Paranal: Important recent changes regarding instrumentation and facilities

This section describes important changes which took place during Periods 104 and 105, as well as changes expected to take place during Period 106.


Proposal anonymisation:

In sight of the future deployment of Dual-Anonymous Peer Review (DAPR), ESO strongly encourages applicants to formulate the scientific rationales of their proposals following the anonymisation rules and examples described in this link, which also gives a detailed description of the DAPR paradigm. Period 106 will be used as a dry run, both to make the community aware of the upcoming implementation of DAPR and to test its practical, procedural and policy aspects. In Period 106, proposal anonymisation will not be mandatory. Nevertheless, applicants should take advantage of the dry run to practice with the new writing style required by the dual-anonymous paradigm, following the guidelines provided in the link above.

p1: A new tool for proposal preparation and submission:

A new tool for the preparation and submission of observing proposals was deployed in Period 105. It represents the first part of a broader re-haul of the ESO Phase 1 system (p1) that also entails a significant modernisation of the Observing Programmes Committee, refereeing process, and related tools. The new p1 system is web-based, resembles the recent p2 tool. Users are encouraged to get familiarised with the new system using the p1demo tool well before the proposal deadline.

Large Programs:

Large Programmes, those that require 100 hours or more, are accepted for Period 106. Since Period 104, Large Programmes can only be submitted in even Periods, i.e., Periods with the proposal submission deadline in March/April. A number of instrument restrictions for Large or Monitoring Programs apply. We refer the reader to Sect. 4.4 in the Call for Proposals Period 106.

Introduction of turbulence categories:

With the advent of instruments using new adaptive optics (AO) modes, new turbulence parameters need to be taken into account in order to properly schedule observations and ensure that their science goals are achieved. These parameters include the coherence time and the fraction of turbulence taking place in the atmospheric ground layer, in addition to the seeing. Such a change was initiated for the SPHERE instrument in Period 103.

In Period 105, the new turbulence constraints were standardised to the turbulence conditions required by all instruments and modes, whether they are seeing-limited or AO-assisted. Users are encouraged to read the general description of these changes for Phase 1 and Phase 2 on the Observing Conditions webpage, as well as instrument User Manuals for specifics per instrument. The Exposure Time Calculators have been updated to reflect these changes.

In addition, interested readers should refer to the article by Martinez et al. 2010 (The ESO Messenger 141, 5) "On the Difference between Seeing and Image Quality" which describes the meaning of these two quantities.

Proposals for exceptional weather conditions:

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 field of the p1 proposal preparation tool. Users requiring excellent seeing conditions should apply for the 10% best turbulence conditions following the new handling of atmospheric constraints. The proposals should describe why the science can only be achieved in such conditions.

Rapid Response Mode (RRM) activation policy:

The RRM policy changed in Period 105: on-going observations on any VLT instrument can be interrupted by an RRM triggered on that specific UT, even if the trigger requires a change of focus, unless the relevant program is specifically protected against an RRM trigger (in case of strictly time-critical programmes). The change will be initially deployed for UT2 and UT3 at the start of Period 105, and likely deployed for all UTs during Period 105. It will be fully deployed to all UTs at the latest at the start of Period 106. More information on the Rapid Response Mode can be found on the Phase 2 page.

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

  • KMOS:
    • Since the beginning of May 2019, Arm #14 shows an hard point in the linear driver. It is thus parked and in simulation. KMOS is available for Science Operations with 23 arms. No intervention is currently planned in Period 106.

UT2 - Kueyen

  • VISIR:
    • The instrument will have been moved from UT4 to UT2 at the end of Period 104 and ready at the start of Period 105. On UT2, VISIR is offered with all functionalities.

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,  turbulence category for SPHERE proposals must be better than 85%, or smaller for targets which cannot be observed at low airmass.
    • CRIRES was installed on the UT3-Melipal Nasmyth B focus in Period 104. Its commissioning will take place in Period 104 and Period 105.
    • The calibration plan does not include observations of telluric standard stars. Please refer to the User Manual regarding the correction of telluric lines.
    • In order to to balance the allocated time between telescopes, XSHOOTER will have been moved to UT3 at the end of Period 104.

UT4 - Yepun

  • HAWK-I:
    • Monitoring and Large Programmes as well as Target of Opportunity proposals are accepted for both HAWK-I in NoAo and in AO mode with GRAAL in Period 106.
  • MUSE:
    • In Period 106, the AO supported MUSE Narrow Field Mode (NFM-AO) is offered for Monitoring and Large Programmes for the first time.
    • Target of Opportunity proposals are accepted for MUSE in all its modes.

Incoherent combined focus

    • The 4-UT mode is offered 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 106), with a minimum duration for each individual observing slot of one half-night.
    • Proposals requesting 4-UT mode must in particular justify its use compared to the 1-UT mode.
    • Monitoring or Large Proposals are accepted for 1-UT mode only.  These observing modes for 4-UT mode are still under consideration and are not accepted.

Visitor focus

  • The UT1 Nasmyth A focus is available for Visitor Instruments since mid-November 2019.
  • Potential users of a visitor focus are requested to consult the VLT Visitor Instruments page.


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 both GRAVITY and PIONIER without restrictions, while some restrictions are in effect for MATISSE in Period 106.
  • Since Period 104, for each observing run, one or more of the following observation types (snapshot, time series, imaging and astrometry) which best describe the proposed observations shall be specified in the instrument configuration section of the proposal. See the VLTI user manual for more details.


  • Since Period 104, AT configurations will be requested by generic names ("small", "medium", "large" and "astrometric") rather than explicit AT station 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 during the nights of 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 106, 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 106 VLTI manual for further details.


  • Two VLTI visitor foci are available in Period 106. Potential users are requested to consult the VLTI Visitor Instrument page.
  • GRAVITY is offered on all AT configurations as well as on all four UTs with the visible (MACAO) and infrared (CIAO, off-axis only) adaptive optics system in Service and Visitor modes.
    • The on-axis mode of the IR wavefront sensor CIAO cannot be offered for the time being due to critical problems that were identified during its commissioning.
    • The limiting magnitudes in effect for use with the ATs have been increased by 1 magnitude owing to the performance of NAOMI, since P104.
    • 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). 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 at least two weeks before the proposal deadline.
  • MATISSE is offered in Period 106 on the UTs (supported by the visible MACAO AO system) and on the short, medium and large AT configurations in Service and Visitor modes.
    • For the first time, MATISSE is offered with restricted use of GRAVITY as an external Fringe Tracker, the so-called GRA4MAT mode. The mode can be used on the ATs only and provided that chopping is not required. The characterization of the GRA4MAT mode will continue in P106.
    • The instrument offers the choice of various spectral resolving powers covering either L and/or M-band (depending on the resolving power) and N-band. In P106, for the first time, the HIGH+ spectral setting (R = 3666) can be used for L-band or M-band observations in conjunction with the GRA4MAT mode (see previous item).
    • Monitoring and Large programmes are offered in all settings in Period 106, except for the new HIGH+ spectral setting. Additionally, Large or Monitoring Programmes requesting the GRA4MAT mode will not be accepted.
  • PIONIER is offered on all ATs configuration and UTs + MACAO in both service and visitor mode. The limiting magnitudes of PIONIER have been updated following the improvements thanks to the installation of NAOMI. The execution times have been adapted accordingly. See the overview or overheads pages.

Survey telescopes and instruments


  • The seven second generation VISTA Public Surveys are in their last stages and most are expected to finish towards the end of Period 105, the remaining running into P106. In addition there is one ongoing Large Programme targeting the COSMOS field (RA~10h). This means that large fraction of time will be available on VISTA during Period 106 and Period 107, and ESO encourages the submission of Normal, Monitoring and Large proposals for VISTA.
  • All observing time is available in Period 106 within the full range of atmospheric conditions. Particularly encouraged are open time proposals requesting any weather conditions (THIN and turbulence category 85% or 100%). No restrictions regarding the RA range are implied.
  • Large and Monitoring Programmes are accepted in Period 106.  PIs should consider that such programmes can run at most over 2 semesters due to the planned installation of 4MOST on VISTA. Proposals that waive proprietary rights are encouraged.


  • Normal, Large and Monitoring proposals are accepted on the VST in Period 106 without restriction on the conditions. PIs should consider that such programmes can run at most over 2 semesters as the contract between INAF and ESO ends at the end of Period 107. Proposals that waive proprietary rights are encouraged.