Important recent changes (since Period 97) regarding Paranal instrumentation and facilities

This section describes important changes which took place during Period 97 and 98, as well as changes expected to take place during Period 99.

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:

Astronomical Site Monitor

Upgraded software for the Astronomical Site Monitor (ASM) as well as a new Differential Image Motion Monitor (DIMM) entered operation early in Period 97. Additionally, the system includes atmospheric turbulence profile data from the Multi Aperture Scintillation Sensor (MASS),  which delivers more accurate values of coherence time and isoplanatic angle, and the SLOpe Detection and Ranging (SLODAR), which estimates Ground Layer Adaptive Optics (GLAO) potential performance as required for AOF operation.  
As it is located on a 7.5m high platform away from any disturbing infrastructure the new MASS-DIMM is less sensitive to turbulence taking place at the ground layer; therefore the new seeing estimate is expected to be significantly more reliable and, once taking the airmass into account, closer to the one actually probed by the UTs.

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 take place during Period 97 to 98. The spectrograph installation and first commissioning, with one UT, are expected to start during the second half of Period 98 and continue during Period 99.


UT1 - Antu

  • Potential users of FORS2 for transmission spectroscopy are recommended to read Boffin et al. "Regaining the FORS: making optical ground-based transmission spectroscopy of exoplanets with VLT+FORS2 possible again"  Paper 9908-85 presented at SPIE 2016.
  • During Period 99 NACO 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 previous periods, 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

    • The instrument will be unavailable for approximately 6 weeks early in Period 99 to allow for the installation of re-designed Atmospheric Dispersion Correction (ADC) systems.  Updates on the status of the ADCs can be found on the XSHOOTER news webpage.
    • Since Period 98, Large and Monitoring Programmes with XSHOOTER have been accepted again, taking this restriction into account.
    • A new Giraffe readout mode has been available in Visitor Mode only since Period 97. The mode is a high-gain, low noise, slow readout mode that results in higher S/N for faint targets (at the limit of the instrument sensitivity). Details can be found in the User and Template manuals, as well as in the News page.

UT3 - Melipal

    • Since Period 97, the broadband H filter for IRDIS has been offered in IRDIFS mode, while the broadband Ks filter is offered in the IRDIFS_EXT mode.
    • 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.
    • Starting from Period 98, four quadrant phase masks are not offered any more.
    • As described in the CPI section of the SPHERE Overview, 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.
    • The instrument was re-commissioned in January 2016 following a major upgrade.
    • 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 is offered starting with 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 their Phase 1 and Phase 2 preparation.
    • ESO is investigating the use of VISIR for a specific experiment which would require its installation at UT4 Nasmysth A (therefore replacing HAWK-I) during 2018 or 2019. Therefore, VISIR is not available for Monitoring or Large Programmes in Period 99.
    • Service Mode programmes that require 1.2" seeing or worse are actively encouraged.
  • VIMOS:
    • Period 99 is the last period for which VIMOS is offered, and only for Normal programmes.
    • 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). Please note that even though the best observing conditions in the RA ranges above are substantially committed to the ongoing Public Spectroscopic Surveys, some time may be allocated to a very strong, scientifically compelling Normal or DDT programmes. Furthermore, Service Mode programmes that require 1.2" seeing or worse are actively encouraged.


UT4 - Yepun

  • Approximately 67 nights have been committed to activities on the UT4, including commissioning activities listed below and 14 nights of GTO.
  • 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 will take place during the first half of Period 98
    • The installation and a first commissioning run of GALACSI, the adaptive optics module for MUSE,  will take place during the second part of Period 98. Further commissioning runs of GALACSI alone and commissioning of MUSE in the Wide Field Mode with AO will take place during Period 99, with the objective to offer it for Period 100.
  • HAWK-I :
    • Since Period 97, the Rapid Response Mode is also available for the Fast Photometry mode.
    • Since Period 98, Fast Photometry using subwindows can asks for Skyflats and/or Twilight flats with the same subwindows as their science.
    • 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.
    • ESO is investigating the use of VISIR for a specific experiment which would require its installation at UT4 Nasmysth A (therefore replacing HAWK-I) during 2018 or 2019. Therefore, HAWK-I is not available for Monitoring or Large Programmes in Period 99.     
    • An upgrade took place in January 2016. It included the installation of a new pre-optics collimator, new J, H and  K filters, a new baffle at the entrance focal  plane,  and new collimator mirrors. In  addition, the membrane mirror of the MACAO system has been replaced.  While at  time of writing the performance of SINFONI is being assessed, at least the following improvements have been observed:
      • a significant improvement in the overall J-band throughput especially towards shorter wavelengths;
      • increased spectral resolution in all bands ranging from about 4 to 15%;
      • a spot of dead pixels which previously affected the centre of the field of view are now shifted mostly out of the way;
      • the vignetting of slit 32 at the top of the field of view is now much reduced.
    • Further information will be made available through the News page.
    • 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 1n 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-UTs and ATs: Since early Period 97, all UTs Coudé and all ATs are exclusively equipped with Star Separator Systems (STS) in view of operations with GRAVITY.
  • VLTI-ATs:
    • Since Period 96 the AT configurations have slightly changed, but the overall characteristics remain. Three different AT quadruplets are offered since Period 98 (check the VLTI configuration page for more information):
      • small (baselines from 10 to 40m): A0-B2-C1-D0;
      • medium (baselines from 40 to 100m): D0-G2-J3-K0;
      • large (baselines from 60 to 140m): A0-G1-J2-J3.
    • Starting January 2017 and for an estimated 4 months, a refurbishment of each of the 4 ATs will take place in order to increase the throughput of their optical Coudé trains by as much as 1 to 2 magnitudes as well as to improve significantly their operational reliability.
  • AMBER is not offered for Large or Monitoring programmes, as it will be decommissioned in the near future.
  • 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 99, ESO offers GRAVITY on all AT configurations as well as on all four UTs with the visible adaptice optics system (MACAO), in service mode only. The spectro-imaging mode will be available  in single field mode with medium and high spectral resolution and in dual field mode in low, medium and high spectral resolution. In particular, the astrometric mode is not offered in Period 99. 

In addition to the instrument itself, the 4 units of the GRAVITY Coudé Infrared Adaptive Optics (CIAO) system for the VLT Interferometer are being installed and commissioned on each of the 4 UTs during Periods 97 and 98. Commissioning of the astrometric mode on both ATs and UTs with CIAO will continue in Period 99.

  • No VLTI Visitor focus is available in Period 99.

Survey Telescopes and Instruments


  • Two Public Surveys have completed their observations. The remaining four surveys will continue through Period 99, and beyond. Seven new Public Surveys are have been selected and essentially span the whole RA range; their observations will start in Period 99 and 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 99. 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.