Download Kilometer-baseline optical intensity interferometry for stellar surface

10.1117/2.1201605.006504
Kilometer-baseline optical
intensity interferometry for
stellar surface observations
Dainis Dravins and Colin Carlile
By connecting telescopes electronically, rather than optically, atmospheric turbulence is avoided and microarcsecond resolutions may
be achieved.
Much of the progress that has been made in astronomy has been
driven by improved imaging capabilities. Amplitude/phase
interferometers—operating over baselines of a few hundred
meters—are the current state-of-the-art optical facilities.1, 2 Indeed, tantalizing results from such instruments are starting to
reveal some stars as resolved objects. In these images, the stars
may be flattened or deformed because of rapid rotation, engulfed by shells, covered by spots, or surrounded by obscuring clouds.3, 4 Most bright stars, however, require baselines of
several kilometers to enable any kind of surface imaging. Furthermore, since amplitude interferometers require stabilities of
within a fraction of an optical wavelength, this standard of
imaging becomes particularly challenging (especially at shorter
visual wavelengths).
In contrast to amplitude/phase techniques, with optical intensity interferometry, a second-order effect of light waves (the way
the instantaneous intensity fluctuations correlate between light
measured by separate telescopes) is exploited.5 This approach
was first developed by Hanbury Brown and Twiss, who built
an instrument in Narrabri, Australia, to measure stellar diameters. As this was fundamentally a two-photon measurement, it
is often thought of as the first experiment in quantum optics.
The important observational advantage of intensity interferometry is that it is practically insensitive to both atmospheric turbulence and telescope optical imperfections. This means that very
long baselines can be used, and observing can be conducted at
short optical wavelengths. In such setups, the telescopes are only
connected electronically, with no optical links between them.
The noise budget therefore relates to electronic timescales (i.e.,
nanoseconds) and light-travel distances of centimeters (or even
Figure 1. In the technique of intensity interferometry, spatially separated telescopes are used to observe the same source, and the measured time-variable intensities—i.e., I1 (t), I2 (t), I3 (t)—are electronically cross-correlated between different pairs of telescopes.
meters) rather than those of the light wave itself. A time resolution of about 10ns corresponds to a light-travel distance of 3m.
Atmospheric path-lengths and telescope imperfections thus only
need to be controlled within some reasonable fraction of 3m.
In this work,6 we provide an overview of the intensity interferometry technique. We also review how this approach can
be used to improve the resolution of optical measurements in
astronomy. The basic underlying concept of intensity interferometry is illustrated in Figure 1. When the telescopes are close
together, the measured fluctuations are almost simultaneous and
thus correlated in time. With increasing distance between the
telescopes, however, the fluctuations gradually become decorrelated. The rate at which this decorrelation occurs provides a
measure of the spatial coherence and, therefore, of the spatial
properties of the source. Since the signal is electronic, it can be
copied, transmitted, or saved (analogous to radio interferometry). Furthermore, a large array of telescopes can enable the
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formation of many baselines between different telescope pairs,
without any additional logistical effort.7
A 2D ‘diffraction pattern’ (the amplitude of the Fourier transform) of the source can be obtained from intensity interferometry observations (see Figure 2). Although such a pattern places
stringent constraints on the source, specific algorithms must be
applied to reconstruct full images. An example of measurements
of an artificial star, which we made with an array of small telescopes in the laboratory, is shown at the bottom of Figure 2.8, 9 To
make these measurements, we operated the small telescopes as
an intensity interferometer with 180 baselines. These images are
rudimentary, but they represent the first instance of diffractionlimited images being obtained from an array of detached optical
telescopes, connected only by electronic software.
Although the problem of atmospheric turbulence can be circumvented with intensity interferometry, this takes place at an
Figure 3. A simulated image of a hypothetical exoplanet (with a
Saturn-type ring and four moons) transiting across the disk of a relatively close star (Sirius, with a 6 milliarcsecond diameter) is used to
illustrate the significance of a 40 microarcsecond optical resolution. In
this image, an active solar-type chromosphere is assumed to be present
above the stellar surface.
Figure 2. Fourier-plane and image-plane information. The left column
shows 2D ‘diffraction patterns’ that correspond to the images shown
on the right. The Airy diffraction pattern (top left) can be recognized
as originating from a circular aperture. A measured pattern from an
artificial asymmetric binary star (bottom left) is built from intensity
interferometry measurements over 180 baselines between pairs of laboratory telescopes. The reconstructed image is shown on the bottom
right, where the pale gray circle indicates the diffraction-limited spatial resolution realized by an array of optical telescopes connected only
through electronic software (i.e., with no optical links).8, 9
associated cost. For example, a realistic time resolution of a few
nanoseconds is much longer than the coherence time of broadband light (i.e., over which the intensity fluctuations are fully developed). This means that the fluctuations become smeared out
over many coherence periods. Large telescopes are thus required
to obtain precise measurements with good photon statistics. In
fact, such large facilities are currently being installed, although
for a different primary purpose. The largest example of such
a project is the Cherenkov Telescope Array (CTA).10 This kind
of telescope will be used to study cosmic gamma-ray sources
from the observation of visual flashes of ‘Cherenkov light’ initiated by incoming gamma rays in the upper atmosphere. It
is the current plan for the CTA to have some 100 telescopes
spread over an area of a few square kilometers, with a combined light-collecting area of about 10,000m2 .11 Although the
CTA will mainly be devoted to gamma-ray studies, other applications are also envisaged.9 These, secondary, observations
could be conducted during nights with bright moonlight, when
the faint Cherenkov light flashes are difficult to detect.
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If baselines of 2 or 3km could be employed for intensity interferometry at short optical wavelengths, the resolution could
approach 30 microarcseconds. This is an unprecedented value
in optical astronomy and is almost three orders of magnitude
better than the Hubble Space Telescope.12 Such astronomical
imaging—on scales of tens of microarcseconds—has hitherto
been achieved only with radio interferometers that operate between Earth and deep space. For optical astronomy, such values
represent completely novel parameter domains. With the image
in Figure 3, we illustrate the true meaning of such resolutions.
This image is a ‘recognizable’ type of object, i.e., solar-type phenomena projected onto the disk of a nearby star. We also show
a hypothetical exoplanet transiting across the star (Sirius). We
made the planet’s size and oblateness equal to that of Jupiter, but
we also fitted it with a Saturn-like ring and four moons. We have
assumed that the stellar surface is surrounded by a solar-type
chromosphere (shining with a reddish emission line), with protruding prominences and eruptions. It is important to note that
the image is not a simulation of any specific observation, but
is an image degraded to the angular resolution expected to be
obtained with a kilometer-scale complex of air Cherenkov telescopes.
In summary, we have described and demonstrated the technique of intensity interferometry for high-resolution astronomy
observations. In our current work, we are aiming to address the
next milestone, i.e., a full-scale demonstration of a stellar observation with one pair of Cherenkov telescopes that are separated
by a few hundred meters. This is a preparatory step toward the
realization of full-scale interferometric imaging with a dozen or
more baselines. Many technological developments have taken
place since the pioneering experiments of Hanbury Brown and
Twiss. With this technique it should now be possible to achieve
a simple, but fundamentally difficult goal, i.e., to view our
neighboring stars not just as unresolved points of light, but as
extended and fascinating objects.
Colin Carlile has an extensive international background in neutron sources and their instrumentation. His recent planning for
the European Spallation Source in Lund was acknowledged with
an honorary doctorate. He is now beginning research in astronomical instrumentation.
References
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Interferometry, p. 325, Cambridge University Press, 2006.
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Stellar Systems, pp. 325–373, Springer, 2013.
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SPIE 9907, 2016.
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2012. doi:10.1016/j.newar.2012.06.001
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doi:10.1016/j.astropartphys.2013.01.007
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Author Information
Dainis Dravins and Colin Carlile
Lund Observatory
Lund, Sweden
Dainis Dravins is a professor of astronomy. His research involves precision observations in optical astronomy, including
spectroscopy of solar and stellar atmospheres, stellar radial velocities, methods to search for exoplanets, photometry through
the atmosphere, astrometry, and quantum optics.
c 2016 SPIE