Important recent changes (since Period 98) regarding Paranal instrumentation and facilities

This section describes important changes which took place during Period 98 and 99, as well as changes expected to take place during Period 100.


Excellent condition proposals:

Seeing at Paranal is better than 0.5" approximately 5% of the time, while episodes of precipitable water vapor smaller than 0.5mm are encountered 6 to 7 nights per year (see Kerber et al. 2014, MNRAS 439, 247) mostly during the southern winter. ESO encourages the unique science that can be carried out under one or both of these conditions, which should be stressed 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.

P2PP in Visitor Mode:

ESO is modifying the Phase 2 preparation tool (p2pp) and visitor Observing Tool (vOT) used by visiting astronomers at Paranal in order to allow for more agile preparation and execution of Visitor Mode observations. A successful trial at UT2 Kueyen took place during Period 98 and commissioning will be extended in Period 99 to include Visitor Mode observations on the other UT telescopes. In the future, further modi cations will encompass Service Mode observations as well as the possibility to create and modify OBs and execution sequences using scripts rather than via a user interface. Further details will be provided through the Paranal Science Operations webpage

Seeing and Image Quality:

  • The seeing information to provide in Box 3 of the proposal form is the seeing in the V band at zenith. This ensures that the scheduling tool uniformly takes the seeing into account. Service mode users for approved programmes will enter the Image Quality for the airmass and wavelength of interest required for their observations as a constraint during Phase 2. The article by Martinez et al. "On the Dffi erence between Seeing and Image Quality" describes the meaning of these two quantities and further information can be found on the Observing Conditions webpage.

The Exposure Time Calculators have been modi fed accordingly and clearly distinguish between the two quantities, using a slightly modi ed version of the formula given in the above article. The ETCs also report on the chances of achieving the input seeing conditions and the resulting image quality.

Exposure Time Calculators:

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 stars 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 star along the entrance slit of a spectrograph allows to increase the total signal collected in a single spectrum by a factor of up to 100 relative to a perfectly shaped image for bright sources in 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).
  • The installation and commissioning of the Coudé train for ESPRESSO started in Period 97 and are expected to be completed in Period 99. The spectrograph installation and first commissioning, with one UT, are expected to start during Period 100 and will continue in Period 101.


UT1 - Antu

  • NACO:
    • During Period 100 is offered for the following modes
      • NAOS:
        • VIS & IR wavefront sensing;
        • Pupil tracking;
      • CONICA:
        • Imaging without chopping, in NoAO;
        • Cube mode;
        • Focal plane AGPM coronography, with the  Lp, NB_3.74 or NB_4.05 filters;
        • Wollaston/HWP polarimetry;
        • SAM/SAMPOL.
      • In contrast to Period anterior to 99, observations should preferably be executed in Service Mode, except for SAM/SAMPOL. PIs requesting Visitor Mode for the other modes should carefully justify their request.
    • Large and Monitoring Programmes with NACO will not be accepted as the availability of several modes cannot be guaranteed in the coming periods.

UT2 - Kueyen

    • Atmospheric Dispersion Correction (ADC) will be available again after the installation of re-designed systems in May 2017.  Updates on the status of the ADCs can be found on the XSHOOTER news webpage.
    • Since Period 98, Large and Monitoring Programmes with XSHOOTER are accepted again.
    • [The following sentence was reworded on March 18th for improved clarity.] Starting from Period 100,
      • the Observatory will supply telluric standard stars only for cases where the requested S/N of the science spectrum in a single OB is 10 or below;
      • for all other cases if the user requires telluric standard star observations they must be accounted for in their time request at Phase 1.
    • Users are encouraged to use the molecfit tool, which allows users to fit and correct 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 98, all IRDIS_LSS observations are allowed in Service Mode, allowing the spectroscopic follow-up of sources of interest with guaranteed atmospheric conditions.
    • Since Period 98, four quadrant phase masks are not offered any more.
    • Starting from Period 100, 
      • Rapid Response Mode is offered for the ZIMPOL imaging and polarimetry modes, IRDIS classical imaging mode and IRDIFS imaging mode.
      • Sparse Aperture Mask (SAM), a 7-hole mask design inserted in the pupil plane (Lyot Wheel), is offered. Sparse aperture masking works only in Pupil Tracking mode in order to stabilise the Fizeau interferometrer created by the 7 apertures. SAM can be used with the modes IRDIS_CI, IRDIS_DBI, IRDIS_DPI, IRDIFS, IRDIFS_EXT and ZIMPOL_I.
      • IRDIS_DPI is now available in Pupil Tracking mode.
      • Targets with R>11 are allowed in Service Mode. Since they require the best possible conditions, users should request them with a seeing contrainst <= 1.0". For R> 11, H>9 targets, coronagraphs should be avoided as SPHERE might be operated without precise drift control on the focal-plane mask (DTTS-free mode).
    • Since Period 98, the offered modes  include normal imaging, burst-mode imaging, low-resolution specctroscopy, and long slit and cross-dispersed high-resolution spectroscopy in both Service and Visitor Modes. Sparse Aperture Masking and coronagraphy using the Annular Grooves Phase Mask (AGPM) or 4-Quadrant Phase Mask (4QPM) are offered in Visitor Mode only. Details on the offered modes and configurations are provided in the User Manual.
    • An M-BAND filter has been offered starting since Period 98.
    • Starting from Period 99, acquisition is now allowed in the K-band in imaging mode.
    • The PWV must be provided as an observing constraint during Phase 1 and Phase 2 preparation.
    • ESO is consolidating the technical  implications of a specific experiment which require the installation of VISIR, slightly modified, at UT4 Cassegrain (therefore replacing SINFONI) during the first semester 2019: thanks to the improved image quality provided by the deformable secondary mirror, VISIR will then have the performance required to search for potentially habitable planets around Alpha Cen. VISIR will not be made available for open time when attached to UT4.
    • Service Mode programmes that require 1.2" seeing or worse are actively encouraged.
  • VIMOS:
    • Public Spectroscopic Surveys started in Period 94 and substantially use all the dark and grey time in good conditions (PHO/CLR, seeing < 1.0") in RA ranges 00-05:30h and dark time in RA ranges 08-12h (also see the Public Survey webpages).
    • Service Mode filler programmes that require 1.2" seeing or worse and/or bad transparency are actively encouraged, but will not be carried over as the instrument will be decommissioned in late Period 100 or early Period 101.

UT4 - Yepun

  • Adaptive Optics Facility
    • The installation of the Laser Guide Stars Facility was completed and commissioned during Periods 97 and 98.
    • The installation of the the Deformable Secondary Mirror and recommissioning of the telescope took place during the first half of Period 98.
    • The installation of GALACSI, the adaptive optics module for MUSE, is taking place during March 2017, and will be immediately with a first commissioning run. takes place during the second part of Period 98 and continue during Period 99.
  • MUSE
    • The installation and a first commissioning run of of MUSE+GALACSI, in Wide Field Mode with AO takes place during the second part of Period 98. Further commissioning of GALACSI alone and commissioning of  MUSE in the Wide Field mode with AO will take place during Period 99. This combination will be offered in Period 101, with the possibility that some programmes be executed late in Period 100.
  • HAWK-I :
    • Starting with Period 100, users can request the same subwindows for skyflats and/or twilight flats as for the science data.
    • Since Period 98, the standard star observing procedure has changed: it will make use of 2MASS Touchstone fields including around 100 standard stars.
    • Since Period 98, a new version of the pipeline is available, developped by CASU. This new pipeline, similarly to the old version, does NOT process the FastPhotometry windowed mode data.  
    • Since Period 99, observations of moving target can take advantage of the Adaptive Optics (AO) capabilities of SINFONI. Adaptive optics offers an improvement in image quality and/or in S/N of the final spectra compared to noAO. Bright asteroids and comets (R < 14 mag) can be used as the AO reference "stars" in NGS mode. For fainter moving objects (14 < R < 17 mag), the science target itself can serve as a Tip-Tilt "star" in LGS mode. For science targets that have differential tracking rates higher than 0.04"/s in either RA or DEC, visitor mode is mandatory if AO-aided observation is requested. The R magnitude for comets is referred to as nuclear magnitude ("N-mag"). The user should check the news section of the SINFONI web page for any updates.

Visitor focus

  • No focus for Visitor Instrument is available on the VLT since Period 95.

VLTI instruments and facilities

  • 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.
  • VLTI-ATs:
    • The refurbishment of each of the 4 ATs continues during Period 99 in order to increase the throughput of their optical Coudé trains as well as to improve their operational reliability signi cantly. New limiting magnitudes will be assessed at the end of the intervention.
    • No operations will take place with the VLTI for approximately 1 month during the second part of Period 100, in order to carry out the following:
      • installation of the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) in the VLTI laboratory,
      • installation, veri cation and commissioning of the first of the New Adaptive Optics Module for Interferometry (NAOMI), the low order adaptive optics system for the ATs.
  • ESO encourages users of  AMBER to utilize Periods 100 and 101 to complete any on-going programme considering that it will likely be decommissioned in Period 102 to make space for a visitor focus in the VLTI laboratory. AMBER is not offered for Large or Monitoring programmes.
  • GRAVITY is a four-telescope beam combination instrument for the VLTI, operating in the K band, with three different spectral resolutions. It provides a spectro-imaging mode as well as an astrometric mode. GRAVITY uses its own fringe tracker to stabilize fringes on- or off-axis.
    • In Period 100,
      • ESO offers GRAVITY on all AT configurations as well as on all four UTs with the visible adaptive optics system (MACAO), in service mode only.
      • The spectro-imaging mode is available, in single and dual field mode, with low-, medium- and high-spectral resolution.
      • The astrometric mode is not offered.
      • GRAVITY is now offered for LP and MP for all offered modes with ATs and UTs+MACAOs.
    • In addition to the instrument itself, the four units of the GRAVITY Coudé Infrared Adaptive Optics (CIAO) system for the VLT Interferometer have been installed and commissioned for Galactic Center observations on each of the four UTs, and only available for GTO observations. Commissioning of the CIAOs for general purpose observations is still continuing. It is expected that the off-axis mode of CIAO will be offered in Period 101,  and the CIAO on-axis mode in Period 102.
    • Limiting magnitudes both at the bright and faint ends have been updated following performance assessment during commissioning: see the GRAVITY instrument description web page.
  • No VLTI Visitor focus is available in Period 100.

Survey Telescopes and Instruments


  • It is foreseen that 3 VISTA public surveys from the first cycle will have completed their observations at the start of Period 100. The remaining surveys will continue through Period 100, and beyond. Seven new Public Surveys have been selected and essentially span the whole RA range; their observations are starting in Period 99 and they are expected to last until 2019.  See the Public Survey page for details. The fraction of the VISTA observing time devoted to Public Surveys is similar to the one of previous periods. A small fraction will be available for open time projects.


  • Normal and Large programme proposals are accepted on the VST in Period 100. However, in order to ensure the timely progress of the ongoing Public Surveys both Normal and Large programmes are restricted to filler programmes that request poor weather conditions (i.e., no moon constraints, and/or seeing>1.2", and/or thin/thick clouds). Target of Opportunity proposals requesting a short amount of time with OmegaCAM will also be considered.
  • Please check that you use the most recent version of the ETC when preparing your proposal as throughputs have recently been updated.