Important recent changes (since Period 96) regarding Paranal instrumentation and facilities

This section describes important changes which took place during Period 96 and 97, as well as changes expected to take place during Period 98.


  • Seeing and Image Quality:  Starting from Period 96, the  seeing information  to provide in Box  3 of a  proposal is the seeing in the V band  at zenith.  This change now 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 constraint during Phase 2. The Messenger article `On the Difference between Seeing and Image Quality: When the Turbulence Outer Scale Enters the Game' by P. Martinez et al. describes the meaning of these two quantities.
    Accordingly, the Exposure Time Calculators have been modified and clearly distinguish between the two quantities, using a slightly modified version of the formula given in the above article.

Exposure Time Calculators:

Astronomical Site Monitor

Pending final acceptance, an upgraded software for the Astronomical Site Monitor (ASM) as well as a new Differential Image Motion Monitor (DIMM) will enter operation during Period 97. Additionally, the system will include atmospheric turbulence profile data from the Multi Aperture Scintillation Sensor (MASS),  which delivers more accurate values of coherence time and isoplanatic angle and 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 will take place during Period 97 and 98. The spectrograph installation and first commissioning, with one UT, are expected to take place during the second half of Period 98.


UT1 - Antu


  • CRIRES was taken out of operations as of July 17th, 2014 to undergo a major upgrade to transform it into a cross-dispersed echelle spectrograph. It is therefore not offered in Period 98. First light of the upgraded CRIRES is expected to take place during the second quarter of 2018.
  • Instead, NACO has been offered on the Nasmyth A focus since Period 94, with a limited list of offered modes.
    • During Period 98, only the following modes are offered:
      • NAOS:
        • VIS & IR wavefront sensing;
        • Pupil tracking;
      • CONICA:
        • Imaging without chopping, in NoAO; usage of the Mp filter is only possible in VM;
        • Cube mode;
        • Focal plane AGPM coronography, with the  Lp, NB_3.74 or NB_4.05 filters;
        • Wollaston/HWP polarimetry;
        • SAM/SAMPOL, only in VM.
    • The following modes are not offered: LGS, SDI+, 4QPM/Lyot coronography, APP imaging and spectroscopy, prism spectroscopy, and, since Period 97, grism spectroscopy, and coronagraphy other than the AGPM coronagraphy.
    • As for February 2015, the Aladdin2 detector (installed in CONICA at the end of 2014) features a spurious background noise that limits the sensitivity of NACO when long individual exposure times (DIT) are used. This extra noise is caused  by an imperfect thermal shielding of the detector electronics. Nearly wavelength-independent, the contribution of the background noise can be modeled as a brighter sky: about -0.6 mag/arcsec through the Ks-band filter and -0.7 mag/arcsec through the H-band filter. There is no significant degradation at long wavelengths (L’, M’) as observations anyway require short DITs due to the nigh ambient background (sky + optics).
    • Following the intervention in October 2015, all objectives are available. However, the detector still suffers from the excess background. There is also no signal in every eigth column of the lower left quadrant  (quadrant 2). Tests are still ongoing to characterise the ETC, please check the News page for updates.
    • 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


    • Re-designed Atmospheric Dispersion Correction (ADC) systems will be installed during the second half of Period 98; the instrument will therefore be unavailable for approximately 6 weeks during that time. Updates on the status of the ADCs can be found on the XSHOOTER news webpage. Starting from Period 98, Large and Monitoring Programmes with XSHOOTER are accepted again, taking this restriction into account.
    • Starting from Period 96, two additional templates in slit and IFU modes are available for mapping of an extended object with a sequence of offsets and with or without any sky position. In parallel, the behavior for the FixedSky and GenericOffset  is slightly changed. The FixedSky offset template allows one to observe the object and a defined, allways identical, sky position. The GenericOffset template allows one to observe an object or a sequence of objects and a sequence of sky positions, which can be different, but the number of sky observations must be equal to the number of object observations. These changes allow a better support by the data reduction pipeline.
    • Improved measures of the instrument efficiency at all settings have been acquired and are used to feed the ETC. As a result, the current ETC outputs a signal-to-noise ratio that can be significantly different from the output obtained using previous versions under the same conditions. The current ETC is expected to provide more accurate predictions of the signal-to-noise ratio. Compared to the previous ETC version, the predicted signal-to-noise ratios are generally lower in blue settings and higher in red settings. Significant differences (less than a factor 2) are expected in the most blue and most red settings.
    • A redetermination of the focus of GIRAFFE has been carried out. As a result, the spectral resolution has significantly improved, as summarized in the FLAMES News page.
    • 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


    • Starting from Period 96, the IRDIS DPI mode is offered in Visitor Mode only.
    • Starting from Period 97, the broadband H filter for IRDIS is offered in IRDIFS mode, while the broadband Ks filter is offered in the IRDIFS_EXT mode.
    • Starting from 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.
  • VISIR is offered again since Period 95.
    • Following a major upgrade, VISIR has been fully re-commissioned in January 2016, including the burst and the Sparce Aperture Masking (SAM) modes. These two modes are offered starting with Period 98 in addition to the previously offered modes.
    • The offered modes therefore include normal imaging, burst-mode imaging, low-resolution specctroscopy, and long slit and cross-dispersed high-resolution spectroscopy in both Service and Visitor Modes. Spare 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
    • Only Large and Monitoring programmes involving imaging, low-resolution specctroscopy, and long slit and cross-dispersed high-resolution spectroscopy will be accepted.
    • The PWV must be provided as an observing constraint during their Phase 1 and Phase 2 preparation.
  • VIMOS:
    • Large and Monitoring Programmes are not offered in Period 98 for operational reasons compounded by the ongoing Public Spectroscopic Surveys.
    • Public Spectroscopic Surveys started in Period 94 and substantially use all the dark and grey time in good conditions (PHO/CLR, seeing < 1.000) 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 programme. Furthermore, Service Mode programmes that require 1.2" seeing are actively encouraged.


UT4 - Yepun

  • Adaptive Optics Facility
    • The following activities occurred during Period 96 and 97:
      • installation of GRAAL on the telescope and re-commissioning of HAWK-I took place in August 2015;
      • commissioning of GRAAL with the first unit of the Laser Guide Stars Facility took place in October 2015;
      • installation of the 3 other LGSUs took place in February 2016; their commissioning as well as the commissioning of the Laser Guide Stars Facility as a whole takes place during Period 97.
    • During Period 98 the following activities affect operation on UT4.
      • the installation of the the Deformable Secondary Mirror and recommissioning of the telescope will take place during the first half of Period 98. As a consequence UT4 will not be available for approximately 3 months.
      • GALACSI, the adaptive optics module for MUSE, is expected to be installed on Nasmyth B during the second part of Period 98.
  • HAWK-I :
    • Starting from Period 96, users of Fast Photometry may control the adapter focus from the acquisition template therefore allowing out-of-focus observations.
    • An upgrade took place in January 2016. It included 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.
  • MUSE:
    • Starting from Period 96, MUSE has been available for observations in Rapid Response Mode.


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 will be 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 for 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 early November 2016 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. Each AT will be sequentially unavailable for operations for approximately one month, during which the VLTI-AT can only be operated with the other 3 ATs. Therefore, the number of nights when the 4 ATs are available for regular operations in Period 98 is expected to be very small, except for March. Indeed, due to the GRAVITY GTO and commissioning runs, only approximately one week will be available for regular operation in October. Only programmes requesting AMBER and PIONIER with three ATs will be accepted for observations between November and February (although, whenever possible, PIONIER observations will be executed with 4 ATs); imaging programmes are strongly discouraged.
      When one or two refurbished ATs are re-commissioned and put in operation, different polarisation states between old and new AT Coud´e optics can be expected due to the different coatings: their impact on the performance of PIONIER and AMBER is unknown.
      A more detailed schedule of the refurbishment will be available end of June and further updated end of August.
  • VLTI-UTs: The AT refurbishment will have no impact on operation with the UTs. Note that UT4 will not be available for operation in October, November
    and December due to the installation of the Deformable Secondary Mirror and re-commissioning of the telescope.
  • The VLTI laboratory underwent major modifications during Period 95 and reopened for science operations in Period 96. As of Period 96, the available instruments are AMBER (with or without FINITO), PIONIER, and, starting as of period 98, GRAVITY.
  • AMBER is  offered again since Period 96. Since it may be decommissioned in the near future, it is not offered for Large or Monitoring programmes.
  • PIONIER is offered again, as an ESO instrument, since Period 96: therefore, it is no longer mandatory to contact the instrument team or to include them as co-I before submitting the proposal. It is available in both service and visitor modes.  Follow late breaking news from technical time results in March, 2015:
    • The instrument can track fringes on visibilities higher than 5%  (V=0.05)
    • The LOW detector gain has been commissioned. The brightest  targets that can be observed in service mode are H=-1mag. Since no suitable calibrators might be available for the LOW gain within the recommended area around the target,  cross-calibration with other gain values has been tested and is  within the typical precision of the instrument. In that case, calibrators well fainter than the target can be used.
    • Since the precision of cross-calibrating different gains is only guaranteed when using LOW and MEDIUM gain, but not the HIGH gain, targets brighter than H~2.5 should have calibrators brighter than H~2.5.
    • Tests on the UTs have shown no improvement of sensitivity or precision over the ATs. UTs can only be requested if use of ATs is excluded (e.g. no guide star available).
  • 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. Starting as of Period 98, ESO offers GRAVITY on all AT configurations in service mode only. In this period the spectro-imaging mode will be available, both in single and dual field modes, with medium and high spectral resolution. In particular, the astrometric mode is not offered in Period 98. However, because of the AT refurbishment
    described above, GRAVITY is only offered for observations during the months of October 2016 and March 2017. In addition to the instrument itself, the 4 units of the GRAVITY Coudé Infrared Adaptive Optics (CIAO) system for the VLT Interferometer will be successively installed on each of the 4 UTs during Period 97. Commissioning of GRAVITY with CIAO will take place during Period 97 and Period 98.
  • No VLTI Visitor focus is available in Period 98.

Survey Telescopes and Instruments


  • New Public Surveys are planned to start in P98 (see the recent Call for Letters of Intent). 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 98. 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.
  • The installation of an improved baffling mechanism including specially designed system of ribs was completed in April 2015. Measurements show that most of the stray-light component has been reduced. Also, the brightness of the background is at least 10% fainter than before the installation of the ribs and baffles.