffleit contains 15,994 parallaxes for 8,112 stars and showed that the most recent parallaxes had standard errors of ±0.004" in comparison with the “Third Catalogue” value of ±0.016 ".

SLIDE 21

The Modern Era IV.

First “Discovery” of Extrasolar Planets

Peter van de Kamp (1901-1995)

• Trained at Groningen and Lick, he worked with

Mitchell at UVa before becoming director of the Sproul Observatory at Swarthmore College in 1937.

• Sproul developed into a major center for parallax and

proper motion determinations as well as for astrometric studies of binary stars.

• With its record proper motion, Barnard’s star was

placed on the Sproul program to determine the star’s secular acceleration.

• Based on 2,316 plates from the 24-in Sproul refractor

taken during 1938-1961, van de Kamp in 1963 announced that Barnard’s star exhibited a 1µm sub- motion due to a 1.6 MJup planet in an eccentric orbit with an orbital period of 24 years.

• In 1974, van de Kamp announced that the perturbation

was due to two sub-Jupiter mass planets in circular

• rbits.
• Never confirmed by observers at Van Vleck, Allegheny,

McCormick and the U.S. Naval Observatories.

• Van de Kamp always maintained the reality of his

discovery, citing the long time-span of his data base.

The Chattanooga Times, 22 April 1963

SLIDE 22

The Modern Era V.

Reflectors Rejoin and Outshine the Old Refractors

Kaj Strand (1907-2000)

• Trained at Copenhagen, he subsequently worked at

Leiden and then at Sproul on photographic

• bservations of double stars.
• Following WWI, he held positions at Yerkes and

Dearborn before joining the U.S. Naval Observatory in 1958, becoming its Scientific Director in 1963.

• Responsible for the development of the USNO 61-in

astrometric reflector, located in Flagstaff.

• Led development of an automatic measuring engine

SAMM.

• USNO still produces the best ground-based

parallaxes, with accuracies of ±1.0 mas.

U.S. Naval Observatory photos

SLIDE 23

Trigonometric Parallax

• The stellar parallax is the apparent motion of a star due to our changing perspective as the

Earth orbits the Sun.

• parsec: the distance at which 1 AU subtends an angle of 1 arcsec.

d(pc) =

1 p(") Relative parallax - with respect to background stars which actually do move. Absolute parallax - with respect to a truly fixed frame in space; usually a statistical correction is applied to relative parallaxes.

SLIDE 24

Trigonometric Parallax

Measured against a reference frame made of more distant stars, the target star describes an ellipse, the semi-major axis of which is the parallax angle (p or π ), and the semi- minor axis is π cos β, where β is the ecliptic

• latitude. The ellipse is the projection of the

Earths orbit onto the sky. Parallax determination: at least three sets of

• bservations, because of the proper motion
• f the star.

Van de Kamp

SLIDE 25

1838 - F. W. Bessel - 61 Cygni, 0.31 +- 0.02 ( modern = 0.287) 1840 - F. G. W. Struve for Vega (α Lyrae), 0.26 (modern = 0.129) 1839 - T. Henderson for α Centauri (thought to be Proxima!), 1.16 +- 0.11 (modern = 0.742) All known stars have parallaxes less than 1 arcsec. This number is beyond the precision that can be achieved in the 18th century. Tycho Brahe (1546-1601) - observations at a precision of 15-35. Proxima Cen - 0.772 - largest known parallax (Hipparcos value) 1912 - Some 244 stars had measured parallaxes. Most measurements were done with micrometers, meridian transits, and few by photography.

Parallax Measurements: The First Determinations

SLIDE 26

Parallax Measurements: The Photographic Era

Observatory Telescope* Percentage (%)**

Yale (Johannesburg, South Africa) 26-in f/16.6 15.5 McCormick (Charlottesville, VA) 26-in, f/15 15.4 Allegheny (Riverview Park, PA) 30-in, f/18.4 15.1 Royal Obs. Cape of Good Hope (now SAAO) 24-in, f/11 13.9 Spoul (Swarthmore, PA) 24-in. f/17.9 10.4 USNO (Flagstaff, AZ) 61-in, f/10 reflector 6.6 Royal Obs. Greenwich 26-in, f/10.2 6.1 Van Vleck (Middletown, CT) 20-in, f/16.5 4.7 Yerkes (Williams Bay, WI) 40-in, f/18.9 3.6

• Mt. Wilson (San Gabriel Mountains, CA)

60-in, f/20 reflector 3.5 * All are refractors unless specified otherwise ** by 1992; other programs, with lower percentages are not listed Source: nchalada.org/archive/NCHALADA_LVIII.html

Accuracy: ~ 0.010 = 10 mas

SLIDE 27

Catalog Date #stars σ(mas) Comments YPC 1995 8112 ±15 mas

• Cat. of all π through 1995

USNO pg To 1992 ~1000 ±2.5 mas Photographic parallaxes USNO ccd From 92 ~150 ±0.5 mas CCD parallaxes Nstars & GB Current 100? ± 2 mas Southern π programs Hipparcos 1997 105 ±1 mas First modern survey HST FGS 1995-2010 ? 100? ±0.5 mas A few important stars SIM 2016? 103 ±4 µas Critical targets & exoplanets Gaia 2016? 109 ±10µas Ultimate modern survey

van Altena - MSW2005

Parallax Measurements: The Modern Era

SLIDE 28

Parallax Precision and the Volume Sampled

Photographic era: the accuracy is 10 mas -> 100 pc; Stars at 10 pc: have distances of 10 % of the distance accuracy Stars at 25 pc: have distances of 25 % of the distance accuracy By doubling the accuracy of the parallax, the distance reachable doubles, while the volume reachable increases by a factor of eight.

• Nearest star (Proxima Cen)

0.77 arcsec

• Brightest Star (Sirius)

0.38 arcsec

• Galactic Center (8.5 kpc)

0.000118 arcsec 118 µas

• Far edge of Galactic disk (~20 kpc)

50 µas

• Nearest spiral galaxy (Andromeda Galaxy)

1.3 µas

Parallax Size to Various Objects

SLIDE 29

Future Measurements of Parallaxes: SIM and GAIA

SIM ! 25 kpc! (10%)! SIM! 2.5 kpc! (1%)!

You are here

SIM(planetquest.jpl.nasa.gov) 1% 10%

SIM 2.5 kpc 25 kpc GAIA 0.4 kpc 4 kpc Hipparcos 0.01 kpc 0.1 kpc

SLIDE 30

Proper Motions: Barnards Star

Van de Kamp

SLIDE 31

Proper Motions

• reflect the intrinsic motions of stars as these orbit around the Galactic center.
• include: stars motion, Suns motion, and the distance between the star and the Sun.
• they are an angular measurement on the sky, i.e., perpendicular to the line of sight;

thats why they are also called tangential motions/tangential velocities. Units are arcsec/ year, or mas/yr (arcsec/century).

• largest proper motion known is that of Barnards star 10.3/yr; typical ~ 0.1/yr
• relative proper motions; wrt a non-inertial reference frame (e. g., other more distant

stars)

• absolute proper motions; wrt to an inertial reference frame (galaxies, QSOs)

µ("/yr) = VT (km /s) 4.74d(pc)

V2 = VT

2 + VR 2

SLIDE 32

Proper Motions

µα = dα dt µδ = dδ dt

µα - is measured in seconds of time per year (or century); it is measured along a small circle; therefore, in order to convert it to a velocity, and have the same rate of change as µδ , it has to be projected onto a great circle, and transformed to arcsec. µδ - is measured in arcsec per year (or century); or mas/ yr; it is measured along a great circle.

SLIDE 33

µ2 = (µα cosδ)2 + µδ

2

Proper Motions

SLIDE 34

Proper Motions - Some Well-known Catalogs

High proper-motion star catalogs > Luyten Half-Second (LHS) - all stars µ > 0.5/yr > Luyten Two-Tenth (LTT) - all stars µ > 0.2/year > Lowell Proper Motion Survey/Giclas Catalog - µ > 0.2/yr High Precision and/or Faint Catalogs Ø HIPPARCOS - 1989-1993; 120,000 stars to V ~ 9, precision ~1 mas/yr Ø Tycho (on board HIPPARCOS mission) - 1 million stars to V ~ 11, precision 20 mas/ yr (superseded by Tycho2). Ø Tycho2 (Tycho + other older catalogs time baseline ~90 years) - 2.5 million stars to V ~ 11.5, precision 2.4-3 mas/yr Ø Lick Northern Proper Motion Survey (NPM) - ~ 450,000 objects to V ~ 18, precision ~5 mas/yr Ø Yale/San Juan Southern Proper Motion Survey (SPM); 10 million objects to V ~ 18, precision 3-4 mas/yr.

SLIDE 35

For this, it is convenient to use a local system of celestial coordinates centered at a certain point A (0,

0, 0).

The equatorial coordinates

• f a point in the vicinity of A are

0 +

, 0 + The image of this region of the celestial sphere

• n an ideal focal surface is planar
• ne has to transform the differential coordinates

and into linear coordinates.

SLIDE 36

It is done by a conic projection from the center

• f the unit

celestial sphere on A. Ax, Ay are tangents to the declination small circle => increasing right ascensions, along the celestial meridian, the positive direction =>N This local system of coordinates = standard coordinates The transformation differential coordinates => standard coordinates gnomonic or central projection

SLIDE 37

MSW 2005 1

Transforming Measured to Standard Coordinates:

Models for wide-field astrographs and simplifications for long-focus telescopes

• T. M. Girard (Yale Univ.)

SLIDE 38

MSW 2005 2

References

• A. König, 1962, “Astronomical Techniques”, Edited by W. A. Hiltner, [University
• f Chicago Press] (Chapter 20)
• P. van de Kamp, 1962, “Astronomical Techniques”, Edited by W. A. Hiltner,

[University of Chicago Press] (Chapter 21)

• P. van de Kamp, 1967, “Principles of Astrometry”, [Freeman & Company],

(Chapters 5 and 6)

• L. Taff, 1981, “Computational Spherical Astronomy”, [Wiley-Interscience]
• personal class notes, Astro 575a, 1987 (taught by W. van Altena)

SLIDE 39

MSW 2005 3

Wide-Field vs. Long-Focus Telescopes

parallaxes, binary-star motion, relative proper motions positions, absolute proper motions uses 10 20 /mm 50 100 /mm scale f/15 f/20 f/4 f/10 f-ratio > 10 m 2 4 m focal length < 2° 2° 10° field of view

Long-focus Wide-field

SLIDE 40

MSW 2005 4

A Typical Astrometric Reduction

The goal is the determination of celestial coordinates () for a star or stars of interest on a plate or other detector.

• 1. Extract reference stars from a suitable reference catalog.
• 2. Identify and measure target stars and reference stars on the plate.
• 3. Transform reference-star coordinates to standard coordinates.
• 4. Determine the plate model (e.g., polynomial coefficients) that

transforms the measured x,y’s to standard coordinates. Use the reference stars, knowing their measures and catalog coordinates, to determine the model.

• 5. Apply the model to the target stars.
• 6. Transform the newly-determined standard coordinates into

celestial coordinates.

SLIDE 41

MSW 2005 5

Relation Between Equatorial and Standard Coordinates

Standard coordinates

(aka tangential coordinates, aka ideal coordinates)

• A. The coordinate system lies in a plane tangent to the

celestial sphere, with the tangent point T at the

• rigin, (0,0).
• B. The “y” axis, , is tangent to the declination circle that

passes through T, (+ toward NCP).

• C. The “x” axis, , is perpendicular to , (+ toward

increasing R.A.)

• D. The unit of length is the radius of the celestial sphere
• r that of its image - the focal length. (In

practice, arcseconds are commonly used.)

SLIDE 42

MSW 2005 6

Equatorial & Standard Coordinates (cont.)

• tan
• cos

tan sin tan

SLIDE 43

MSW 2005 7

Equatorial & Standard Coordinates (cont.)

• cos

sin cos cos sin cos sin sin cos sin sin cos cos cos sin sin cos

• Spherical triangle formed by star S,

tangent point T, and north celestial pole NCP. Tangent point is at o,o Star is at

SLIDE 44

MSW 2005 8

Equatorial & Standard Coordinates (cont.)

Equatorial from Standard:

• cos

cos cos sin sin cos sin cos cos sin cos cos cos sin sin sin cos

• 2

2

1 cos sin sin sin cos tan

• Standard from Equatorial:

SLIDE 45

MSW 2005 9

Corrections to Measured Coordinates

A. Correct for known “measuring machine” errors – repeatable deviations (offsets and rotation) from an ideal Cartesian system. Direct and reverse measures can be used to calibrate such effects. B. Correct for instrumental errors – plate scale, orientation, zero-point, plate tilt, higher-order plate constants, magnitude equation, color equation, etc. (To be discussed.) C. Correct for “spherical” errors – refraction, stellar aberration, precession, nutation. (To be discussed.)

SLIDE 46

MSW 2005 10

“Spherical” Errors: Atmospheric Refraction

where is the true zenith distance, i.e., the arclength ZS and is the parallactic angle.

• 2

' tan

, tan

• z

SLIDE 47

MSW 2005 11

“Spherical” Errors: Atmospheric Refraction (cont.)

Refraction varies by observing site, and with atmospheric pressure and temperature. See R. C. Stone 1996, PASP 108, 1051 for an accurate method of determining refraction, based on a relatively simple model. Importantly, refraction also varies with wavelength! Differential Color Refraction (DCR) can introduce color equation, an unwelcome correlation between stellar color and measured position. See R. C. Stone 2002, PASP 114, 1070 for a discussion of DCR and a detailed model for its determination. (In practice, it is sometimes incorporated into the plate model.) " 2 . 58 , 460 17 ) , (

• F

F

T P T P

SLIDE 48

MSW 2005 12

“Spherical” Errors: Stellar Aberration

If is the angle between the star and the apex

• f the Earth’s motion,

" 5 . 20 , sin

• c

v c v

• where
• ...

sin 2 ) ( cos " 5 . 20 ) (

2

• Thus, differentially across a field of size ,

Note: For = 5°, the maximum quadratic effect has amplitude ~100 mas. In practice, this would be absorbed by general quadratic terms in the plate model, which would almost certainly be present for such a large field.

SLIDE 49

MSW 2005 13

“Spherical” Errors: Precession & Nutation

As both precession and nutation represent simple rotations, these are almost never applied explicitly. They are effectively absorbed by the rotation terms in the plate model, which are always present. Note: The equinox of the reference system is therefore, in a practical sense, arbitrary. It is typically chosen to be that of the reference catalog for convenience. Of course, the tangent point must be specified in whatever equinox is chosen.

SLIDE 50

MSW 2005 14

The Plate Model

Often, a polynomial model is used to represent the transformation from measured coordinates, (x,y), to standard coordinates, Note: The various terms can be identified with common corrections...

... ...

12 11 3 10 2 9 2 8 3 7 2 6 5 2 4 3 2 1 12 11 3 10 2 9 2 8 3 7 2 6 5 2 4 3 2 1

• CI

b m b x b yx b x y b y b x b yx b y b b x b y b CI a m a y a xy a y x a x a y a xy a x a a y a x a

• )

, , , , ( ) , , , , (

i i i i k i i ref i i i i i k i i ref i

CI m y x b CI m y x a

• For each reference star, i, calculate deviation, Minimize 2.

SLIDE 51

MSW 2005 15

The Plate Model (cont.)

• solution
• solution

Correction ... ... higher order terms... y*CI x*CI color magnification CI CI color equation y*m x*m coma m, (m2, m3...) m, (m2, m3...) magnitude equation y*(x2+y2) x*(x2+y2) cubic distortion y*(px+qy) x*(px+qy) plate tilt constant constant zero point x y

• rientation

y x scale

SLIDE 52

MSW 2005 16

The Plate Model (cont.)

Some Helpful Hints The simplest possible form should be used. The modeling error is thus kept minimal. Reference stars are usually at a premium! (Rule of thumb: Nref > 3*Nterms.) Pre-correct measures for known (spherical) errors. Update reference star catalog positions to epoch of plate material, i.e., apply proper motions when available. Uniform distribution of reference stars is best. Avoid extrapolation. Iterate to exclude outliers, but trim with care. Plot residuals versus everything you can think of!

SLIDE 53

MSW 2005 17

When The Plate Model Is Just Not Enough

“Stacked” differences wrt an external catalog can uncover residual systematics. A comparison between preliminary SPM3 positions (derived using a plate model with cubic field terms) and the UCAC. The resulting “mask” was used to adjust the SPM3 data, field by field. (See Girard et al. 2004, AJ 127, 3060)

SLIDE 54

MSW 2005 18

Magnitude Equation – The Astrometrist’s Bane

Magnitude equation = bias in the measured “center” of an image that is correlated with its apparent brightness. It can be particularly acute on photographic plates, caused by the non- linearity of the detector combined with an asymmetric profile, (due to guiding errors, optical aberrations, etc.)

• Difficult to calibrate and correct internally
• Reference stars usually have insufficient magnitude range
• Beware of “cosmic” correlations in proper motions
• In clusters, magnitude and color are highly correlated

NOTE: Charge Transfer Efficiency (CTE) effects can induce a similar bias in CCD centers. (More often, the CTE effect mimics the classical coma term, i.e., x*m).

SLIDE 55

MSW 2005 19

Magnitude Equation – The Astrometrist’s Bane (cont.)

SPM (and NPM) plates use

• bjective gratings, producing

diffraction image pairs which can be compared to the central-order image to deduce the form of the magnitude equation. A comparison of proper motions derived from uncorrected SPM blue-plate pairs and yellow-plate pairs indicate a significant magnitude equation is present. Using the grating images to correct each plate’s individual magnitude equation, the proper motions are largely free of bias.

SLIDE 56

MSW 2005 20

Long-Focus Telescope Astrometry

Plate tilt often negligible, but should be checked Distortion can be significant for reflectors; usually can be ignored for refractors - constant over the FOV Refraction usually ignored unless at large zenith angle, or for plate sets with a large variation in HA Aberration small, ignored Magnitude equation usually present! Can be minimized by using a limited magnitude range. Color equation DCR will be present. Careful not to confuse with magnitude equation for cluster fields. Color magnification generally not a problem over the FOV Coma images are often affected, but variation across FOV is slight and can be neglected in general (but check)

Traditional Simplifications - due to scale & small field of view

SLIDE 57

MSW 2005 21

Long-Focus Telescope Astrometry (cont.)

A Parallax and Binary-Motion Example: Mass of Procyon A & B (Girard et al. 1999, AJ 119, 2428)

Overview: The plate material consisted of 250 (primarily) long-focus plates, containing >600 exposures and spanning 83 years. Magnitude-reduction methods were used during the exposures to bring Procyon’s magnitude close to that of the reference stars. Linear transformations between plates, putting all onto the same standard coordinate system. Astrometric orbit and parallax were found.

SLIDE 58

MSW 2005 22

Long-Focus Telescope Astrometry (cont.)

A Relative Proper-Motion Example: Open Cluster NGC 3680 (Kozhurina-Platais et al. 1995, AJ 109, 672)

Overview: The plate material consisted of 12 Yale-Columbia 26-in. refractor plates, spanning 37 years. Explicit refraction correction was needed as plates were taken at two observatories, Johannesburg and Mt. Stromlo. Plates exhibited magnitude and color equation which affected the derived relative proper motions. The cluster’s red giants were displaced in the proper-motion VPD.

SLIDE 59

MSW 2005 23

Long-Focus Astrometry (NGC 3680 Example cont.)

Before After

SLIDE 60

MSW 2005 24

In lieu of a summary slide...

Some Helpful Hints The simplest possible form should be used. The modeling error is thus kept minimal. Reference stars are usually at a premium! (Rule of thumb: Nref > 3*Nterms.) Pre-correct measures for known (spherical) errors. Update reference star catalog positions to epoch of plate material, i.e., apply proper motions when available. Uniform distribution of reference stars is best. Avoid extrapolation. Iterate to exclude outliers, but trim with care. Plot residuals versus everything you can think of!

SLIDE 61

Astrometry Through the Ages I.

Hellenic Beginnings Hipparchus ( ~190-120 BC)

• Born in Nicaea (Turkey), but probably

worked in Rhodes and Alexandria

• Only one minor, original work survives,

and he is mostly known from the writings

• f others, particularly from Ptolemy’s

Almagest.

• Accomplishments include:
• Early work in trigonometry
• Introduced 360º circle into Greece
• Discovered precession from

comparison of historical measures of length of the year

• 46 "/yr (Hipparchus)
• 36 "/yr (Ptolemy, 300 years

later)

• 50.26 "/yr (Modern value)
• Developed a “lunar theory”
• Determined eclipse period
• Calculated lunar distance
• Produced a star catalog of ~850 stars

Farnese Atlas: Dr. Gerry Picus, courtesy Griffith Observatory

SLIDE 62

Astrometry Through the Ages II.

Greco-Roman Culmination Claudius Ptolemy (~85-165 AD)

• Observed from Alexandria between 127-141 AD
• Greatest work: The Mathematical Compilation

The Greatest Compilation Al-majisti Almagest (Greek to Arabic to Latin).

• The Almagest presents:
• Lunar and solar theories
• Eclipses
• Motions of the “fixed” stars
• Precession
• Planetary theory
• Incorporating epicycles and

eccentrics.

• One of the most successful theories
• f all time.
• A catalog of 1,000 stars
• Some successors (Tycho, Delambre,

Newton) accused Ptolemy of stealing his catalog from Hipparchus and precessing it by 300 years.

SLIDE 63

Astrometry Through the Ages III.

Islamic Contributions Ulugh Beg (1393-1449)

Archnet.org

• Grandson of Tamarlane who was appointed

ruler of Samarkand (in present day Uzbekistan) by his father in 1409.

• Built a madrasah (center of Islamic higher

learning) with a great observatory 50 m in diameter and 35 m high featuring a large marble sextant.

• Ulugh Beg’s accomplishments:
• Determined the length of the year to

within 51 sec of actual value.

• Created the first major new star catalog

since Ptolemy, the Zij-I Sultani, containing 992 stars.

• Advanced trigonometry and calculated

value of sin 1° accurate to one part in 1015.

• Forcibly “retired” by his son who had him

murdered in 1449.

SLIDE 64

Astrometry Through the Ages IV.

The Copernican Revolution The Usual Suspects:

• Nicolaus Copernicus (1473-1543) – Challenged the

prevailing thought that dated to Aristotle while still keeping the practical aspects of Ptolemaic modeling.

• Tycho Brahe (1546-1601) – The greatest pre-

telescopic observer (and overall fascinating guy).

• Johannes Kepler (1571-1630) – Tycho’s brilliant

assistant who outshone his master intellectually.

• Galileo Galilei (1564-1642) – Sought to measure

stellar parallax and suggested that “double stars” might present that possibility.

• Isaac Newton (1642-1727) – Not usually thought of

as an astrometrist, but he revolutionized the theoretical basis of the field.

SLIDE 65

Astrometry Through the Ages V.

Taking Catalogs to their Next Level

John Flamsteed (1646-1719)

• First Astronomer Royal and builder of Greenwich Observatory.
• Attempted to provide Newton with observations toward

understanding the orbit of the Moon, but never quite got Newton what he needed. They grew to dislike one another enormously.

• Published the catalog Historia Coelestis Britannica in 1725 with

3,000 star positions of unprecedented accuracy.

• Feuded extensively with Newton’s protégé Edmond Halley, who

ironically succeeded him as Astronomer Royal.

• “… an intemperate man even by the standards of an intemperate

age.” Dictionary of Scientific Biography (New York, 1970-90). Edmond Halley (1656-1742)

• Born into a prosperous family, Halley’s father provided him with

instruments and introductions.

• Worked as an assistant during his brief undergraduate career to

Flamsteed, but dropped out of Queen’s College Oxford.

• Sailed to St. Helena to attempt the first southern star catalog
• Observed the transit of Mercury on 7 Nov 1677.
• Published his catalog of 341 southern stars in 1678.
• Anticipated Newton’s proof of the inverse square law of attraction.
• Black balled by Flamsteed from the Savilian Chair at Oxford.
• Discovered stellar proper motion by comparison with Ptolemy.
• Instrumental in Newton publishing The Principia.

SLIDE 66

Astrometry Through the Ages VI.

More Newtonian Era Advances

Jean-Dominique Cassini (1625-1712)

• Began career in his native Italy (Giovanni Domenico Cassini) and
• riginally accepted the geocentric theory.
• At Bologna, he measured rotation periods of Jupiter and Mars and

first suggested (then rejected) the finite speed of light.

• Louis XIV invited him to oversee the construction of the Paris
• Observatory. Cassini became a French citizen in 1673.
• Paris accomplishments:
• Four new moons of Saturn
• Famous gap in Saturn’s rings; suggested true nature of rings
• Paris meridian
• First accurate value of solar parallax (from observations of

Mars made at Cayenne and Paris).

• Rejected Newtonian theory and believed Earth prolate.

James Bradley (1693-1762)

• Originally trained for the Church but resigned to take the Savilian

Chair in Astronomy at Oxford in 1721.

• Announced the discovery of the abberation of star light in 1729.
• Announced the discovery of nutation after following the moon for

an entire Saros cycle.

• Became the third Astronomer Royal in 1742 and modernized the

equipment at Greenwich.

SLIDE 67

• First reference ever to duplicity is that of C. Ptolemy (2nd C

AD) who used the term to describe and Sgr (which is optical). Ptolemy did not mention the other obvious case of Alcor and Mizar.

Double Stars – Beginnings I.

• Oldest recognized system discovered

telescopically is the “Trapezium” in Orion in 1619 by Johannes Cysat of Ingolstadt (1587 - 1657).

• First physical binary resolved by the telescope was Mizar,

generally credited to Giovanni Ricciolo (1598 – 1671) in about

• 1650. It now appears that Galileo had actually resolved the

star as early as 1617.

• Galileo was interested in double stars as a means for proving

the heliocentric theory. He assumed systems were optical in nature.

SLIDE 68

Double Stars – Beginnings II.

• First evidence for physical nature was presented by J.

Mitchell in 1767 and C. Mayer in 1779 whose catalogs of

• bservations showed relative motion. Others, for example

Bode, held onto the accidental alignment hypothesis and advanced the utility of double stars for parallax and proper motion studies.

• In 1667, G. Montanari noticed brightness change in Algol

(ras Al gul) although the Arabic name probably resulted from that phenomenon. In 1783, J. Goodricke attributed the variation as being due to either large spots or an eclipse by a giant planet (which was not proven until 1889).

• By end of 17th C, Centauri, Cruxis, Geminorum and

Virginis had all been discovered. They were still generally considered to be chance alignments, although Lambert mentioned the possibility of physical association in 1761.

SLIDE 69

First great observer of double stars was Wilhelm (William) Herschel (1738-1822). Between 1779 and 1784, he made his first series of observations of ~700 objects. On 9 June 1803, he presented the paper “Account

• f the Changes that have happened, during the

last Twenty-five Years, in the relative Situation

• f Double-Stars; with an Investigation of the

Cause to which they are owing.” He used Gem (Castor) to argue persuasively for orbital rather than any other kind of motion.

Double Stars – Beginnings III.

Herschel’s 20 ft telescope, now on display at the Air and Space Museum in Washington

SLIDE 70

Friedrich Georg Wilhelm Struve (1793- 1864) published Mensurae Micrometicae in 1837 as the first systematic systematic program of discovery and synoptic

• bservation to deduce orbital motion.

Introduced the use of (,) as standard measured quantities for binary stars.

Double Stars – 19th Century I.

Used the 9-inch Fraunhofer refractor at the Dorpat Observatory in Russia (Estonia) to discover 3,134 pairs.

• N

E

• Nine-inch refractor by Fraunhofer on display at the Deutsches

Museum in Bonn (identitical to that used by Struve)

SLIDE 71

Double Stars – 19th Century II.

John Frederick William Herschel (1792-1871)

• bserved double stars from the Cape of

Good Hope, South Africa, during 1834- 1838. Southern double star work would continue well into the 20th C by Innes, Van den Bos and Finsen at Johannesburg and Rossiter at Bloemfontein. Alvin Clark resolved Sirius in 1862 (with an 18-inch refractor originally ordered by the University of Mississippi but diverted to Dearborn Observatory of Northwestern University because of the Civil War).

Lick Observatory 3-m telescope image of Sirius A and B Chandra Image of Sirius B and A

SLIDE 72

Double Stars – “Modern” Era I.

Sherburne Wesley Burnham (1838-1921) was a court reporter with a keen interest in double stars and observed at Dearborn, Washburn, Lick and Yerkes, eventually discovering 1,336 doubles. He specialized in discovering “close” pairs ( as small as 0.2 arcsec and large m pairs. In 1906, he published A General Catalogue of Double Stars Within 120o of the North Pole, with 13, 665 star. (Typically referred to as the “BDS” catalog.) Robert Grant Aitken (1864-1951) joined Lick Observatory in 1895 and served as Director during 1930-35. He initiated a survey for duplicity of all stars north of = -22o that resulted in 4,400 new pairs, 3,100 of which he discovered. In 1932, he published A New General Catalogue of Double Stars Within 120o of the North Pole (the “ADS”) with 17,180 entries. His book The Binary Stars (1935 with several Dover reprints) is a classic of the field.

SLIDE 73

Double Stars – “Modern” Era II.

Charles Worley (1935-1997) – Surveyed nearby faint dwarfs for companions in 1960 at Lick and went on to observe close pairs at the 26-inch USNO refractor until his death. He transferred the Lick “Index Catalogue” to the USNO and initiated the “Washington Double Star Catalogue” (WDS) which continues today under the direction of Brian Mason and Bill Hartkopf. Worley retired his micrometer and initiated a speckle interferometry program at the USNO in the 1990’s.

Worley with GSU speckle camera at Lowell 26-inch refractor

Other Productive Techniques:

Photography – Some 100,000 (many are spurious or optical) CPM stars – Willem Luyten discovered ~2,000 common proper motion pairs from the Palomar Sky Survey. These are probably the most common type of binary, but orbital periods are too long to measure. Speckle Interferometry – First systematically introduced in 1975. Approximately 35,000 measures to date (majority from CHARA and USNO) and ~300 new, bright and close pairs. HIPPARCOS – 51,500 measures

SLIDE 74

Interferometry

Michelson Perfects Extremely Narrow Angle Astrometry

Albert Michelson (1852-1931)

• Born in Prussia, his family emmigrated to the

U.S. when he was two. He would become the first American scientist to win the Nobel Prize (in 1907 “for his optical precision instruments and the spectroscopic and metrological investigations carried out with their aid.”)

• While he did not invent interferometry, his

experiments at Mt. Wilson on the then new 100- inch telescope showed great promise for measuring stellar diameters and resolving close binary star systems.

SLIDE 75

Michelson’s 20-ft Interferometer on the Mt. Wilson 100-inch

A.A. Michelson & F.G. Pease, ApJ, 53, 249, 1921

SLIDE 76

Michelson’s 20 ft Interferometer

On Display in CHARA’s Mt. Wilson Exhibit Hall

SLIDE 77

Pease’s 50-ft Interferometer on Mt. Wilson

An Instrument Ahead of its Time and Technology

Adapted from a diagram by Peter Lawson, JPL

SLIDE 78

Speaking of Mt. Wilson ...

By the way, watch where you step!

SLIDE 79

The Discovery of Speckle Interferometry

Labeyrie’s Brilliant Insight into “Seeing”

On the occasion of Labeyrie receiving the Franklin Medal in Philadelphia (April 2002). Deane Peterson (SUNY Stonybrook) and yours truly lectured at a special symposium in Labeyrie’s honor. Antoine Labeyrie first proposed the technique of speckle interferometry as a graduate student in Meudon before going to Stonybrook on a postdoc.

SLIDE 80

ADS 11483

G2V+G2V, 1985.51, Sep = 1.74 arcsec GSU Speckle Camera @ CFH Telescope

Atmospheric Turbulent Layers Telescope Entrance Pupil Telescope Focal Plane Star A Star B

Binary Star Speckle Interferometry

SLIDE 81

Micrometer Observations Speckle Observations

0.1 arcsec

Enhanced Measurement Accuracy

Visual Binary Peg: P = 11.6 yr, a = 0.25 arcsec

SLIDE 82

An Interferometric Renaissance

Hanbury Brown & Labeyrie Reawaken Interferometry

Built by Labeyrie and his collaborators in the mid-1980’s in the south of France, the Grand Interféromètre à 2 Télescopes incorporates Labeyrie’s novel ideas for telescope design, path length compensation, and beam combination. The Intensity Interferometer was developed by Robert Hanbury Brown (1916-2002) and his colleagues and operated at Narrabri, NSW, Australia during 1963-1972 with the sole purpose of accurately measuring the angular diameters of 32 bright, southern stars.

SLIDE 83

Other Optical Interferometers I.

The NRL/USNO Mark III Set the Modern Standard

The Mark III Interferometer, built by Michael Shao, Mark Colavita and their collaborators, was operated on Mt. Wilson from 1986 to 1992. It was the third of a series of technological stepping stones and successfully demonstrated in a productive scientific program what has become the modern standard for interferometric subsystems. The Mark III is the direct ancestor to the Palomar Testbed Interferometer, the Keck Interferometer and the Space Interferometry Mission.

SLIDE 84

Other Optical Interferometers II.

The USNO NPOI as an Astrometric Instrument

Operated by the U.S. Naval Observatory in collaboration with Lowell Observatory on Anderson Mesa in northern Arizona, the Navy Prototype Optical Interferometer has the dual role of serving positional astronomy using its “astrometric subarray” surrounded by the longer baseline imaging array.

U.S. Naval Observatory photo

SLIDE 85

Other Optical Interferometers III.

These Things are Complicated!

An interior view of the delay line laboratory of the CHARA Array, operated on Mt. Wilson, California, by Georgia State University with support from the National Science Foundation, hints at the terrific overhead imposed on interferometers by the requirements of fringe detection.

Photo by Steve Golden, GSU/CHARA

SLIDE 86

Four Centuries of Progress

From Tycho to SIM

60 arcsec precision 1 micro-arcsec precision

SLIDE 87

Some Useful References

Fundamentals of Astrometry J. Kovalevsky & P.K. Seidelmann, Cambridge, 2004. Observing and Measuring Visual Double Stars R. Argyle, Springer, 2004. Modern Astrometry 2nd Ed., J. Kovalevsky, Springer, 2002. Parallax A.W. Hirshfield, Freeman, 2001. Astrometry of Fundamental Calalogues H. Walter & O. Sovers, Springer, 2000. Relativity in Astrometry, Celestial Mechanics and Geodesy M.H. Soffel, Springer, 1989. Astrometry W. van Altena, Ann. Rev. Astr. Astp., 21, 1983. Vectorial Astrometry C.A. Murray, Hilger, 1983. Stellar Paths P. van de Kamp, Reidel, 1981. Double Stars W.D. Heintz, Reidel, 1978. Astronomy of Star Positions H. Eichhorn, Ungar, 1974. Principles of Astrometry P. van de Kamp, Freeman, 1967. A Compendium of Spherical Astronomy S. Newcomb, Dover reprint, 1906. see also The Double Star Library at ad.usno.navy.mil/wds/dsl.html