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Measuring the size of small things

that shine in the sky at night

Stellar Diameters

Atmospheric Cherenkov Telescopes

Table top stars

The future

Stephan LeBohec,

Micah Kohutek and

Jamie Holder

March 5 th 2006

Galileo Galilei

(1564-1642)

Dialogue Concerning

the Two Chief World Systems,

1632

Portrait of Galileo Galilei,

by Ottavio Leoni (1578-1630).

http://webexhibits.org/calendars/year-text-Galileo.html

“I hung up a light rope in the direction of a star . . . and then by

approaching and retreating from this cord placed between me and the star, I

found the point where its width just hid the star from me. This done, I

found the distance of my eye from the cord, which amounts to the same

thing as one of the sides which includes the angle formed at my eye and

extending over the breadth of the cord.”

“And . . . the apparent diameter of a fixed star of the first magnitude is no

more than 5 seconds.”

d

θ < 5”

or

1mm at 40m

D

How far from the sun do we need to travel for it to appear no

brighter than a bright star? From that distance, what would

be its angular diameter?

Mars

d=227x10 6 km

r=3400km

Earth

d=150x10 6 km

Sir Isaac Newton, 1642-1727

Stars are 10 5 times further

away than the sun

π x 3400 2

L MARS

= L SUN

x = L SUN

2 x 10 10

4π x (227 x 10 6 ) 2

Stars are 10 5 smaller than

the sun, or

a few milli-arc-seconds

Light behaves like a wave

Visible light: wavelength ~500nm, frequency ~5x10 14 Hz.

Electric field

Magnetic field

Light intensity ~ (Electric field ) 2

Huygens principle

Christiaan Huygens

1629-1695

500 x 10 -9 m / 1 x 10 -3 m = 5 x 10 -4 > 1 x 10 -3 m / 40 m = 2.5x10 -5

so Galileo could not have done the measurement he describes!

Diffraction limits telescopes angular resolution

William HERSCHELL

1738-1822.

Thomas YOUNG

1773-1829.

Visibility 1

Hippolyte FIZEAU

1819-1896.

I Max

- I Min

Visibility =

I Max

+ I Min

Baseline

Baseline = 1.22 λ / θ

θ=1mas and λ=500nm ⇒ D=100m

Michelson stellar interferometry

1890: Measure of one of Jupiter's satellite diameter

1920: with Pease, Measure of Betelgeuse diameter (47mas)

Albert Michelson

1853-1931

Mount Wilson Hooker Telescope

1930: 50 foot interferometer

1932: The technique is abandoned for being too difficult

with 20 foot Interferometer

Cambridge

Optical

Aperture

Synthesis

Telescope

Images of Capella (COAST)

September 13 th 1995 September 28 th 1995

Images of Betelgeuse (COAST)

700nm (1997) 1290nm (1997)

Colloquium by John Monnier on Thursday March 23 rd at 4PM

IOTA J, H, K 7 6 30

ISI N 0 10 50

KeckInterfero

meter K 10 85 85

Keck Aperture

Masking J, H, K, L 2 0.5 9

MIRA 1.2 R, I 3 30 30

NPOI visible V, R, I 5 5 300

PTI J,H,K 7 86 110

SUSI B, V, R, I 5 5 640

VLTI near

infrared

VLTI mid

infrared

J, H, K 12 46 130

N 4 46 130

"dual-star"

capable

Used for

interferometry

a few weeks

per year

Used for

interferometry

a few weeks

per year

New Interferometers and Improvements to Existing

Interferometers

Limiting

magnitude Minimum Maximum

Waveband agnitude baseline (m) baseline (m) Comments

CHARA J, H, K 12 70 400 2005?

LBTI near

infrared J, H, K >20 0 22 2006?

MRO R, I, J, H, K 14 7 400 2008?

VLTI near

infrared using

4 ATs and

PRIMA

VLTI near

infrared using

3 UTs and

PRIMA

J, H, K 13 8 200

Operating

every night

2007?

J, H, K 16 46 130 2007?

SPACE INTERFEOMETRY MISSION . . .

scheduled for launch in 2011

Cosmic

Rays

Protons, Nuclei, ?, ...

Electromagnetic

.

γ

.e + e - E

E/2

Atmospheric Showers

p. or Nucl.

+ -

X 0

E/4

1 atmosphere ~ 28 X 0

E/8

E/16

E~80x10 6 eV

1TeV

(10 12 eV)

5ns

~100 photons/m 2

130m

3.10 11 eV or 300 GeV gamma

ray

400

m

~

2 o

Http://www.mpihd.mpg.de/hfm/CosmicRay/ChLight/ChLat.html

The Whipple observatory on Mount Hopkins

MMT

10M

2200m

a.s.l.

The 10M Very High Energy Gamma Ray Telescope

7m

10m

7m

7m

Atmospheric Showers Cherenkov Images

p(?)

muon

gamma?

Event rate ~ 30Hz

~ Gamma ray rate ~ 0.1Hz

Undergrad seminar by David Kieda on March 23 rd at 11:50AM

Robert Hanbury Brown, 1916-2002

1950: Intensity Interferometry

1953: Radio measurements of CasA and CygA

1955: Search light measurement of Sirius

1963: Measurement of Vega from Narrabri (Australia)

1972: Operation of Narrabri interferometer stops

32 stars measured

from Narrabri

magnitudes < 2.5

0.41mas < diameters < 3.24mas

10 of them in the main sequence

How does it work?

D.C. Component

“Wave noise”

selected by A.C. coupling

More generally:

- c(d) is the Fourier transform

of the light distribution across

the source.

- Measuring c(d) is measuring

the source size and shape.

Table top test bench

Our first intensity

interferometer

Inside the black

box

The electronics

Control and digitization

Integrator

Mixer

Amplifiers

Toward a

Digital

Intensity

Interferometer

Installation on VERITAS telescopes

Analog optical fibers to central location

Time delay

Interferometric observation during full moon

What science could we do?

Stellar diameters

Pulsating stars

Stellar surface

features

Binary stars

Extra solar planets

Wolf-Rayet stars

Be Stars

Circum-stellar disks

. . .

Km baseline, 1 mas

VERITAS base lines