Transcript Intensity interferometry

SPIE 7734, Optical and Infrared Interferometry II, San Diego, June 2010
STELLAR INTENSITY INTERFEROMETRY
Astrophysical targets for sub-milliarcsecond imaging
Dainis Dravins, Hannes Jensen
Lund Observatory, Sweden
Stephan LeBohec, Paul D. Nuñez
The University of Utah, Salt Lake City
Intensity interferomety:
What, Why & How?
Narrabri intensity interferometer
with its circular railway track
R.Hanbury Brown: BOFFIN. A Personal Story of the Early Days
of Radar, Radio Astronomy and Quantum Optics (1991)
Flux collectors at Narrabri
R.Hanbury Brown: The Stellar Interferometer at Narrabri Observatory
Sky and Telescope 28, No.2, 64, August 1964
INTENSITY INTERFEROMETRY
Intensity interferometry
Pro: Time resolution of 10 ns, say, implies 3 m light travel
time; no need for more accurate optics nor atmosphere.
Short wavelengths no problem; hot sources observable
Con: Signal comes from two-photon correlations,
increases as signal squared; requires large flux collectors
MAGIC
VERITAS
H.E.S.S.
AIR CHERENKOV TELESCOPES
CANGAROO III
CTA, Cherenkov Telescope Array (2018?)
An advanced facility for ground-based gamma-ray astronomy
Digital intensity interferometry
 Cherenkov telescopes: Large flux collectors
 Fast digital detectors & high-speed signal handling
 Combine optical telescopes in software
 Huge number of baselines, no loss of digital signal
 65 CTA telescopes: N(N-1)/2 = 2080 baselines
 Filled (u,v)-plane enables sub-milliarcsecond imaging
S/N in intensity interferometry
PROPORTIONAL TO:
 Telescope areas (geometric mean)
 Detector quantum efficiency
 Square root of integration time
 Square root of electronic bandwidth
S/N in intensity interferometry
PROPORTIONAL TO:
 Telescope areas (geometric mean)
 Detector quantum efficiency
 Square root of integration time
 Square root of electronic bandwidth
 Photon flux per optical frequency bandwidth
S/N in intensity interferometry
PROPORTIONAL TO:
 Telescope areas (geometric mean)
 Detector quantum efficiency
 Square root of integration time
 Square root of electronic bandwidth
 Photon flux per optical frequency bandwidth
INDEPENDENT OF:
 Width of optical passband
OBSERVATIONS IN INTENSITY INTERFEROMETRY
Stellar diameters for different temperatures and different apparent magnitudes.
Dashed lines show baselines at which different diameters are resolved.
Original estimate of possible S/N as function of the stellar temperature
•
For stars with same angular diameter but decreasing temperatures (thus decreasing fluxes),
telescope diameter must successively increase to maintain the same S/N.
When the mirrors become so large that the star is resolved by a single mirror, S/N drops.
For stars cooler than a given temperature, no gain results from larger mirrors.
R.Hanbury Brown, R.Q.Twiss: Interferometry of the intensity fluctuations in light III. Applications to astronomy,
Proc.Roy.Soc.London Ser.A, 248, 199 (1958)
SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY
Verification of simulation software against
classical observations by Hanbury Brown et al.
Left: Sirius observed with the Narrabri stellar interferometer
(R.Hanbury Brown, J.Davis, R.J.W.Lake & R.J.Thompson; MNRAS 167, 475, 1974)
Right: Simulated observations with Narrabri instrumental parameters
SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY
Squared visibility (“diffraction pattern”), of a stellar disk of angular diameter 0.5 mas.
Z = normalized second-order coherence
SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY
Squared visibility from a close binary star.
Left: Pristine image; Right: Logarithm of magnitude of Fourier transform
OBSERVATIONS IN INTENSITY INTERFEROMETRY
Simulated measurements of a binary star with CTA-B telescope array
Left: Short integration time (noisy); Right: Longer integration time.
Color scale shows normalized correlation.
SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY
Limiting magnitude
mv = 3
mv = 5
mv = 7
Simulated observations of binary stars of visual magnitudes 3, 5, and 7.
Total integration time: 20 hours; wavelength 500 nm, time resolution 1 ns, quantum efficiency = 70%
Array: CTA D
SIMULATED OBSERVATIONS OF BINARY STARS OF DIFFERENT SIZE
(mV = 3; Teff = 7000 K; T = 10 h; t = 1 ns;  = 500 nm; QE = 70%, array = CTA B)
Top: Reconstructed and pristine images; Bottom: Fourier magnitudes.
Already changes in stellar radii by only a few micro-arcseconds are well resolved.
SIMULATED OBSERVATIONS IN INTENSITY INTERFEROMETRY
S/N independent of spectral passband
SIMULATED OBSERVATIONS OF ROTATIONALLY FLATTENED STAR WITH EMISSION-LINE DISK
Left: Pristine image, 0.4 mas across with 10 µas equatorial emission-line disk, 6 times continuum intensity
Center: Observed magnitude of the Fourier transform in continuum light
Right: Same for a narrow-bandpass filter at He I  587 nm emission
Stellar magnitude: mv = 6, Teff = 7000 K; T = 50 h, QE=70%; Array = CTA I
Top: STARS SHAPED BY RAPID ROTATION; Middle: DISKS & WINDS; Bottom: STELLAR SURROUNDINGS
NON-RADIAL PULSATIONS & VELOCITIES ACROSS STELLAR SURFACES
Observations through very narrow bandpass filters, spanning one spectral line
(might require ordinary telescopes rather than Cherenkov ones)
Simulated observations of a Cepheid-like star undergoing non-radial pulsations
mV = 3.4; Teff = 7000 K; Δt = 1 ns;  = 500 nm; Array = CTA B
Left: Pristine image; Right: Observed Fourier magnitude
“Our
local Universe is teeming with stars, but despite 400 years
of telescopic observations, astronomy is still basically
incapable of observing stars as such!
Although we can observe the light radiated by them, we do not
(with few exceptions) have the capability to observe the stars
themselves, i.e., resolving their disks or viewing structures
across and outside their surfaces (except for the Sun, of
course!).
In 2009, we
celebrated
400 years of
telescopic
astronomy
One can just speculate what new worlds will be revealed once
stars no longer will be seen as mere point sources but as
extended and irregular objects with magnetic or thermal spots,
flattened or distorted by rapid rotation, and with mass ejections
monitored in different spectral features as they flow towards
their binary companions.
It is not long ago that the satellites of the outer planets passed
from being mere point sources to a plethora of different worlds,
and one might speculate what meager state extragalactic
astronomy would be in, were galaxies observed as point
sources only.”
(Dravins & LeBohec, SPIE Proc. 6986, 2008)