Paranal: Important recent changes regarding instrumentation and facilities

This section describes important changes which took place during Periods 102 and 103, as well as changes expected to take place during Period 104.


Excellent condition proposals:

Seeing at Paranal is better than 0.5" for 5-10% of the time, while episodes of precipitable water vapor smaller than 0.5mm are experienced 6 to 7 nights per year (see Kerber et al. 2014, MNRAS 439, 247) mostly during the southern hemisphere winter. ESO encourages the unique science that can be carried out under one or both of these conditions: proposals designed to take advantage of one of these conditions should include the mention "Excellent condition proposals" in the \SpecialRemarks macro in the ESOFORM proposal form. Users requiring excellent seeing conditions should specify 0.4" in the \ObservingRun macro in their proposal. The proposals should describe why the science can only be achieved in such conditions.

Preparation tool in Service and Visitor Mode:

Since Period 102, the new web-based Phase 2 Proposal Preparation tool, p2, is used for the preparation of all observations on Paranal. Users are invited  to
familiarise themselves with the tool via a demo web interface that  does not require having a scheduled observing run. Further information is available on the p2 web pages.

Target of Opportunity observations:

The implementation of p2 had lead to a change in the procedure for triggering Target of Opportunity observations since Period 103. Successful proposers of ToO runs still have to prepare (usually dummy) OBs for their observations well ahead of the beginning of an observing Period. However, before triggering the ToO, the PI of the programme (or one of his/her delegates) will now use p2 to directly update the OB with the relevant coordinates and exposure times, insert configuration files if necessary, and attach a finding chart. The service observer will then execute the specified OB.

If real time assessment of the observations can be beneficial for their scientific output, the Paranal Observatory Eavesdropping Mode can be requested at the time of the trigger; it will then be activated by the service observer at the start of the execution of the OB.

Further details will be available on the Phase 2 Target of Opportunity Procedures webpage.

Note that Rapid Response Mode observations are executed following a different procedure; in particular, it does not involve p2 at the time of the trigger (see the Phase 2 Rapid Response Mode Procedures webpage).


UT instruments and facilities

Telescopes: all UTs

  • The concept of "Virtual Image Slicer" has been developed and implemented on the UTs. The Virtual Image Slicer consists in elongating the stellar images in a given direction by the introduction of a small amount of astigmatism thanks to the Active Optics of the telescope. Alignment of the major axis of the elongated stellar image along the entrance slit of a spectrograph increases the total signal collected in a single spectrum by a factor of up to 100 relative to a perfectly shaped image for bright sources within comparable execution time,  as overheads would otherwise be much longer than the actual observing (shutter) time. The use of the "Virtual Image Slicer" is only allowed in Visitor Mode. It must be explicitly mentioned in box 8b 'Observing Mode Justification'. Details can be found in Guisard, Sterzik & Munoz Proc. SPIE 9145, Ground-based and Airborne Telescopes V, 914544 (July 22, 2014).

UT1 - Antu

  • NACO: will be decommissioned at the end of Period 103.  It is therefore not offered in Period 104.
  • FORS2:
    • Since Period 102, the high-time resolution modes (imaging: HIT-I, spectroscopy: HIT-S, and multi-object spectroscopy: HIT-MS) are no longer offered.
  • KMOS:
    • Since the failure of Arm #4 in August 2018, KMOS is now available for Science Operations with 23 arms. Updates on arm functionality can be found on the KMOS news webpage.

UT2 - Kueyen

  • XSHOOTER: the calibration plan does not include observations of telluric standard stars. Please refer to the User Manual regarding the correction of telluric lines.

UT3 - Melipal

    • As described in the CPI section of the SPHERE Overview web page, the quality of the correction strongly depends on the seeing: in particular,  seeing constraints for SPHERE proposals must be better than 1.2", or smaller for targets which cannot be observed at low airmass.
    • Since Period 102,
      • dithering is offered for IRDIS/DPI;
      • the IRDIS/LSS mode is offered also in service mode;
      • a new SLLC apodizer is offered for the IRDIS/LSS mode.
    • Since Period 103,
      • a new acquisition template (to be used in the second or later OB of a concatenation) allows one to alternate Adaptive Optics (AO) observations on two stars of similar magnitudes (delta mag < 2) and separation < 30 arcminutes, with only one minute gap between integration sequences, by eliminating the need to repeat the AO acquisition;
      • the constraint on the Atmospheric Turbulence Model for Phase 2 OB preparation now combines constraints on coherence time and seeing. The combinations of values correspond to excellent, good, median and poor conditions and better match the expected SPHERE performance. The ETC has been updated accordingly.
  • VISIR will be moved back from UT4 to UT3 likely during the second half of Period 104 (see below). On UT3, VISIR is offered with all functionalities.
  • SINFONI will be decommissioned during Period 103. It is therefore not offered in Period 104.
  • Following its upgrade into a cross-dispersed spectrograph, CRIRES will be installed on the UT3-Melipal Nasmyth B focus and commissioned in Period 103.

UT4 - Yepun

  • HAWK-I :
    • is offered on UT4 during the start of Period 104 with restricted capabilities, as it has been modified for the use the Deformable Secondary Mirror for the NEAR experiment. The use of VISIR with adaptive optics is not offered for regular operations, but the possibility of further scienti c exploitation of this experimental mode in a limited, dedicated campaign is under consideration (such campaign would be open to the community through a special call). Hence, during Period 104 while VISIR is installed at the Cassegrain focus of UT4, regular operations is only possible for observations
      • in the M- and N-bands,
      • with a fixed chop throw of 6 arcsecs,
      • Only Large and Monitoring programmes accomodating these restrictions will be accepted in Period 104.
      • VISIR will be moved back from UT4 to UT3 likely during the second half of Period 104. On UT3, VISIR is offered with all functionalities.
  • MUSE
    • The combination of the  Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging (GALACSI) in its Laser Tomographic Adaptive Optics (LTAO) mode with MUSE in Narrow Field Mode (MUSE-NFM AO mode) has been offered since Period 103.
    • Large and Monitoring programmes for MUSE are accepted in MUSE-WFM NoAO and MUSE-WFM AO modes. But they are not accepted in MUSE-NFM AO mode, as its characterisation must be improved.
    • Target of Opportunity proposals are offered for MUSE in all modes.

Incoherent combined focus

    • The 1-UT mode has been o ffered since Period 102. ESPRESSO OBs for the 1-UT mode can be executed from any UT. Two observing modes are available:
      • (1) the High Resolution (HR) mode providing a resolving power of 140 000, and
      • (2) the Ultra High Resolution mode (UHR), providing a resolving powerof 190 000.
    • The 4-UT mode is offered since Period 103.
      • It provides a medium resolving power (MR) of 70 000.
      • It is offered in in Visitor Mode only. Observations will be scheduled in groups of consecutive nights. Users must request a total time that is an integer multiple of half-nights (corresponding to 4 hours in Period 104), with a minimum duration for each individual observing slot of one half-night.
      • Proposals requesting ESPRESSO in 4-UT mode must in particular justify its use compared to the 1-UT mode.
      • Monitoring and Large Proposals for ESPRESSO in 4-UT mode will not be accepted.
    • The instrument throughput is expected to increase following an intervention taking place in April 2019. Proposals that aim at comparing radial velocity measurements before and after this intervention should consider that it will likely introduce a radial velocity offset.

Visitor focus

  • The UT1 Nasmyth A focus is available for Visitor Instruments starting mid-November 2019. Potential users are requested to consult the VLTI Visitor Instruments page.
  • The UT3 Cassegrain focus is available for Visitor instruments during the first months of Period 104, while VISIR is installed on UT4.

VLTI instruments and facilities  


  • For new users to VLTI needing assistance to prepare their VLTI proposals, the community supported VLTI Expertise Centres - disseminated throughout Europe - can offer in-depth support. They also offer support for advanced data reduction and interpretation.
  • ESO aims to increase the fraction of service mode for VLTI observations. PIs requesting visitor mode for VLTI should carefully justify the need for this mode.
  • Proposers should be aware that there is a minimum time limit of 1 night per baseline configuration for Visitor Mode runs requiring VLTI-AT observations. Proposers requiring shorter runs per baseline configuration should specify Service Mode observations. These restrictions do not apply to the VLTI-UT baselines.
  • Monitoring and Large Programme proposals on the VLTI-UTs and VLTI-ATs are accepted for GRAVITY, MATISSE, and PIONIER in Period 104.
  • Starting from Period 104, for each observing run, one or more of the following observation types which best describe the proposed observations shall be specified in the instrument configuration section of the proposal:
    • snapshot: standalone concatenations (CAL/SCI, CAL/SCI/CAL or CAL/SCI/CAL/SCI/CAL, depending on what each instrument offers) without further links to other observations in terms of time links or filling the uv plane;
    • time series: time series of concatenations, that are repeated once or more often over the period;
    • imaging: a set of concatenations with different baseline configurations to fill the uv plane for the purpose of image reconstruction, in which case special care is taken at execution to fill uniformly the uv plane;
    • astrometry: GRAVITY dual-feed observations with the purpose of extracting astrometric information.


  • The commissioning of NAOMI, the New Adaptive Optics Module for Interferometry - a low-order adaptive optics system for the ATs - was completed in Period 102.
  • As of Period 104, AT configurations will be requested by generic names ("small", "medium", "large" and "astrometric") rather than explicit configurations. The standard configurations used for a given period are detailed on the VLTI configuration web page and should be used for phase1 and phase2 preparation. For operational reasons, observations may take place (although rarely) on “relocation configurations” which occur the nights during a transition between two standard configurations. A criteria of at least 50% baseline length overlap will be used. This scheme will be primarily used for imaging programmes. The overlap in baseline length between standard and relocation configurations is detailed on the aforementioned web page.
  • In Period 104, ESO will continue a scheme to optimise operations for aperture synthesis with the VLTI. This scheme only applies to service mode proposals using ATs with PIONIER, GRAVITY and MATISSE. The reader is referred to the Period 104 VLTI manual for further details.


  • GRAVITY is offered on all AT configurations as well as on all four UTs with the visible (MACAO) and infrared (CIAO) adaptive optics system (both on-axis and off-axis) in Service and Visitor modes.
    • For dual-fi eld observations, swapping between two targets has been possible since Period 102.
    • The limiting magnitudes for the ATs have been increased by 1 magnitude due to NAOMI.
    • In dual-field mode, there is no longer any constraint on the magnitude difference between the objects used for the science and fringe-tracking channels.
    • ESO invites proposals with the goal of performing astrometric measurements, a capability of GRAVITY which is still under development (see, e.g., GRAVITY Collaboration 2017 A&A 602, A94 or The ESO Messenger 170, 10). Proposers who wish to use the astrometric capability and contribute to its development are invited to consult the GRAVITY webpage and contact the astrometric team.
  • MATISSE, the Multi AperTure mid-Infrared SpectroScopic Experiment is a second generation instrument operating on the VLT interferometer in the L (3-4m), M (4.6-5μm) and N (8-13μm) bands. MATISSE resolves features as small as 3.5mas in the L-band and as small as 8mas in the N-band with the ATs, and 5mas and 12.5mas with the UTs, respectively. Its commissioning started in Period 100 and will continue in Period 103 and 104.
    • Based on early commissioning results, MATISSE is offered with a limited set of its capabilities in Period 104:
      • spectral resolving powers of R=34, 506, and 959 in the L-band and R= 30 and 218 in the N-band; M-band observing and additional resolving powers will be offered in later periods only;
      • the instrument combines either four UTs (with MACAO only) or four ATs in standard VLTI configurations, providing six visibilities and four closure phases in one observation.
    • MATISSE is offered in Service and Visitor modes.
    • Monitoring and Large programmes for MATISSE are offered starting from Period 104.
    • Various technical activities totalling approximately 2 months will take place during Period 103:
      • implementation and commissioning of GRA4MAT to use GRAVITY as a fringe tracker for MATISSE,
      • intervention on the cryo-cooling system, and
      • replacement of the Very High Resolution grating.
  • PIONIER execution times for calibrated visibility measurements have been adapted to target brightness, see the overview or overheads pages.
  • No VLTI Visitor foci are available in Period 104.

Survey telescopes and instruments


  • All first cycle VISTA surveys, except for one, have completed their observations. Seven second generation Public Surveys, essentially spanning the whole RA range are ongoing and are expected to last until the end of Period 104. See the Public Surveys page for details. The fraction of the VISTA observing time devoted to Public Surveys is ~70%, slightly less than in previous periods, due to the good current survey progress and the completion of the first generation public surveys on VISTA.
  • Therefore, approximately 30% of VISTA observing time is available in Period 104 for open time proposals within the full range of atmospheric conditions. Particularly encouraged are open time proposals requesting any weather conditions (THIN and seeing >1.2") which can fill up to 10% of the observing time. No restrictions regarding the RA range are implied, but it should be considered that in Period 104 fewer time is available for RA ~0–3hrs, due to remaining  public survey observations. PIs of Large and Monitoring Programmes should consider that such programmes can run at most over 4 semesters due to the installation of 4MOST.
  • Monitoring and Large Programme proposals will be accepted on VISTA/VIRCAM for the first time in Period 104. Observations will be more likely to take place in Period 105 as more public surveys will have finished their observations by then. Proposals that waive proprietary rights are encouraged. When necessary, priority will be given to complete the ongoing Public Surveys.


  • Normal, Monitoring and Large Programme proposals are accepted on the VST in Period 104 without restriction on atmospheric conditions. PIs of Large and Monitoring Programmes should consider that such programmes can run at most over 4 semesters as the contract between INAF and ESO ends at the end of Period 107. Proposals that waive proprietary rights are encouraged. When necessary, priority will be given to complete the ongoing Public Surveys.