Download Measuring the size of small things Stellar Diameters Atmospheric Cherenkov Telescopes

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 5th 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=227x106km
r=3400km
Earth
d=150x106km
Sir Isaac Newton, 1642-1727
Stars are 105 times further
away than the sun
π x 34002
LMARS = LSUN x
= LSUN
4π x (227 x 106)2
2 x 1010
Stars are 105 smaller than
the sun, or
a few milli-arc-seconds
Light behaves like a wave
Visible light: wavelength ~500nm, frequency ~5x1014Hz.
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.
IMax - IMin
Visibility =
IMax + IMin
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 13th 1995
September 28th 1995
Images of Betelgeuse (COAST)
700nm (1997)
1290nm (1997)
Colloquium by John Monnier on Thursday March 23rd at 4PM
IOTA
J, H, K
ISI
N
KeckInterfero
K
meter
Keck Aperture
Masking
J, H, K, L
MIRA 1.2
R, I
NPOI visible
V, R, I
PTI
SUSI
J,H,K
B, V, R, I
7
0
6
10
30
50
10
85
85
2
3
5
0.5
30
5
9
30
300
7
5
86
5
110
640
VLTI near
infrared
J, H, K
12
46
130
VLTI mid
infrared
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
Maximum
magnitude Minimum
Waveband agnitude baseline (m) baseline (m)
J, H, K
12
70
400
CHARA
LBTI near
J, H, K
infrared
MRO
R, I, J, H, K
VLTI near
infrared using
4 ATs and
J, H, K
PRIMA
VLTI near
infrared using
3 UTs and
PRIMA
J, H, K
>20
14
0
7
Comments
2005?
22
400
2006?
2008?
13
8
200
Operating
every night
2007?
16
46
130
2007?
SPACE INTERFEOMETRY MISSION . . .
scheduled for launch in 2011
Cosmic
Rays
Protons, Nuclei, ?, ...
Atmospheric Showers
Electromagnetic
.
γ
.e+ e-
X0
Hadronic
p. or Nucl.
E
+-
E/2

E/4

E/8
E/16
E~80x106eV
1 atmosphere ~ 28 X0
1TeV
(1012eV)
5ns
~100 photons/m2
130m
400
m
3.1011eV or 300 GeV gamma
ray
Http://www.mpihd.mpg.de/hfm/CosmicRay/ChLight/ChLat.html
~
2o
The Whipple observatory on Mount Hopkins
MMT
10M
2200m
a.s.l.
The 10M Very High Energy Gamma Ray Telescope
7m
7m
10m
7m
Atmospheric Showers Cherenkov Images
p(?)
gamma?
muon
Event rate ~ 30Hz
~ Gamma ray rate ~ 0.1Hz
Undergrad seminar by David Kieda on March 23rd 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