The first `OHANA fringes with the Keck telescopes (Forwarded)

Observatoire de Paris
Paris, France

Contact:
Guy Perrin, Observatoire de Paris, LESIA
Tél: 33 1 45 07 79 63
Fax: 33 1 45 07 71 02

13 January 2006

The first `OHANA fringes with the Keck telescopes

Towards kilometric arrays of telescopes in the near-infrared

An international team of astronomers has demonstrated the first coherent
coupling of large telescopes with single-mode fibers in the near infrared.
The test, performed with the two 10 m Keck telescopes of the Mauna Kea
Observatory in Hawaii, is the first step of the `OHANA project whose aim
is to turn the observatory seven largest telescopes into a gigantic imager
of 800 m diameter. This first experiment is a milestone on the way to
future very large arrays of telescopes connected with single-mode fibers
to produce astronomical images with sub-milliarcsecond angular resolution.
Very high angular resolution and sensitivity are required to study very
compact sources such as the environment of black holes in other galaxies
or the inner regions of exoplanetary systems where life may have
developed. The results are published in the January 13, 2006 issue of the
Science magazine.

The ability of a telescope to resolve small-scale structures of
astronomical sources is ultimately limited by its size. The larger the
telescope, the higher its angular resolution. The current largest
telescopes have diameters of 8 to 10 m. The size of the next generation
telescopes will increase to a few tens of meters with 100 m appearing as a
technological limit for single-aperture systems. This limit can be
overcome if the light of distinct collectors is coherently recombined. The
distance between telescopes then gives the resulting angular resolution.
This technique known as astronomical interferometry was first suggested by
Hyppolite Fizeau in the XIXth century and demonstrated by Albert A.
Michelson with a single telescope as a beam combiner. A new era started in
1974 when Antoine Labeyrie first demonstrated interferometric fringes
between separate apertures. Since then, about twenty interferometric
optical observatories have been in operation and have allowed resolving
stellar surfaces and environments otherwise point-like to telescopes. With
the advent of very large telescopes equipped with adaptive optics, the
Keck Interferometer and the Very Large Telescope Interferometer have
inaugurated the era of very sensitive high angular resolution with the
very first studies of the environment of the core of active galactic
nuclei. Higher angular resolutions are needed to resolve the core which
hosts a super-massive black hole.

Current interferometers have baselines at most a few hundred meters long.
Propagating beams over large distances requires a large number of mirrors,
which together with the effects of diffraction induce a low throughput.
Single-mode fibers can transmit light over even larger distances with
potentially lower losses. They may therefore play a key role to build very
large arrays of telescopes of kilometric or larger size. The effectiveness
of single-mode fibers for this purpose has been studied by Observatoire de
Paris and Institut de Recherche en Communications Optique et Micro-ondes
in the framework of the `OHANA (Optical Hawaiian Array for Nanoradian
Astronomy and which also means family in the Hawaiian language) project.
300 m long single-mode fiber cables have been shown to convey the
coherence of light in 300 nm large bandwidths in the near infrared with
transmissions from 50% to 95%. `OHANA aims at connecting the seven largest
telescopes of the Mauna Kea Observatory in Hawaii (Figure 1). The
resulting array will have a diameter of 800 m and will provide
unprecedented angular resolution in the near infrared.

The first interferometric test has been performed between the two 85 m
apart Keck telescopes in the astronomical K. band (2-2.3 µm) with fluoride
glass single-mode fibers manufactured by the France based company Le Verre
Fluoré. Two 300 m long fibers were used in the experiment thus simulating
a 500 m long baseline with respect to beam propagation. The fiber inputs
were directly set at the adaptive optics corrected Nasmyth focus of each
telescope and were routed down to the interferometric laboratory in the
basement of the telescopes building (Figure 2). Prior to mixing, the beams
output from the fibers were delayed with classical optical delay lines to
match the optical paths of the two beams in order to detect the narrow
fringe pattern (Figure 3). No long stroke fibered delay line is yet
available and this remains a field of research to further develop for
future large arrays. Despite cloudy conditions, first fringes were
detected on the 4.6 K band magnitude star 107 Herculis (Figure 4).

This first successful `OHANA test is the beginning of the effort to
characterize each `OHANA baseline before completing the entire array. The
next baseline will link the Canada-France-Hawaii and Gemini North
telescopes. This success also opens the way to the design of dedicated
large interferometric facilities in the optical domain comprising a large
number of individual telescopes separated by kilometric distances and
which will bring a far more accurate vision of our universe than current
facilities.

The `OHANA project led by Paris Observatory is a collaboration between the
Mauna Kea observatories (Canada-France-Hawaii Telescope Corporation, W.M.
Keck Observatory, Subaru Telescope, Gemini Observatory, United Kingdom
Infra-Red Telescope, Infra-Red Telescope Facility), the Institute for
Astronomy of the University of Hawaii, the National Optical Astronomical
Observatory, the National Astronomical Observatory of Japan, Institut
National de Rercherche en Communication Optique et Micro-Ondes and
Institut National des Sciences de l'Univers du CNRS.

See also
http://www.keckobservatory.org/news/science/060113_ohana/index.html

Reference:

Perrin G., et al., 2005
"The first `OHANA fringes with the Keck telescopes"
Science, 2006, 13 January issue

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/jan06/ohana-f1.jpg (48KB)]
The Mauna Kea observatory and the 7 telescopes of the project 'OHANA:
Optical Hawaiian Array for Nanoradian Astronomy. ~OHANA means also
"family" in hawaïan language. © Richard Wainscoat

[Figure 2:
http://www.obspm.fr/actual/nouvelle/jan06/ohana-f2.jpg (36KB)]
Experiment set-up (1). Routing of single-mode fibers from the Keck Nasmyth
foci down to the basement beam combination laboratory. Fibers are attached
to the telescope structure like regular cables and go through the cable
wrap system to prevent any damage when the telescopes rotate in azimuth.
The remaining fiber lengths (approximately 200 m for each arm) are wrapped
on spools in the basement (see Figure 3). © W.M. Keck Observatory et
Observatoire de Paris

[Figure 3:
http://www.obspm.fr/actual/nouvelle/jan06/ohana-f3.jpg (50KB)]
Experiment set-up (2). Sketch of the interfacing of the output of the
'OHANA fibers with the Keck interferometer delay lines and beam combiner.
Only one beam/fiber is shown for the sake of clarity. The two beams follow
equivalent paths. The fiber is placed at the focus of an off-axis parabola
(OAP) to produce a collimated beam. The beam is reflected with a flat
mirror (M10) towards the Long Delay Line. It is then launched into the
Fast Delay Line. The beam size is reduced in a Beam Compressor and feeds
the Beam Combiner.

[Figure 4:
http://www.obspm.fr/actual/nouvelle/jan06/ohana-f4.jpg (42KB)]
Fringes on the star 107 Herculis. The interference fringe signal is the
high frequency 200 µm large pattern. The low frequency intensity
fluctuations are due to vibrations in the top panel. The signals have been
high-pass filtered in the bottom panel to remove the low frequency
vibrations.