undergraduate seminar

Measuring the size of small things
that shine in the sky at night
●
Stellar Diameters
●
Atmospheric Cherenkov Telescopes
●
Cross-road
●
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
Hadronic
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