Observing in Service Mode:
Philosophy and Scheduling

General Philosophy of Service Observing

• Maximize science efficiency by executing the programmes with highest scientific priority first and under the required observing conditions;
• Maximize operational efficiency by sharing calibration data between programmes, and by helping infrequent users of complex facilities in optimizing the use of the allocated observing time;
• Maximize the scientific use of telescope time by having appropriate programmes ready for execution under a broad range of observing conditions;
• Maximize the scientific productivity of the facility by means of the reuse of the data, made possible by building uniform datasets accessible through an archive.

The key concept in optimizing the use of the available time to produce the highest scientific return is Flexible Scheduling, i.e., the capability to continuously adapt the observing schedule to the external, often unpredictable conditions. For this reason, and unlike in the case of space observatories, it is not generally possible to foresee the precise date when a given observation or programme will be carried out.

An obvious limitation of Service Mode observing is the impossibility for users to take decisions on the execution of their programmes in real time, for example based on a specialized scientific assessment of the outcome of a new observation. Programmes in which real-time decisions are needed for their scientific success are thus deemed unsuitable for Service Mode observing, and are normally carried out in Visitor Mode instead.

In order to meet the goals stated above while also serving the needs of a diverse astronomical community, it is necessary to have a structured system in place that guides and monitors programmes throughout their entire lifecycle, from the initial submission of the observation programmes to the use of their data for archival research in the future. The ESO Data Management and Operations Division (DMD) has been given the responsibility to develop the VLT Data Flow System (DFS) to supply the tools and services necessary to achieve these goals.

• The Visiting Astronomers Department (VISAS),
• The DMD User Support Department (USD),
• The Paranal Science Operations Team or the La Silla Science Operations Team,
• The Data Flow Operations (DFO) Department and its Quality Control (QC) Group, and
• The Science Archive Facility (SAF).

The Programme Priority Groups

• Group A: High Priority: These programmes are considered to have the highest scientific value and are executed first as observing conditions allow. ESO does every possible effort to complete programmes in this Group. This includes the possibility of carrying over a limited number of programmes in this category to the next semester, in case that their degree of completion at the end of the allocated semester is too low for the fulfillment of the scientific goals. Target-of-Opportunity (ToO) programmes cannot be considered for carryover status.


• Group B: Medium Priority: Programmes in this group have lower scientific priority than Group A and are executed only when no Group A programme can be executed. ESO tries to complete all programmes in this Group, but incomplete programmes are terminated at the end of the allocated semester. The likelihood that any given Group B programme is executed decreases as more stringent observing constraints are requested.


• Group C: Low Priority: These programmes have lower scientific priority than those in Groups A and B but have relaxed constraints (see below), which allows them to be scheduled when the external conditions are not suitable for the execution of any programmes in Groups A and B. Most programmes in Group C would not have been scheduled if Visitor Mode had been the only observing mode offered by the Observatory. However, the flexibility of service mode scheduling allows in this way a scientifically valuable use of the telescope time even under conditions that would be unsuitable for the execution of higher-ranked programmes.

Scheduling Service Mode Programmes: observing constraints

• Programme priority group,
• Observing constraints imposed by the Principal Investigator, and
• Actual observing conditions.

Every requested observation has multiple observing constraints. Typical observing constraints are:

• the target coordinates
• the allowable lunar phase range
• the allowable moon-to-object angular separation
• the allowable airmass range
• the allowable image size range
• the allowable sky transparency
• for Adaptive Optics instruments (currently NACO and SINFONI), the Strehl ratio on the reference star.
• for VLTI instruments, the local sidereal time range and the availability of the desired baseline

The Observing Constraints are specified by the user at Phase 2 for each Observation Block. Since the execution conditions required by each programme are an important ingredient in the process of building up the Long Term Schedule of an observing semester, and thus determines which programmes can or cannot be scheduled, users are not allowed to specify at Phase 2 constraints that are more strict than those specified in the original proposal. Users can however relax the constraints in order to enlarge the range of conditions under which their programme can be scheduled, and thus the chances of execution.