Service Mode Rules and Recommendations for Observation Blocks

Preparing Observation Blocks

Both Visitor and Service Mode programmes are carried out at all ESO telescopes by executing Observation Blocks (OBs) provided by the users. OBs are constructed and submitted to ESO using the Phase 2 Proposal Preparation (P2PP) tool for UT1, UT3, UT4, VLTI and La Silla instruments. In Period 89 VISTA, VST and UT2 instruments users have to prepare the OBs using the new P2PP version 3 (P2PP3) Tool.

Please refer to the P2PP version 2 User Manual (or P2PP3 User Manual) and to the User Manuals of the different instruments for more specific information on the structure and contents of OBs, and how to build OBs for different instruments. A number of tutorials describing step-by-step the construction of OBs for different instruments is available.

Service Mode OBs: rules and advices

It is important to keep in mind the Service Mode policies and the following rules and guidelines when designing a Service Mode programme or when preparing a Phase 2 package:

  • Some observing strategies cannot be supported in Service Mode; in particular, real-time decisions about the sequencing of OBs, complex OB sequencing, or decisions based on the outcome of previously executed OBs (like adjustment of integration times or execution of some OBs instead of others).
  • OBs are only executed once. If you want to repeat an identical observation multiple times, you must submit multiple OBs. This requirement applies to standard stars as well.
  • OBs are normally executed non-contiguously. Since efficient Service Mode operations require continuous flexibility to best match the OB constraints with actual observing conditions, OBs for a given programme are normally scheduled non-contiguously. Therefore, users should not expect their OBs to be executed on a specific sequence or in a linked way, unless a sound scientific justification (to be approved with a Phase 2 Waiver and indicated in the README file) exists. Exceptions to this rule are cases in which one OB observing a calibrations source needs to be executed contiguously to a science OB. In Period 89 on VISTA, VST and UT2 instruments it is possible to put OBs into a concatenation scheduling container to enforce their contiguous execution.
  • Multi-mode, multi-configuration OBs are normally not permitted in Service Mode. Although multiple configurations within one OB may sometimes reduce overheads, scheduling and calibrating such OBs is extremely inefficient and can increase the calibration load to an unsustainable level. Examples of such multi-configuration OBs are those combining imaging and spectroscopy in a single OB, spectroscopy with multiple grisms or central wavelength settings, or imaging with a large number of filters (although most imagers allow multiple broadband filters in one OB). Multi-configuration OBs are accepted only if duly justified and authorized by means of a Phase 2 Waiver Request.
  • OB execution times must be below 1 hour. Long OBs are more difficult to schedule and execute within the specified constraints because of the unpredictable evolution of the observing conditions. For this reason,OBs taking more than one hour to execute time are accepted by ESO only in exceptional cases and provided that a Phase 2 Waiver Request is submitted and approved. In such cases, ESO will consider the OB successfully executed if the constraints were fulfilled during the first hour of execution, even if conditions degrade after that time.
  • For UT2, VST and VISTA users in Period 89: concatenation scheduling container execution time must be below 1 hour. Only in exceptional cases, and provided that a Phase 2 Waiver Request is submitted and approved, concatenations longer than 1 hour may be submitted. In such cases, ESO will consider the concatenated OBs successfully executed if the constraints were fulfilled during the first hour of execution, even if conditions degrade after that time.
  • User-provided calibration OBs that need to be executed contiguously with science OBs need to be indicated in the User comments entry of the relevant science OBs for UT1, UT3, UT4 and VLTI instruments. For UT2, VISTA and VST instruments such contiguous execution of OBs needs to be specified via concatenation scheduling container.
  • Time constraints must be indicated in the OBs. If you intend to observe time-critical events or monitor a target at specific time windows, you need to indicate this under the Time Intervals tab of the OBs. This information needs to be provided also in the corresponding section of the README file. Specifying time windows as broad as possible will reduce the possibilities that your OBs are not executed because of higher priority programmes or because the external conditions did not allow the observations during the interval that you specified.
  • Specify the weakest possible Constraint Set values. OBs that can be executed under a broad range of conditions are easier to schedule, especially if they belong to priority groups B or C. In particular, if photometry is needed of a field, it is normally sufficient to obtain a short integration under photometric conditions (transparency = PHO) and carry out the rest of the integration with OBs having a transparency = CLR constraint.

Additional Service Mode Requirements for FLAMES

Instrument Comments: Reference Stars Magnitudes, Hour Angle, and S/N Ratio

It is mandatory to specify in the Instrument Comments field in P2PP (this is the one just below the User Comments field) the following FLAMES specific information (an example of the syntax is provided below):

  • magnitude of the VLT Guide Star of each field (in the range R=9-11mag)
  • magnitudes of the Fiducial Stars (in the range R = 8 - 15 mag with a recommended maximum delta R = 3 mag)
  • hour angle range in which the field configuration is valid
  • targeted S/N ratio per single exposure at some reference wavelength

Recommended Syntax: GS=mag; FACB=mag1,mag2,mag3,mag4; HA=+/-...h; S/N=... @ ...nm

Example: GS=10.5; FACB=10.4,11.1,11.2,12.6; HA=+/-3h; S/N=70 @ 580nm


ARGUS is the large integral field unit of FLAMES available on plate 2 only. For a technical description of this unit, please read the FLAMES User Manual and the ARGUS commissioning report. For the preparation of OBs please note the following:

  • The ARGUS long axis is along the North-South direction for a Position Angle of 0 degrees, with the PA entered in FPOSS being measured from North to East.
  • The fifteen ARGUS sky fibres are single fibres only.
  • The BIGGER field of view is obtained with the scale 1:1. The SMALLER field of view is obtained with scale 1:1.67.
  • Screen flats and a specphot standard are taken by the Observatory for all  ARGUS observations. The specphot standards are selected from the lists available here.
  • The ARGUS fast template, which enables observations of different targets without reconfiguring the plate, is only available in Visitor Mode. This template saves on overheads, but only if (a) The ARGUS sky fibres are at the same radius and/or not used; (b) The plate scale is the same for the two observations.
  • When using the FLAMES_giraf_obs_argoff template, the offsets are relative to the telescope position. See the FLAMES Template Reference Guide for more details.
  • Recall that for ARGUS, the SSN, FPS, and PSSN numbers increase from right to left in the current (and only) version of the fibre table. This goes in the opposite direction with respect to Medusa/IFU fibres.

Total Execution Time

The differential atmopsheric refraction affecting FLAMES observations depends on the airmass, and therefore on the time when the observations are going to be carried out. For this reason, the prepared field has to be configured on the Fiber Positioner plate for the expected mid-time of the science exposures defined in the OB. To properly determine this configuration time, the total execution time of the OB must be known.

In order to check the execution time of individual/multiple OBs, users can make use of the pull-down menu Reports->Execution Time from the main P2PP GUI. Alternatively, while working in the P2PP View OB window, the total execution time of the OB is also reported in the Execution Time entry (once the exposure time for that observation has been specified). Any time the exposure time is revised, please remember to click on Recalc ExecTime, because the Execution Time entry is not updated automatically.

Configuration Wavelengths

The differential atmopsheric refraction affecting FLAMES observations are wavelength dependent.Therefore, the FLAMES acquisition templates require a configuration wavelength per instrument to be specified by the user in order to correct for it. During the execution of the observation blocks at the telescope, the prepared field will be configured on the Fiber Positioner plate for these wavelengths.

Therefore, the selected configuration wavelengths must match the respective specified wavelength settings in the observation template(s)of the same OB.

Please, keep in mind when selecting the wavelength settings in combined mode (Giraffe + UVES), that a large difference in the configuration wavelengths will lead to increased fibre entrance losses of the UVES fibres depending on the airmass and length of the actual observation.

The differential atmospheric refraction will affect long observations depending on the declination of the center of the field, the duration, and the airmass at the beginning of the exposure. This is described in the User Manual Sec.4.4, where Fig.14 shows the relative displacement of an object at a given distance from the field center for fixed values of object declination. A more flexible way to assess the impact on observations can be found by using the FLAMES Airmass Plotter, which delivers plots customized to the particular field under study.

Observing with the SimCal fibers

The FLAMES-Giraffe observing templates allow to specify the use of a simultaneous Thorium-Argon calibration lamp during the science exposure (possible values "ON" or "OFF"). If "ON" then the lamp is switched on for the entire science exposure, with intensities adjusted by the use of ND filters. Use of the lamp enables higher radial velocity accuracy (compared to the "OFF" option).

The intensity of the calibration spectra is adjusted according to the total exposure time. However, for wavelengths larger than 700nm, the object spectra adjacent to the 5 simultaneous calibration spectra can be contaminated by strong Argon lines. See here for an example of how this affects the observed images at 769.1nm. However, you may still choose to switch the lamp off even with blue settings if the objects are faint. If high wavelength accuracy is required and you are worried about contamination, then with a Waiver, you can bracket your main science target (with the lamp OFF), by short 60s exposures with the lamp ON.

FPOSS Requirements and Recommendations

Guide and Fiducial Stars

For a successful execution of your program, we highly recommend to choose the VLT Guide Stars and the Fiducial Stars within the following magnitude limits:

Reference Star R magnitude Comments
VLT Guide Star 9 - 11 select guide stars close to 9 mag (but not brighter) if your constraints allow a seeing of >1.4". Guide stars up to 12mag may be considered on a case-by-case basis, but experience has shown that the telescope active optics can fail to close (especially if the seeing degrades), leading to loss of time.
Fiducial Stars 8 - 15 the maximum difference in magnitude among the selected stars should not exceed 3 mag

Further requirements:

  • make sure that the Guide Star is selected inside the 25 arcmin field of view;
  • make sure that the Guide Star can be clearly identified in the 30" field of view of the guide probe;
  • make sure that the Fiducial Stars are isolated objects so that they can be clearly identified and well centered in the 2" field of view of the FACB fibres;
  • make sure that all four FACB fibres are allocated to Fiducial Stars even if only a minimum of 3 is required by FPOSS;
  • it is crucial that all reference stars (VLT Guide Star and Fiducial Stars) are given in the same astrometric system as the science targets.

Fibre Status Information

While configuring your field, please pay attention that none of your highest priority targets is allocated to the fibres in the Fibre Status table. In that table, RP refers to the retractor position, which is the "fibre number" or "pivot number" visible in FPOSS when you allocate the fibres. The FPS number is the progressive fibre position number in the slit.

Validity Check

Please keep in mind that Allocation OK in the Messages window of the FPOSS Control Panel means that your successful configuration is valid only at meridian. The safety margins encoded in the FPOSS tool are chosen to avoid fibre-fibre collisions at configuration time on the Fiber Positioner over a hour angle range of +/-4 hours for most of the configured fields.

However, as part of the basic FPOSS allocation sequence it is mandatory to check the actual hour angle range for which a configuration is valid (step Check HA range). This validity interval is recorded in the Target Setup file and must be reported in the Instrument Comments field in P2PP (cf the details given above). Try to maximise the valid hour angle range before saving the Target Setup file to increase the execution probability of your observation. In most cases this can be achieved by deallocating the few fibres which are found to be the most sensitive for collisions.

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