Service Mode Rules and Recommendations for Observation Blocks

Preparing Observation Blocks

Observations at all ESO telescopes are carried out by executing Observation Blocks (OBs) provided by the users. OBs for Service Mode runs with Paranal Instruments must be made with p2. For (designated) Visitor Mode observation preparation, please follow dedicated Visitor Mode Guidelines.

Please refer to the Phase 2 step-by-step preparation with p2 page and to the User Manuals of the different instruments for more specific information on the structure and content 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 may be scheduled non-contiguously. Therefore, users should not expect their OBs to be executed in a specific sequence or in a linked way. Usuers who do require contigous observations might use concatenation scheduling container (see definition below) by follwing the rules applied to this case.
  • Multi-mode, multi-configuration OBs are normally not permitted in Service Mode. Although multiple configurations (e.g. combining imaging and spectroscopy) within one OB may sometimes reduce overheads, scheduling and calibrating such OBs is inefficient and can increase the calibration load to an unsustainable level. 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 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.
  • 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, longer concatenations 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 specified via concatenation scheduling containers.
  • Time constraints must be indicated in the OBs. If a user intends to observe time-critical events or monitor a target at specific time windows, this must be indicated under the Time Intervals tab of the OBs. Please note that absolute (UT) time constraints refer to the interval in which the OB can be started, whereas for Local Sidereal Time (LST) time intervals, the time interval refers to the entire duration of the OB. For monitoring observations it is often more appropriate to put OBs in a time-link container. 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 observing conditions did not allow the observations during the interval that you specified.  Usage of absolute time intervals must be scientifically justified in the README file. Please read carefully the time-critial OB execution policy.
  • Specify the weakest (most relaxed) possible Constraint Set values. OBs that can be executed under a broad range of conditions are easier to schedule. In particular, for photometric calibration 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.
  • Nested scheduling containers (see definition below) should be used only if strictly necessary as they increase complexity for observations and scheduling.

Time-linking of OBs

Some OBs must be executed within precise time windows for scientific reasons, rather than any time when the external conditions (moon, seeing, transparency...) would allow the execution. The following types of time-dependencies can be recognized:

  • Absolute time constraints, meaning that an OB must be executed at specific dates that can be predetermined. An example is the observation of a binary star at a precise phase of its period or a planetary transit observation.
  • Relative time links, implying that an OB must be executed within a time interval after the execution of a previous OB, but not necessarily at a fixed date. Examples of this are monitoring observations of a variable source at some pre-defined intervals.

Both types of time-dependency are implemented within p2. Whereas absolute time constraints are available at the level of single OBs, the relative time links are implemented within the new "Time Link" container.

Within a Time Link container, the user can define a series of OBs, having the earliest and latest time when a given OB in the series must be executed with respect to the preceding OB. The time-related information is stored in a database, from where it is retrieved by scheduling tools available to the operator on the mountain in order to build up a short-term schedule that properly takes these constraints into account.

If an OB with absolute time constraint or time-linked OB that acquired an absolute time constraint following execution of a previous OB in sequence (i.e. OB to be observed after earliest from and before latest from time of the previous OB in the sequence) is not successfully completed within the specified time interval, it will expire and get status F(ailed). Such an OB is not observable any more and policy for time-critical OB execution applies.

If the time-linked OB expired in the middle of a time-link sequence, the sequence execution continues as follows:

  • If the failed OB is not the very first OB of the time link, it had the absolute time window corresponding to delay from the previously executed OB. After it expired the next OB acquires an absolute time window by adding the relative minimum and maximum time delays to an assumed hypothetical execution for the failed OB in the middle of its constraint window. 
  • If the failed OB is the very first OB of the time link, the failure can only occur if this OB has one or more absolute time constraints defined and all of them have expired. In this case the next OB acquires an absolute time window by adding its relative minimum and maximum time delays to an assumed hypothetical execution of the failed OB at the end of its last absolute time interval. 

It should be noticed that, depending on the length of the relative time intervals, and the delays between them, a failure of an OB in a sequence may result in a cascade of failing OBs.

Concatenation of OBs

In some cases it may be desired to execute the OBs consecutively, with no other observations in between. This has been implemented in p2 within the "Concatenation" container. The Concatenation container consists of two or more OBs that must be executed "back-to-back" without breaks. The sequence of the execution of OBs in a Concatenation typically follows the sequence as they are listed in the p2 window. 

Definition of groups of OBs

Groups of OBs are used to express the preference to complete observations of a given group of OBs before continuing with other OBs (or groups of OBs) within the same observing run. This is the most loose scheduling container concept, and the priority for execution of the group with respect to other groups within the same run is defined though group priority that has values 1-10 (1 top, 10 lowest priority) as for the user priority for loose OBs. The priority for execution of OBs within the given group is regulated through the OB group contribution. 

If OBs within the group, whose observation started, are not observable (constraints are not fulfilled), it is possible to start observations of another group. After that group score defines which groupo will be given priority in case both groups of OBs are observable again. 

Definition of Nested Containers

For Service Mode observations that use VLT or VLTI instruments on Paranal it is now possible to design more complex observing strategies in the p2 tool with nested scheduling containers. For example a science case that requires time-monitoring of a set of concatenations of science+telluric OBs can be expressed as time-links of concatenations. For the VLTI imaging observations use of groups of pairs of science+calibrator is mandatory, such that the group defines the set of concatenations that contribute to the same image or uv plane. 

Additional Service Mode Requirements for FLAMES

Using gaia coordinates

We recommend where ever possible, to use the latest gaia DR coordinates (currently gaiaEDR3) for Guide and FACB stars, thus implying that your science targets must also be in the the same astrometric system.

Please note though that you must specify EPOCH 2000 coordinates.

If you obtain gaia catalogue(s) from Vizier, RAJ2000 and DEJ2000 columns are included and these are the coordinates you should use (along with proper motions).

If you obtain your gaia catalogue(s) directly from ESA, please see this web page concerning propagation of coordinates to other EPOCHs.

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

It is mandatory to specify in the Instrument Comments field in p2 (this is one of three fields to be filled in the Obs. Description tab) 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). Please note, FPOSS has not been updated to reflect this change made in 2009, therefore please ignore the warnings in FPOSS about Guide Star brightnesses and be sure to select stars consistent with the updated rules documented here, in the User Manual and below.
  • 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
  • average targeted S/N ratio per single exposure at central 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

Note that OBs are not classified with respect to the measured S/N, when compared to the S/N provided by users. However, it is very useful to know what is the targeted S/N in order to identify OBs with potential issues. For example, if the measured S/N is less than about half the targeted S/N, users might be contacted to verify the acquired data as soon as possible.


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

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

Differential atmopsheric refraction affecting FLAMES observations is wavelength dependent. Therefore, the FLAMES acquisition templates require a configuration wavelength per instrument (GIRAFFE and/or UVES) 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.

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 longer 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

Additional 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.

Proper motion

As most astrometric catalogues are based on an epoch of 2000.0, as time passes the issue of proper-motion becomes increasingly important. If the Guide Star, and/or one or several of the FACB stars has significant (see below) proper-motion it can complicate the acquisition, even to the point of making it impossible to complete the acquisition. Even if the acquisition can be completed, FACB coordinates which fail to account for proper motion may cause the mis-alignment of every target fibre with the sky. Finally even if guide and/or FACB stars have negligible proper motion and/or any significant proper motion is properly accounted for, proper motion of the science targets may cause misalignment of the fibre with individual targets.
It is thus strongly recommended to check all targets (guide, FACB and science stars) for proper motion, and where appropropriate include the relevant additional information in the FPOSS fld file.
You should include proper motion information in the FPOSS fld file for any target whenever the magnitude of the proper motion at the end of the relevant period would exceed 0.1 arcsec (A-ranked programmes may want to consider the magnitude at the end of the relevant period plus 2, to cover the possibility of carryover). For example, for P110 programmes, the end of the period is at 2023.25, therefore check if:
     (2023.25 - 2000.0) * sqrt(PropRA*PropRA + PropDec*PropDec) > 0.1 arcsec
                  23.25 * sqrt(PropRA*PropRA + PropDec*PropDec) > 0.1 arcsec
for any of your guide, FACB or science target stars. In case it is then you can have FPOSS take proper account of the proper motion by adding the appropriate information in the FPOSS fld file as follows:
Lines that look like:
<targ_name> HH MM SS.SSS  ±DD MM SS.SS  <type>  <priority>  <mag> 0

should look something like this:

<targ_name> HH MM SS.SSS  ±DD MM SS.SS  <type>  <priority>  <mag> 0 (PM-RA=x.xxxxx) (PM-DEC=y.yyyyy)
The 0 (which the FPOSS user manual describes as a 'program id') is not used by FPOSS, but is nonethelsee REQUIRED in the FLD file to insure that the Cooment field (with the propoer motions) is properly recognised and decoded. The Proper motion values x.xxxxx and y.yyyyy should be in arcsec per year.
Further details are provided in the FPOSS user manual (see section 5.1).

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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