Scheduling Service Mode observations for execution is determined by three main factors:

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

The Observing Constraints are the set of criteria related to the target position and external conditions required by a particular observation that controls when that observation can be executed. For more details, see the section about the critical observing constraints.

Once the observation has been executed, the comparison between the specified constraints and the actual execution conditions determines whether or not the observation met the user-specified requirements.

The Service Mode observations on all Paranal telescopes are assisted by the short term scheduling algorithm that is part of the Observing Tool. The algorithm contains two parts: (1) filtering that determines whether an observation could be successfully carried according to the requested constraints (e.g. target is above the specified airmass, and fullfills the seeing, moon, PWV and other constraints), and (2) ranking that sorts all observable Observation Blocks (OBs) according to their relative priorities that are combined observability priority (OBs with most demanding constraints get higher priority when achievable), priority of their programme priority group, the user priority for OBs within the same observing run, and, for OBs that are organized in groups, the so-called group score. The ranking priorities for OBs and Scheduling Containers are based on the algorithm developed for the ESO survey telescopes (Bierwirth et al. 2010), and implementing through observation preparation with the p2 tool. 

The following articles provide further information on the VLT Operation Model with particular emphasis on Service Mode scheduling and execution:

• The Messenger article "Service Mode Scheduling: A Primer for Users" by D. Silva (2001) 
• The SPIE article "New observing concepts for ESO survey telescopes" by T. Bierwirth et al. (2010)
• The Messenger article "Fifteen Years of Service Mode Operations: Closing the Loop with the Community" by F. Primas et al. (2015)
• The Messenger article "The Scientific Return of VLT Programmes" by M. F. Sterzik et al. (2015)
• The SPIE article "The evolution of observing modes at ESO telescopes" by S. Marteau et al. (2016)
• The Messenger article "Should I stay, or should I go? Service and Visitor Mode at ESO's Paranal Observatory" by M. Rejkuba et al. (2018)

This page is valid for all instruments