CTA Consortium meeting – DESY, Berlin/Zeuthen; May 2010

Dainis Dravins & Hannes Jensen

Lund Observatory, Swedenwww.astro.lu.se/~dainis

HUNDRED TIMES SHARPER THAN HUBBLE !

Intensity Interferometry withVarious CTA Configurations

ANGULAR RESOLUTION IN ASTRONOMY

1 arcsec

1 mas

100 µas

10 mas

100 mas

10 µas

Observing stars…(and not only starlight)

ESO Paranal

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 µmGreen = 1.6 – 1.75 µmRed = 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)

Many stars becomeresolved surface objectsfor baselines 100-1000 m

Intensity interferometryPro: 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

Narrabri intensity interferometerwith its circular railway trackR.Hanbury Brown: BOFFIN. A Personal Story of the Early Daysof Radar, Radio Astronomy and Quantum Optics (1991)

Flux collectors at NarrabriR.Hanbury Brown: The Stellar Interferometer at Narrabri ObservatorySky and Telescope 28, No.2, 64, August 1964

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.

OBSERVATIONS IN INTENSITY INTERFEROMETRY

Squared visibility (“diffraction pattern”), of a stellar disk of angular diameter 0.5 mas.

Z = normalized second-order coherence

OBSERVATIONS IN INTENSITY INTERFEROMETRY

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

OBSERVATIONS IN INTENSITY INTERFEROMETRY

Different array layouts

VERITAS Fourier plane coverage during 8 hours, as a star moves through the zenith

Projected baselines changewith Earth rotation

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.

OBSERVATIONS IN INTENSITY INTERFEROMETRY

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.

CTA I

CTA D

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.

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.

CTA B

Left to right: About one half, one quarter, one eight of the telescopes retained.

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).Stellar magnitudes mV=3 (left column), mV=5, and mV=7 (right).

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).Half of all telescopes (left column), one quarter, and one eight

(right).

Subsets of CTA B

Subsets of CTA D

Subsets of CTA I

CTA candidate configurations examined

Conf.B Conf.D Conf.INumber of telescopes 42 57 77Unique baselines 253 487 1606Shortest baseline 32 170 90 metersLongest baseline 759 2180 2200 metersResolution range @ 500 nm 0.16-3.9 0.05-0.75 0.06-1.4 mas

Resolution range denotes smallest and largest angular sizes that can be resolved with the array (= 1.22 D/)

Evaluation:All configurations B, D, I provide dense sampling of the (u,v)-

planedue 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.

Digital intensity interferometry

Very fast digital detectors, very fast digital signal handling,

and the quantum-optical theory of optical coherencenow enable very-long-baseline optical interferometry

by combining distant Cherenkov telescopes in software

OBSERVATIONS IN INTENSITY INTERFEROMETRY

Diameters of brighter stars that are observable with intensity interferometry.

Stellar diameters for different temperatures and different apparent magnitudes.Dashed lines show the baselines at which different diameters are resolved.

OBSERVATIONS IN INTENSITY INTERFEROMETRY