VLTI Configurations Overview - 89
In Period 87 (April 1st, 2011 - September 30th, 2011), 88 (October 1st, 2011 - March 31st 2011)and 89 (April 1st, 2012 - September 30th 2012)
the following configurations are offered with the VLTI.
For configuration offered in P86 please look at the following link
Configurations offered using the UTs
AMBER is offered on all four available UT triples:
MIDI is offered on all six UT baselines:
Configurations offered using the Auxillary Telescopes
Starting with P85 the Auxillary Telescopes are offered in 4 telescopes configurations. For each of these quadruplets, all possible 2 telescopes and 3 telescopes configurations can be used.
At the time of Phase I, user are only requested to provide informations on which of the available quadruplets they wish to use for observations. The decision on which specific baselines will be used at the time of the observation will be decided at the time of the Phase II or in preparation of the visitor run.
The 3 offered quadruplets are:
For reference the available configuration for AMBER and MIDI are:
AMBER is offered for 12 triples:
MIDI is offered with 16 baselines:
For details on the offered modes pls. refer to the Call for Proposals for Period 87 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 E.g. an AT on the A1 station has severe sky restrictions to the north due to the proximity of UT1. There are also restrictions on D0and C1 (see the site layout above). On the UT1-UT4 baseline fringes cannot 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.
| AT station | Elevation limit | Lower Azimuth Limit | Upper Azimuth Limit |
|---|---|---|---|
| A1 | 34 | 163 | 195 |
| D0 | 48 | 112 | 156 |
| C1 | 37 | 144 | 177 |
For the very longest baselines we also suffer from insufficient Delay Line stroke.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.
Figures illustrate the Delay Line restrictions for all baselines are available when clicking on the baseline name in the list above. The VLTI can not preset to areas which are indicated by the yellow dots. North is up and east is right. The yellow circles correspond to elevations of 65, 55, 42, 30 and 20°. Remember that the telescopes do not point below 20°.The yellow circles also indicate the 1.1, 1.2, 1.5, 2.0 and 2.9 airmass limits.
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 guidestars 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 Photo diodes) 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".
For a FLI above 85% guiding will be impossible if the target is too close to the moon.
- For a star brighter than 9th magnitude guiding is not possible for a moon distance smaller than 10 degres.
- For a star fainter than 9th magnitude, guiding is not possible for a moon distance smaller than 20 degres.
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 2007accumulated 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 was offered only on the Auxiliary Telescopes for the first time with the Call for Proposals for Period 80. FINITO is now offered with the UTs with the Call for Proposals for Period 82.
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", τ0above 2.5ms, and airmass < 1.5. The limiting correlated magnitude for FINITO on the ATs is H = 5 and the minimum visibility in the H band is 15%. The limiting correlated magnitude for FINITO on the UTs is H=7with a minimum visiblity of 10%.
These numbers were determined with a seeing < 0.8" and τ0 above 2.5ms.