VLTI Configurations Overview

In Period 79 (April 1st, 2007 - September 30th, 2007)
Period 80 (October 1st, 2007 - March 31st, 2008)
and Period 81 (April 1st, 2008 - September 30th, 2008)
the following configurations are offered with the VLTI.

AMBER is offered on all four available UT triples:

  • UT1-UT2-UT3
  • UT1-UT2-UT4
  • UT1-UT3-UT4
  • UT2-UT3-UT4
  • and on four different AT triples:
  • E0-G0-H0
  • A0-D0-H0
  • D0-H0-G1
  • A0-K0-G1
  • MIDI is offered on all six UT baselines:

  • UT1-UT2-57m
  • UT1-UT3-102m
  • UT1-UT4-130m
  • UT2-UT3-47m
  • UT2-UT4-89m
  • UT3-UT4-62m
  • and on ten different AT baselines:
  • E0-G0-16m
  • G0-H0-32m
  • E0-H0-48m
  • D0-H0-64m
  • A0-H0-96m
  • A0-K0-128m
  • D0-G1-72m
  • H0-G1-72m
  • A0-G1-90m
  • K0-G1-90m
  • For details on the offered modes pls. refer to the Call for Proposals for Period 81 More information about the VLTI site layout, the stations and baselines can be found here.

    Observational constraints

    Telescope shadowing and limited Delay Line strokes

    Please note that some AT configurations suffer from shadowing by the UTs and the very longest baselines from insufficient Delay Line stroke. E.g. an AT on the A0 station has severe sky restrictions to the north due to the proximity of UT1. There are also restrictions on D0 and E0 (see the site layout). On the UT1-UT4 baseline fringes can not be obtained on objects close to the southwestern horizon. Even more severe restrictions are present for the baselines A0-H0 and A0-K0 to the east and G1-K0 to the northeast.

    Elevation limits for AT stations shadowed by UTs. In the table azimuth 0° is south and 90° is west.
    AT stationElevation limitLower Azimuth LimitUpper Azimuth Limit
    A048165209
    D048112156
    E03898133

    The following figures illustrate the Delay Line restrictions for the baselines mentioned above. The VLTI can not preset to areas which are indicated by the black points. North is up and east is right. The circles correspond to elevations of 70, 50, 30 and 10°. Remember that the telescopes do not point below 20°.
    UT1UT4 DL restrictions UT1-UT4
    A0H0 DL restrictions A0-H0 A0K0 DL restrictions A0-K0
    G1H0 DL restrictions G1-H0 G1K0 DL restrictions G1-K0

    In all cases, please verify your observations with VisCalc.

    Telescope guiding

    The Coude foci of the UTs are equipped with MACAO (Multi Application Curvature Adaptive Optics) units, which can be used with natural guide stars with 1 < V < 17, seeing < 1.5", τ0 > 1.5ms and airmass < 2. The distance of the natural guide star from the science target is restricted to be within 57.5".

    The ATs are equipped with STRAP (System for Tip/tilt Removal with Avalanche Photodiodes) units, which provide tip-tilt correction for targets brighter than V=13.5. The distance of the guide star from the science target is restricted to be within 57.5".

    Please note that it is mandatory that users provide guide stars. The science target may be used as a guide star, if it is bright enough at visible wavelengths. In case you can not find a suitable guide star, have a look at 2MASS. We have very often found suitable guide stars there. The guide star coordinate accuracy should be better than 1". In case there are no guide stars with your target, the planned observations could still be attempted in visitor mode.

    Proper motions

    Proper motions are an issue with VLTI since many sources we observe are bright, thus usually close and have proper motions which can not be neglected. A source with a proper motion of 0.1"/year has by early 2007 accumulated a coordinate error of 0.7". Observing this source on the longest baselines of 130m these 0.7" translate to 450µm optical path difference, which is more than five times the coherence length in AMBER low resolution mode. The fringes are not only far away from where they are expected, but they will also move with time due to the coordinate error. Thus, users should pay special attention to proper motions when entering object coordinates.

    The Fringe Tracker FINITO

    FINITO is offered on the Auxiliary Telescopes for the first time with the Call for Proposals for Period 80.

    FINITO scans the center of the fringe packet in H band with high speed and sends a cophasing signal to the VLTI Delay Lines via a dedicated high speed link. Due to the short individual exposure times (between 0.5ms and 2ms) and the need to measure the fringe in every individual scan, the sensitivity of FINITO is somewhat worse than for a science instrument that slowly records the fringe and can reject some data. FINITO operates on two channels, i.e. tracks three baselines.

    The full potential of FINITO comes with the science instrument using higher spectral resolution. Since the fringes are "frozen" in OPD space, the science instrument can integrate longer and/or one can stack the individual fringes during postprocessing.

    Currently, FINITO operations are feasible for seeing below 1.2", τ0 above 2.5ms, and airmass < 1.5. The limiting correlated magnitude for FINITO is H = 3 and the minimum visibility in the H band is 15%. These numbers were determined with a seeing < 0.8" and τ0 above 2.5ms.

     
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