Important recent changes (since Period 101 and 102) regarding Paranal instrumentation and facilities

This section describes important changes which took place during Periods 101 and 102, as well as changes expected to take place during Period 103.


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 leads to a change in the procedure for triggering Target of Opportunity observations starting in Period 103. Successful proposers of ToO runs will 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:
    • Period 103 will be the last full period of NACO operations. Large and Monitoring Programmes with NACO will therefore not be accepted.
    • 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. 
    • The VIS WFS has been decomissioned and is not offered in P103. Tests are ongoing to see if the IR-WFS + VIS dichroic combination will be offered in VM and dVM.
    • CONICA still suffers from several detector defects which compromise all but the upper left quadrant.  Check the NACO news page for the current status.
    • During Period 103 NACO is offered for the following modes
      • NAOS:
        • 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.
  • FORS2:
    • Since Period 101, two additional narrow-band filters have been offered in Visitor Mode. These are 'FILT_753_8+89' (central wavelength of 7528 Å, FWHM 60 Å), and 'FILT_621_5+87' (central wavelength of 6195 Å, FWHM 60 Å). A full characterisation of these filters is ongoing, and potential users are encouraged to contact in advance of proposal submission.
    • 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:
    • After an intervention in May 2018  KMOS is fully operational, Arm 1 and 3 have been replaced and arm 17 fixed. KMOS is now available for Science Operations with all 24 arms. Updates on arm functionality can be found on the KMOS news webpage.

UT2 - Kueyen

    • Since Period 101, the calibration plan does not include observations of telluric standard stars. Instead, users are encouraged to use the Molecfit tool, which can provide accurate telluric line corrections in most cases. Since Period 102, the observatory obtains one telluric standard star with the slit widths used for each science observation during the night, which can be used to improve the telluric correction. No airmass and no time window constraints will be imposed on these telluric standard star observations. The X-shooter News web page and User Manual list a set of criteria for which observation of a telluric standard star is recommended and for which execution time must be included in the phase 1 proposal.

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.
    • Starting from 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 repeeat 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.
    • Zelda mask day calibrations may offer AO performance improvements by correcting non-common path wave front errors. The corrections are expected to be applied automatically to all infrared modes during Period 103.
  • VISIR is not offered in Period 103. It will be modified and installed on UT4 to allow it to use the Deformable Secondary Mirror for the NEAR experiment.
    • As a consequence of the previous point, SINFONI will be dismounted from UT4 during the second half of Period 102 and moved to UT3. As a consequence, no observations requiring the Laser Guide Star can be executed during the Period 103.
    • Large and Monitoring programmes for SINFONI will not be accepted in Period 103 as the instrument will be decommissioned during the period - possibly as early as June 2019 - so that SPIFFI can be integrated in ERIS.
  • VIMOS was decomissioned at the end of Period 101, and is therefore not offered any more.
  • Following its upgrade into a cross-dispersed spectrograph, CRIRES will be installed on the UT3-Melipal Nasmyth B focus and commissioned in Period 102 and Period 103. Depending on successfull commissioning, CRIRES will be o ffered in Period 104.

UT4 - Yepun

  • MUSE
    • The Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging (GALACSI), the Adaptive Optics module for MUSE, was commissioned during Period 99. This combination has been off ered since Period 101 in its Ground Layer Adaptive Optics (GLAO) mode for the MUSE Wide Field Mode (WFM). The full capabilities of MUSE+GALACSI in this mode are currently been characterised and interested users should check the instrument News web page for details.
    • Commissioning of GALACSI in its Laser Tomographic Adaptive Optics (LTAO) mode as well as its combination with MUSE in Narrow Field Mode (NFM) was completed during Period 101. It is offered for the first time in 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.
  • HAWK-I :

Incoherent combined focus

    • The Échelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations is located at the Incoherent Combined Coudé Focus (ICCF). Its commissioning in 1-UT mode was completed in Period 101.
    • The 1-UT mode has been o ffered starting in 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, with the diameter of the fibre aperture on sky of 1", and
      • (2) the Ultra High Resolution mode (UHR), providing a resolving powerof 190 000, with the diameter of the fibre aperture on sky of 0.5".
    • The 4-UT mode is offered starting from Period 103 in groups of consecutive nights in Visitor Mode only.
      • it provides a medium resolving power (MR) of 70 000 with the diameter of the fibre aperture on sky of 1";
      • proposals requesting ESPRESSO in 4-UT mode must in particular justify its use compared to UVES.
      • monitoring and Large Proposals for ESPRESSO in 4-UT mode will not be accepted.
    • The performance of ESPRESSO should improve following an intervention taking place in September and October 2018.

Visitor focus

  • No focus for Visitor Instrument is available on the VLT in Period 103.  

VLTI instruments and facilities

  • General:
    • 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.
    • Due to proposed technical activities and forecasted Guaranteed Time Observations for GRAVITY and MATISSE, the amount of time available for open time on the VLTI-ATs is expected to be in the range of 1 to 2 months in Period 103.
    • Monitoring and Large Programme proposals will be accepted in Period 102 for GRAVITY (astrometry and spectro-imaging) and PIONIER on both the VLTI-UTs and VLTI-ATs.
  • VLTI-ATs:
    • The refurbishment of each of the 4 ATs was completed during Period 101. Their transmission increased by an average of 65% in the near-infrared.
    • The installation, veri fication and commissioning of NAOMI, the New Adaptive Optics Module for Interferometry - a low-order adaptive optics system for the ATs - will be completed in Period 102.
    • Since Period 101, the AT astrometric con guration A0-G1-J2-K0 is off ered for GRAVITY in Dual Field mode only (see the baseline con figuration webpage). In order to limit idle time, Service Mode programmes requesting the Large configuration (A0-G1-J2-J3) might see their observations executed on the astrometric confi guration instead, as the baselines show similar length and sky coverage.
    • ESO is developping a scheme to optimise operations for aperture synthesis with the VLTI. This scheme requires that proposals aiming at imaging reconstruction with GRAVITY, MATISSE or PIONIER with the ATs include the sentence "This program aims at collecting VLTI data for reconstructing images." using the \SpecialRemarks macro. In addition, such proposals should request time corresponding to at least six concatenations (CAL-SCI for GRAVITY and MATISSE, or CAL-SCI-CAL-SCI-CAL for PIONIER) per object and per AT con guration. They should also specify the maximum period over which data can be collected, based on the expected evolution time scale of the target, with a minimum of ten days due to operational constraints.
  • Instruments:
    • AMBER has been decommissioned at the end of Period 101. It is therefore not offered in Period 103.
    • GRAVITY is offered on all AT configurations as well as on all four UTs with the visible (MACAO) and infrared (CIAO off-axis) adaptive optics system in both Service and Visitor modes.
      • For dual-fi eld observations, swapping between two targets has been possible since Period 102.
      • 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 102 and 103.
      • Based on early commissioning results, MATISSE is offered with a limited set of its capabilities in Period 103:
        • Spectral resolving powers of R=34, 506, and 959 in the L-band and R=30 in the N-band. However, 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 will not be accepted in Period 103, as performances need to be consolidated.
      • 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 focus is available in Period 103.

Survey Telescopes and Instruments


  • It is foreseen that further VISTA Public Surveys from the first cycle will have completed their observations at the end of Period 101. The remaining surveys will continue through Period 102, and possibly beyond. Seven second generation Public Surveys, essentially spanning the whole RA range have started observations in Period 99 and are expected to last until 2019.  See the Public Surveys page for details. The fraction of the VISTA observing time devoted to Public Surveys is similar to the one of previous periods. 
  • Approximately 20% of VISTA observing time is available 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.
  • Monitoring and Large Programme proposals will not be accepted in Period 103, as operations on VISTA will be suspended for a planned upgrade during an intervention expected to last approximately 40 days during Period 103.


  • Since Period 101, the zenith avoidance distance is set to 7 degrees to minimise problems with ellipticity near the zenith.
  • Normal, Monitoring and Large Programme proposals are accepted on the VST in Period 103 without restriction on atmospheric conditions.