 |
 |
|
|
Pôle
Astronomie
|
Accueil
Interférométrie
|
|
The
instrument FLUOR on CHARA
|
The instrument FLUOR (Fiber Linked Unit for Recombination) allows a fibered recombination of two beams of a stellar interferometer in the infra-red. The recombination takes place in the triple coupler for fibres MONA , developed by the company "Le Verre Fluoré". It was set up for the first time in 1992, on the site of Kitt Peak, in Arizona.
Overall diagram of the table FLUOR on CHARA
FLUOR constitutive elements:
• FASTSCAN : makes possible to sweep fringes at a high rate.
• ALIU : Unit for the alignment of FLUOR on CHARA, composed of a cubic beam splitter and of a corner cube, which redirect beams to the viewing telescope.
• Injection stages : They allow to inject the whole beam into the fiber; they have three components: a plane mirror that sends the beam in an off-axis parabola which focalise exactly on the center of the optical fiber.
• MONA : Recombine the light of two fibres; gives two photometric signals and two interferometric signals in its four out fibers.
• OUT Stage: create a colimated beam starting from the four out fibers of MONA linked into a strand, to send it into NIRO.
• NIRO : IR camera of CHARA, based on a NICMOS chip.
Installed since on the site of the MT Hopkins, on the interferometer IOTA, FLUOR has demonstated the benefit of the use of optical fibres for spatial filtering to recombine light beams. These last years, its excellent results, both technical and scientific (Measurement precision on the Visibility frequently around 1%), as well as the success of the fibered recombiner VINCI on the site of the VLTI, have made clear the possible great scientific contribution, which FLUOR could get with an interferometer of bigger size, that would allow a better resolution. This led to come into contact with CHARA (Center for High Angular Resolution Astronomy), located on the ground of MT Wilson Observatory. CHARA makes possible the recombination of beams coming from six telescopes of one meter in diameter, and which the baselines are ranging from 34m to 331m (to be compared with 38m on IOTA).
The first contact
was established during the year 2001 by Vincent Coudé du Foresto (Astronomer
at LESIA) and Pascal Bordé (PhD student at LESIA); the removal of IOTA towards
CHARA, from July 10 to July 15 2002, was the occasion of a very particular
adventure, narated in text and images on the website usa-roadtrip.ifrance.com.
The
people having directly worked on the installation of FLUOR on CHARA were:
Vincent
Coudé du Foresto, Antoine
Mérand (PhD student at LESIA), Cyrille
Baudouin and Antonin
Rémond (Trainees engineers from the ENSPM
with the LESIA). Theo
Ten Brumelaar, responsible CHARA on the MT Wilson, Steve
Ridgway, astonomer at NOAO, and Nils
Turner, astronomer CHARA, also took part in the installation
and the first attempts at observation. This last move of FLUOR was accompanied
by a certain number of modifications of which in particular, a simplification
of the general outline, and the addition of two new
injection stages, inspired by the output stage of VINCI. In addition,
improvements were also made on the FLUOR software (Operating System), under
environment Labview and MacOS, to be able to command the instruments of
CHARA, programmed with C and working under Linux; it was also necessary
to develop a new code that would allow to communicate with the PICNIC camera
of CHARA. Then, the Operating System was divided into two parts: an "acquisition"
part, in the laboratory, that allows the reading of the camera and the command
of the FastScan unit; a part "observation", installed in the control room
of CHARA, a hundred meters far away of the laboratory, in charge of the
recovering of the data provided by the acquisition computer, and of the
real time processing of these data. The FLUOR adaptation to CHARA, like
its removal was the subject of the engineer internship of Cyrille Baudouin,
under the supervision of Vincent Coudé du Foresto.
The setting up of the whole optical and electronic components necessary for correct operation of FLUOR took two weeks, and on August 1 2002, the instrument was ready for the debugging of the software part, which took five additional days. Then, on August 6 2002, the first fringes on an artificial source were observed, in spite of a disastrous signal to noise ratio, the result of which was that the attempt at observation on the sky the following day failed. In order to improve the quality of the injection into the fibres (cause of the bad SNR), the following days were entirely dedicated to a complete realignment of the FLUOR table, which leaded to observe fringes on artificial source again Friday August 9, 2002, this time with a much better SNR.
The following week, observations were tried on the sky every night, but as the seeing was very poor, partly because of a fire near the MT Wilson that prevented the opening of the telescopes most of the time. Finally, the night of August 18, 2002, the first fringes were observed on the star alpha of Cassiopee. The results are excellent: a visibility of 90% on the star calibrator and the measurement of the diameter with a precision of 2%.
It should be noted that the results of the early observations with FLUOR should largely be improved in the next weeks of September and October 2002, because the first observations used only one of the new injection stages, the second still not having been delivered then. This led to use one of the former IOTA stages, which was absolutely not optimized for the light beams of CHARA, and caused a great loss of signal during the injection of light into the fibre.
The
next series of observations is planned for the end of September and the
beginning of October. Other improvements are in hand, in particular a total
automation of the beam alignment and an optimization of the injection into
the fibres.
To see the mosaic of the work of the "FLUOR TEAM", click HERE.
Site carried out by Antonin Rémond
| Contacts | Vincent Coudé du Foresto |
