Since the introduction of observing tools that automatically take into account absolute and relative time delay when scheduling observations at the telescope, time-critical Service Mode Observation Blocks (OBs) are handled more rigorously than before. The OB Ranking Algorithm identifies the most time critical OBs that will soon have time windows expiring and assigns them higher execution priority. Nevertheless, it is possible that due to too many conflicting high priority observations or unsuitable weather, some time-critical OBs cannot be executed within their assigned time window. If all time windows have already passed, the OB expires and is marked with the status F(ailed). This status cannot be changed and the OB is lost.

There are following types of time-critical observations:

1. OBs with absolute time constraint window
   

   Setting an absolute time window in an OB implies that the execution of that OB outside of the specified time window(s) is not useful scientifically. Hence if the OB cannot be executed and it expires, this time is lost. Therefore, during Phase 2 submission, the time-windows should be set in the OBs only if strictly necessary and as loosely as possible.

2. OBs with relative time constraints, that are part of a time-link container
   

   Relative time constraints in time-linked OBs should also be set as loosely as possible, according to the science goals. For some runs (e.g. of monitoring nature) the science case may require the completion or quasi-completion of a sequence of OBs. Should one or more OBs be marked with the status F(ailed), it may be possible to submit new OBs to compensate for the epoch(s) that were missed and for which the corresponding OBs have expired, if the observing run is still active. However, this is not an automatic process and requires a scientific evaluation.

   • The PI should ask for permission to submit new OBs by submitting a request via the ESO Operations Helpdesk web form. The request conveyed in this manner shall contain run ID and a short scientific justification.
   • If the submission of new OBs is approved, they will be either added to the end of the existing time-linked sequence or submitted as a new time-linked sequence. Final advice will be provided by the support astronomer.

1. VLTI time-critical imaging observations
   

   The following specific rule applies only to VLTI Imaging Programmes, which are designed to be observed as a group of time-critical observations (concatenations of OBs) within an imaging slot. PIs of VLTI imaging programmes can request to repeat all observations of a time-critical imaging campaign if it was not completed within the requested time. However, this is not an automatic process and requires a scientific evaluation. The request shall be submitted via the ESO Operations Helpdesk web form and shall contain the run ID and a justification that the goal of reconstructing an image cannot be reached with the observations already obtained. The following conditions apply:

   • the run is A-ranked;
   • the time interval during which the image needs to be completed (Imaging Time) is specified in the proposal;
   • the Imaging Time is not shorter than one month, and
   • the guarantee concerns not more than the ESO-recommended number of uv points for imaging (currently 15 concatenations per target). Possible additional points are taken on a best-effort basis, and expire outside the Imaging Time interval.