All programmes allocated observing time in Service Mode are assigned to one of the following priority classes:

• Rank A: High Priority: These programmes are considered to have the highest scientific value and are executed first as observing conditions allow. ESO makes every possible effort to complete programmes in this class. This includes the possibility of carrying over 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, and if they are not time-critical. Large Programmes are automatically considered as high priority programmes. Within group A they are ranked higher than Normal Programmes. Target-of-Opportunity (ToO) runs cannot be considered for carryover status. As Monitoring Programmes span multiple Periods for the purposes of monitoring individual targets/fields this removes the necessity for carryover of most runs. Monitoring Programmes are considered for carryover after their last semester for up to one more visibility period. All PIs of programmes in Rank A are informed about one month prior to the next Call for Proposal deadline about the carryover eligibility of their programme.
• Rank B: Medium Priority: Programmes in this group have lower scientific priority than Rank A and are executed only when no Rank A programme can be executed. ESO tries to complete all programmes in this class, but incomplete programmes are terminated at the end of the allocated semester. The likelihood that any given Rank B programme is executed decreases as more stringent observing constraints are requested.
• Rank C: Low Priority: These programmes have lower scientific priority than those in Ranks 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 Ranks A and B. The programmes in Rank 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.

The webletter (or email in the case of DDTs), prepared by the Observing Programmes Office (OPO), states which priority class your programme was assigned to.

This page is valid for all instruments