1. CTA Consortium meeting – DESY, Berlin/Zeuthen; May 2010
HUNDRED TIMES SHARPER THAN HUBBLE !
Intensity Interferometry with
Various CTA Configurations
Dainis Dravins & Hannes Jensen
Lund Observatory, Sweden

2. ANGULAR RESOLUTION IN ASTRONOMY
1 arcsec
100 mas
10 mas
1 mas
100 µas
10 µas

3. (and not only starlight)
Observing stars…
(and not only starlight)

5. ESO Paranal

6. Actual image of the Mira-type variable T Leporis from VLTI
Image obtained by combining hundreds
of interferometric measurements
Central disc shows stellar surface,
surrounded by a spherical shell of
expelled molecular material
Infrared wavelengths color-coded:
Blue = 1.4 – 1.6 µm
Green = 1.6 – 1.75 µm
Red = 1.75 – 1.9 µm
In the green channel, the
molecular envelope is thinner
The size of Earth’s orbit is marked.
Resolution = 4 milli-arcseconds
(ESO press release 0906, Feb. 2009)

7. resolved surface objects
Many stars become
resolved surface objects
for baselines m

8. Intensity interferometry
Pro: Time resolution of 1 ns implies 30 cm light travel time;
no need for any 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

9. 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)

10. Flux collectors at Narrabri
R.Hanbury Brown: The Stellar Interferometer at Narrabri Observatory
Sky and Telescope 28, No.2, 64, August 1964

11. Intensity interferometry

12. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Visibility (solid) and squared visibility (dashed) as function of baseline at  500 nm.
Inner curves are for a stellar disk of diameter 2 mas; outer for 1 mas.

13. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Squared visibility (“diffraction pattern”), of a stellar disk of angular diameter 0.5 mas.
Z = normalized second-order coherence

14. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Squared visibility (“diffraction pattern”) from a close binary star.
Left: Pristine image; Right: Logarithm of magnitude of Fourier transform

15. Different array layouts

16. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Projected baselines change
with Earth rotation
VERITAS Fourier plane coverage during 8 hours, as a star moves through the zenith

18. Simulated measurements of a binary star with CTA-B telescope array
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.

19. CTA B CTA D CTA I Left: Telescopes for CTA configurations B, D, and I.
Center column: (u,v)-plane coverage for a star in zenith.
Right: (u,v)-plane coverage for a star moving from zenith through 20 degrees west.

20. CTA B Simulated observations of binary stars with different sizes.
(mV = 3; Teff = 7000 K; T = 10 h; t = 1 ns;  = 500 nm;  = 1 nm; QE = 0.7, 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.

21. CTA B
Simulated observations of binary stars with different separations.
(mV = 3; Teff = 7000 K; T = 10 h; t = 1 ns;  = 500 nm;  = 1 nm; QE = 0.7, array = CTA B)
Top: Reconstructed and pristine images; Bottom: Fourier magnitudes.
Stellar diameters and binary separations are well resolved.

22. Subsets of CTA B Subsets of CTA D Subsets of CTA I
Left to right: About one half, one quarter, one eight of the telescopes retained.

23. CTA B
CTA D
CTA I
Simulated observations in the (u,v)-plane of close binary stars.
Full CTA configurations B (top row), D (middle), and I (bottom).
Stellar magnitudes mV=3 (left column), mV=5, and mV=7 (right).

24. Simulated observations in the (u,v)-plane of close binary stars.
Subsets of CTA B
Subsets of CTA D
Subsets of CTA I
Simulated observations in the (u,v)-plane of close binary stars.
Full CTA configurations B (top row), D (middle), and I (bottom).
Half of all telescopes (left column), one quarter, and one eight (right).

25. CTA candidate configurations examined
Conf.B Conf.D Conf.I
Number of telescopes
Unique baselines
Shortest baseline meters
Longest baseline meters
Resolution 500 nm mas
Resolution range denotes smallest and largest angular sizes
that can be resolved with the array (= 1.22 D/)

26. Evaluation:
All configurations B, D, I provide dense sampling of the (u,v)-plane
due to the sheer number of telescopes.
Different declinations of the source or the geographic orientation of the array have negligible effects due to the large number of telescopes.
but…
Arrays such as D are severely crippled by lack of short baselines,
limiting the instrument to studying sources smaller than 0.5 mas.
Many telescopes are required for good Fourier-plane sampling,
and too few telescopes provide poor data.
Best performance among those examined: Configuration I.

29. Digital intensity interferometry
Very fast digital detectors, very fast digital signal handling,
and the quantum-optical theory of optical coherence
now enable very-long-baseline optical interferometry
by combining distant Cherenkov telescopes in software

30. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Diameters of brighter stars that are observable with intensity interferometry.

31. OBSERVATIONS IN INTENSITY INTERFEROMETRY
Stellar diameters for different temperatures and different apparent magnitudes.
Dashed lines show the baselines at which different diameters are resolved.