Important recent changes regarding La Silla and Paranal instrumentation and facilities

This section describes important changes which took place during Periods 112, as well as changes expected to take place during Periods 113 and 114.

Distributed Peer Review:

ESO employs Distributed Peer Review (DPR) for approximately half of the submitted proposals. In this paradigm, first introduced by Merrifield & Saari (2009), all PIs of proposals qualifying for DPR accept to review a number of proposals (N) submitted by their peers during the same cycle. Correspondingly, they accept that their proposals are reviewed by N peers who submitted proposals in the same cycle. More details and background information on DPR at ESO can be found in Patat et al. (2019). Users should familiarise themselves with the DPR rules and guidelines.
In Period 111, N = 10, and the criteria describing the proposals which qualify for DPR are as follows:

  1. All proposals requesting a total time (including overheads) less than 16 hours are assigned to DPR. This time threshold is set to have an approximate 50/50 distribution between DPR and panels, and it is based on the time request statistics compiled in recent cycles.
  2. Exceptions to this general rule are proposals including at least one ToO run, and DDT proposals.
  3. All other proposals submitted during a regular cycle will be reviewed in the classical way by the OPC and the panels.

In Period 113, the review channel (DPR vs. panels) is assigned at the time of proposal submission, based on the above rules. The PI (or delegated PI; dPI) is informed about the assigned review process and prompted to formally accept the conditions at the time of submission. At this stage the PI/dPI can delegate the reviewer’s role to one of the co-Is listed in the proposal. The delegation can also occur when the list of co-Is is specified.

By submitting a proposal qualifying for DPR, the PI/dPI commits to follow the DPR rules (see DPR rules and guidelines).

Scientific keywords:

While preparing their proposals in p1, users must select at least two keywords, and at most five keywords (ten keywords for Large Programmes), except that proposals for Calibration Programmes do not require any keywords. The keywords must be selected in decreasing order of relevance (i.e., the first selected keyword is the most relevant).

Proposal anonymisation:

The proposal anonymisation is mandatory. Failure to abide by the Dual-Anonymous Peer Review (DAPR) rules may lead to the disqualification of the proposal. Applicants must 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.

The fields Background and Expertise and Data Product Delivery Plan (in the case of Large Programmes) are the only fields of the proposal in which information on the proposing team can be disclosed. These fields will not be included in the material distributed to the referees during the proposal review phase and will only be accessible to them after the ranking phase is completed.

Large Programs:

Large Programmes, those that require 100 hours or more, are accepted for Period 114. 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 114. Due to significant demand and existing allocations (approved LPs and GTO), the available observing time on UT4 is highly constrained, particularly within the RA range of 9-14 hours. As there is no time available to schedule any new LP proposal submission targeting this RA range on UT4, such proposals will be formally rejected. Similarly, for UT3, the allocation for the top 20% of turbulence plus CLR sky conditions around RA~17h is overcommitted due to an ongoing approved LP. Proposal submissions should consider the limited availability of observing time under these specific conditions.

Any-weather programmes:

In the interest of maximizing scheduling flexibility and accommodating the often over- and under-subscribed observing conditions, applicants for observing time are strongly urged
to consider essential information provided on our designated webpages: Forecast of telescope pressure, and Any-weather programme statistics. This information can guide your selections and enhance the likelihood of fulfilling your observing objectives. Please take particular note of the current high demand on UT4, especially in the 9<RA<14h range, where securing observing time will be notably challenging.

La Silla

Instruments and Facilities

  • NIRPS: A Laser Frequency Comb for wavelength calibrations is expected to improve further the stability from Period 113 onwards.
  • HARPS can be operated simultaneously with NIRPS starting from Period 111.
  • HARPS: The bluer LFC, covering the range 420nm-689nm is available for Period 114
  • The Visitor (Cassegrain) focus of the 3.6-m telescope is not offered due to the operation of NIRPS. The possibility of offering this visitor focus will be re-evaluated in the future.
  • The Visitor focus of the NTT continues to be available.
  • The installation and commissioning of SoXS - the Son of X-Shooter - will continue in Period 114. As a consequence, SoFI has been decommissioned, and EFOSC2 is expected to be decommissioned during Period 114.
  • The multi-object spectroscopic (MOS) mode of EFOSC2 has been decommissioned in Period 111.
    • ULTRACAM is a high-speed imaging photometer designed to study faint astronomical objects at high temporal resolutions. ULTRACAM employs two dichroic beamsplitters and three frame-transfer CCD cameras to provide optical imaging with a field-of-view of 60 and at frame rates of up to 300 Hz simultaneously in the u'g'r', u'g'i' or u'g'z' bands.
    • This PI instrument is o ffered to the ESO community for up to 5% of the observing time at the NTT in Period 109. Large Programmes will not be accepted.
    • Operation of this PI instrument requires the presence of the instrument team, so ULTRACAM programmes will preferentially be scheduled contiguously on periods of several nights. For questions on the instrument and observation strategies, users shall contact the instrument PI, Prof. Vik Dhillon (vik.dhillon[AT], at least two weeks prior to submitting their proposal.
    • The ULTRACAM consortium is committed to support the PIs and observers from the ESO community that have been awarded telescope time with ULTRACAM. The ULTRACAM team will support the execution of the observations and the subsequent data reduction to allow the scienti c exploitation of the data obtained with ULTRACAM.
    • Proposers must check that their planned observations do not duplicate any protected targets speci fied for ULTRACAM in the Period 113 GTO target protection webpages.




Rapid Response Mode (RRM) activation policy:

The RRM policy changed starting 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 programme is specifically protected against an RRM trigger (in case of strictly time-critical programmes). The change has been implemented at all UTs. More information on the Rapid Response Mode can be found on the Phase 2 page.

UT instruments and facilities

UT1 - Antu

  • KMOS:
    • Since P112, the observatory only provides a telluric in twilight observed at an average airmass. Users must take this into account and provide their own tellurics concatenated to the OBs if a maximum difference of airmass or time is required by their science goals.
  • FORS:
    • Due to decommissioning of the FORS2 Absolute Photometry (FAP) project, the observatory no longer provides photometric standard observations. This will now be performed with the available stars in the field through GAIA photometry, which the users can check in ObsPrep.  In those 10 to 15% of cases where there no suitable reference stars available (either none or saturated), the users are expected to provide the OBs for their photometric standard observations, if they wish to photometrically calibrate their imaging data.

UT2 - Kueyen

  • UVES:
    • With the purpose of producing up-to-date response curves in all offered standard setting, starting from the beginning of P111 flux standards will be taken with bi-weekly cadence in the modes DIC1 346+580, 390+564 and DIC2 437+860, and with monthly cadence in the setting DIC2 390+760 and RED 520, 600.

UT3 - Melipal

    • For the star center observations (obs. Type=C), the WAFFLE pattern amplitude in all modes is calculated automatically and should be left as “AUTO” by default.

UT4 - Yepun

  • MUSE:
    • The extension of the GALACSI NFM tip-tilt limiting J-band magnitude has been commissioned. Details are available on the news page of MUSE.
  • ERIS:
    • All originally planned ERIS modes are now fully commissioned.

Incoherent combined focus

    • A new public version (3.1.0) of the ESPRESSO pipeline is available.
    • The ESPRESSO LFC has been in operations since October 2022. Proposal requesting it for wavelength calibration are accepted, but the users must inform USD and clearly indicate the need to the LFC for their science.  The instrument status is regularly updated in the ESPRESSO news webpage.

Visitor focus

  • The UT1 Nasmyth A focus is not available for Visitor Instruments, due to the installation of the field corrector lens for MOONS.
  • 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.
  • Monitoring and Large Programme proposals on the VLTI-ATs are accepted for both GRAVITY and for MATISSE without exceptions. Such programmes on the VLTI-UTs will be available again after the completion of the GPAO implementation of the Laser Guide Star.


  •  After a first commissioning period of the Gravity+ AO (GPAO) in October 2024,  VLTI-UT science operations will resume in November 2024 in Service Mode, and from January 2025 in Visitor Mode. In this first phase of the GPAO implementation, only the Natural Guide Star modes will be offered: the visible mode (NGS_VIS, replacing the former MACAOs) and the infrared mode (NGS_IR, formerly called CIAOs). Note the limiting magnitude for NGS_VIS of Grp=12.5mag.  See the VLTI manual for details.


  • 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 114 VLTI manual for further details.
  • In P112, we introduced the extended configuration A0-B5-J2-J6, offering the longest baselines on the VLTI platform. It is offered for single-field operations. Due to the redundancy with the astrometric configuation, the previous large configuration A0-G1-J2-J3 is no longer offered.


  • One VLTI visitor focus (VISITOR 1) is available. 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.
    • ESO invites proposals with the goal of performing astrometric measurements (see, e.g., the following articles: Gravity collaboration, 2017 A&A 602, A94 and The Messenger 170, 10). Proposers who need help with the preparation of GRAVITY astrometric observations are invited to contact the ESO Helpdesk well in advance of the deadline.
    • Starting with P110 GRAVITY a new observing mode dual-field wide is offered for the UTs and the small & large configurations of the ATs. This mode allows fringe tracking from a target that is up to 30 arcsec away from the science target by using the VLTI Star Separators to separate the beams. The magnitude limits of the fringe tracker and acquisition camera and the expected sensitivity for the science camera in this mode can be found on the GRAVITY webpage and are based on a preliminary characterisation of the mode. Due to the need of good atmospheric conditions (turbulence class 30% and better), observations are strongly encouraged to be carried out in service mode to ensure that the conditions are met. Note that 'dual-field wide' mode only provides relative measures (differential visibilities and phases), no absolute visibilities, due to atmospheric effects when separation increases. Calibrations of the interferometric transfer function are thus not needed and not offered for this mode.
    • Starting with period P112 GRAVITY single-field and dual-field acquisition templates will each be split into two separate templates: on-axis and off-axis. More information on their use cases can be found in the GRAVITY User Manual.
    • For the GRAVITY dual-field wide mode at the limit of system performance, in particular for crowded fields or embedded objects, it is recommended to confirm positions and magnitudes of AO-guide and fringe tracking stars in images with sufficient angular resolution, not relying purely on catalogs. Such images might also be provided as additional finding charts. We are in the process of further characterizing the performance of the GRAVITY-wide mode as a function of separation of the FT star and the atmospheric conditions. Note that the offered maximum separation of 30 arcsec distance may be optimistic unless the weather is excellent and both the FT and SC stars are bright. With more typical conditions, the maximum separation may be somewhat smaller. A performance estimation tool for this mode is currently under development by the JMMC.
  • MATISSE is offered on the UTs (supported by the visible MACAO AO system) and on the short, medium and large AT configurations in Service and Visitor modes.
    • MATISSE is available with the GRA4MAT mode with ATs fully, and only without chopping for UTs.
    • 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. The HIGH+ mode is only available with the GRA4MAT mode.
    • Monitoring programmes are offered in all settings in Period 111, except for the new HIGH+ spectral setting. Additionally, Large or Monitoring Programmes requesting the GRA4MAT mode will not be accepted.
    • For targets for which photometry may be too noisy, direct measurement of the correlated fluxes is possible for objects with lower fluxes that for visibility measurements. See the MATISSE news page for details. Absolute calibration of correlated fluxes requires special calibrators, as not only their sizes need to be known, but also their SED. Please contact the VLTI Expertise Centre at Leiden Observatory for help with selecting appropriate calibrators and data reduction.
    • As from P112, MATISSE is offered for narrow off-axis mode for exo-planet and similar observations (with GRAVITY as fringe tracker), meaning while GRAVITY tracks fringes on a source, MATISSE can point to a nearby offset position. See the MATISSE news page for details.
  • PIONIER PIONIER is offered on all ATs configuration in both service and visitor mode. See the overview or overheads pages.
    • The cooling pump of the detector is currently not performing, and the temperature sensor of the detector is not functional. This situation may affects the performance of the instrument. While a spare for the pump has been identified, the temperature sensor problem remains. A direct consequence of the instrument status is that no Large nor Monitoring Programs are offered for Period 114.