VLTI Configurations Overview

Since P112, we introduced the extended configuration A0-B5-J2-J6. This triggered a change in the offering of the other standard configurations:

  • The  A0-G1-J2-J3 configuration, previously known as the "large" standard configuration is no longer offered due to redundancy with A0-G1-J2-K0 and scheduling constraints. 
  • Following this, we renamed A0-G1-J2-K0 as  the (new) "large" standard configuration.

AT configurations are requested by generic names rather than explicit configurations using AT positions. The following table shows which instrument / mode can request which configuration.  By clicking on their name, the user can see the sky coverage corresponding to the standard AT positions for a given configuration.

AT Configurations PIONIER, MATISSE, GRAVITY single-feed

GRAVITY dual-feed


yes yes
yes no
yes yes


IMPORTANT NOTE: We successfully tested a new standard AT medium configuration A0-B5-D0-G1 that improves observability, functionality, and baseline-complementarity between configurations. It is planned to put it in operations from P115 and replace the current one (K0-G2 D0 J3 ). Observability on this new medium configuration, in particular of northern targets, might affect some programmes.  Users are advised to adapt their observing strategy accordingly by using the large configuration for example.

Over/under subscription

As a general rule, > 50% turbulence categories (e.g. 70% turbulence category for MATISSE ) and right ascenscions centred on RA=2h (October and Novermber) are regularly undersubscribed. Users are encouraged to request those turbulence categories and RA ranges when possible, in order to maximize the probabiliy of completing their observations.


Instructions for Time Series

Time series of concatenations that are repeated once or more often over the period ;  a cadence of typically 1 week on a given configuration is not realistic given the need to cycle through several configurations in the AT schedule. As a guideline, possible cadences are either a few days in a row or not more than 2-3 times per period per AT configuration.

Instructions for Imaging programmes

  • It is highly recommended to request imaging in service mode in order to benefit from an u,v coverage optimized by ESO and taking advantage of intermediate configurations. Imaging propsals requesting visitor mode are expected to present a strong science case justification.
  • Taking into account the VLTI/AT field of view and the highest spatial frequency, the image that a user can aim at reconstructing at VLTI has approximatively 200x200 pixels. Each 4 AT observations bring 6 Visibilities and 3 (independent) CP, or 9 data points. 8 observations per configuration is a minimum to get enough data. A minimum of two configurations is mandatory to request an imaging programme. A u,v plane filling factor can be defined and computed for a set of AT configurations (including intermediate configurations that are only possible during relocation nights in service mode). In these graphs, this factor is plotted as a function of declination to take into account the observability constraints due to telescope shadowing and delay line limits. Based on these, we recommend requesting the following number of concatenations depending on the declination of the choosen targets:   

              - 6 concatenations per configuration for -75º<dec<-30º

              - 4 concatenations per configuration for dec>-30º

Configurations offered using the Auxiliary Telescopes

The users are requested to provide their chosen AT quadruplet in their proposal at phase 1. The quadruplet cannot be changed after proposal submission.

This new scheme allows a more flexible execution of OBs, for example using intermediate configurations during AT relocations (transition nights between two standard configurations). This scheme will be primarily used for imaging programmes.  The overlap in baseline length between standard and relocation configurations is detailed in this graphic. A criteria of at least 50% of baseline length overlap will be used to decide if OBs can be executed on an another onfiguration than the standard one.

Based on this criteria and also adding the constraint on the North-South telescope configuration that impacts dual-feed OBs execution, this table gives the correspondence between actual AT quadruplets and classification in terms of "small", "medium", "large" and ''extended" configurations.

Starting with P105, alternative baseline configuration can be specified at phase 1 in a keyword list, in order of priority. Each run will, however, be scheduled on only one configuration. Most likely, it will be the first (primary) choice. However, a run may exceptionally be scheduled on an alternative choice if the primary choice of the baseline configuration cannot be scheduled. Because a run will be scheduled on only one configuration, all observing set-ups of a run need to request exactly the same list of baseline configurations. If different configurations are needed, different runs need to be specified, each with one configuration.

Configurations offered using the Unit Telescopes

PIONIER, MATISSE and GRAVITY are offered on the quadruplet UT1-UT2-UT3-UT4.

Configurations from previous periods

For reference, the configurations used in the previous periods can be found on this page.

Observations preparations

For details on the offered modes please refer to the current Call for Proposals and the VLTI manual. In all cases, please verify your observations with ESO tool VisCalc, or alternatively, with the third party preparation software Aspro2, developped with inputs from ESO.