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, unless a sound scientific justification (indicated in the README file and approved with a Phase 2 Waiver in case of a contiguous execution lasting longer than 1 hr) exists. Approved OB sequences should then be prepared as concatenations. Exceptions to this rule are cases in which one OB observing a calibration source needs to be executed contiguously to a science OB. In such a case place both 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 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 multiple 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 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 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. 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.
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.
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.
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.
Additional Service Mode Requirements for VLTI
Special care should be taken when entering the target coordinates in P2. Also, to ease fringe finding during the observations, proper motions of the target (if available) must be entered in P2. The GRAVITY-wide acquisition requires parallaxes. We recommend the use of the Guide Star Catalog, Version 2.4.2 from the STScI, available through VizieR.
To prepare and plan your VLTI observations, we recommend to use the Visibility Calculator VisCalc for PIONIER, the GRAVITY ETC for GRAVITY, and the MATISSE ETC for MATISSE, or third-party tools such as ASPRO. The assessment of the feasibility of an observation requires an estimate of the expected visibility for the chosen VLTI configuration, of the observability of the targets (taking into account airmass, the limited delay line strokes, and shadowing effects), and a check whether science and calibrators in concatenations can be executed back-to-back.
Concatenations of science targets and calibrators
Interferometric observations require the frequent measurement of interferometric calibrators. VLTI OBs should be submitted as sequences of science target OBs (SCI) and calibrator OBs (CAL) making use of concatenation containers in P2. Observations using the GRAVITY-wide mode do not provide absolute calibrations, and should be perfomed with SCI stand-alone OBs. Astrometric observations with GRAVITY may be executed without a calibrator so that stand-alone SCI OBs may be submitted for this purpose. Each concatenation must normally include a SCI-CAL, CAL-SCI or CAL-SCI-CAL sequence. CAL-SCI-CAL-SCI-CAL sequences are generally available for PIONIER. Please also note the VLTI-specific naming rules for science target and calibrator OBs.
The use of nested containers for VLTI observations is offered. These include groups of (SCI/CAL) concatenations and time links of (SCI/CAL) concatenations. P2 tutorials are available to guide you through the creation of a group of concatenations for imaging (VLTI Imaging p2 Tutorial) and a relative time link of concatenations (VLTI time link p2 Tutorial).
Support for imaging observations
As of Period 108, ESO introduces imaging slots (ISLs) in the VLTI-AT telescope schedule, in order to further improve the efficiency of VLTI imaging observations. ISLs are periods of about two weeks of uninterrupted service mode, with flexibility on the exact dates to change the AT configurations. The ISLs are specifically set aside and normally centred around new moon in November, February, May, and August of every year. ISLs are primarily intended to support imaging observations, but they are not restricted to this type of VLTI observations. ISLs are regular SM time, and OBs are executed according to their priority. Likewise, imaging observations are not restricted to ISLs, but can be executed in SM time outside of the ISLs.
In case of imaging observations, the use of groups of concatenations is mandatory to define the set of concatenation that contribute to the same image (the same uv plane).
If the image needs to be completed within a certain period of time, all OBs of the group need to use absolute time constraints within which the image shall be completed, ideally around an ISL. PIs of VLTI imaging programmes can petition to repeat all observations of a time-critical imaging campaign if it was not completed within the requested time under certain conditions as outlined in the Time critical OB execution policies.
A p2 tutorial on VLTI Imaging specifically has been prepared to guide you through the creation of a group of concatenations for imaging.
Community-supported VLTI Expertise Centres
For users to VLTI needing assistance to prepare their VLTI observations, the community supported VLTI Expertise Centres - distributed throughout Europe - can offer in-depth support. They also offer support for observation preparation, advanced data reduction and interpretation.
Additional Service Mode Requirements for GRAVITY
An overview on the pointing restrictions for 4-telescope baselines triplets offered for GRAVITY can be found at the VLTI Configuration Overview page. GRAVITY has its own Exposure Time Calculator, which replaces the general visibility calculator VisCals for GRAVITY obsrvations.
Any sequences of science and calibrator observations are supported for GRAVITY within total concatenation times of 1h (1.5h with waiver). We recommend a standard SCI-CAL sequence for most observations. The new GRAVITY-wide mode does not allow an absolute calibration, so that SCI stand-alone OBs are recommended for this mode. Astrometric observations may also be executed with SCI stand-alone OBs.