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In the same section :
First fringes with the VLT unit telescopes

AMBER observes through three 8m telescopes simultaneously
Saturday 5 June 2004, by Fabien Malbet, Romain Petrov

At the very end of our last commissioning #1 night (the morning of June 1), we obtained fringes on three UTs with the medium resolution mode of AMBER.

This result was somehow harder to obtain than the fringe acquistion with UT2 and UT3. We used UT2 and UT3 without MACAO Adaptive Optics and UT1 without neither AO nor tip-tilt correction. The combination of three telescopes and three delay line was being used for the first time, which implied quite larger unknown offsets between delay lines, but we were confident that the large coherence length of the medium resolution mode of AMBER wOULD allow scanning large optical path differences very rapidly. Unfortunately, the «STRAP unit» (Coudé fine guiding and tip tilt correction) of UT1 was not working with strong effects on the centering of the corresponding VLTI beam and also on the quality of the active optics correction. The VLTI team and later the combined AMBER and VLTI teams had to work hard to achieve a sufficient injection of UT1 beam into the instrument.

It was a difficult night, during which we switched several times between 2 UTs and 3 UTs mode. In 2T mode, we tried to accumulate data, while efforts were made by the Paranal staff to center UT1 beam. We switched to 3 UTs mode when it appeared that we had some chance to achieve the goal of the night which was 3 UTs interferometry. Finally, AFTER 6 am, we obtained a UT1 intensity that was a marginally sufficient fraction of UT2 and UT3 ones. Then, we were lucky to find immediately UT1-UT2 fringes (at 0.7 mm of their theoretical position) and it took us only a few minutes to catch the UT2-UT3 fringes, which had escaped 28 mm away in this new setup. The UT1-UT3 fringes is weak (the object visibility is substantially reduced for this longer baseline) but they are «mathematically» present in the recorded 3 telescope data, if the accumulated SNR is sufficient, which is quite likely.

We should know that soon, when the AMBER team gets some sleep and recuperates some energy to reduce the data. These difficult conditions could finally result in a good test of performances for an (accidentally) weak source with an (accidentally) very low contrast. Conclusion of Andrea Richichi at the end of the night: never give up! He did not and deserves our gratitude for helping us try well beyond «reasonable» hope. Thanks also to the telescope operators for their patience and availability in this apparently despaired situation, to Markus Schoeller for spending a part of the night in the VLTI light ducts and tunnels chasing the UT1 beam and for all the staff who worked hard to provide a usable solution.

UT1, UT2 and UT3 which are used Sunset on the VLTI platform Starting the night R. Petrov & A. Richichi
G. Zins & M. Kiekebush debuuging the software Some spectators from UT4 That’s it! The three beams are finally all on the detector E. Tatulli a little bit tired
G. Zins at the end of the night Fringes on the two short baselines Ready for the group picture? AMBER and ESO team
Champagne bottle is opened! Celebration!

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