The Scheduling Tool (or SCHED), is a set of software tools aimed at supporting ESO's Observing Programme Committee (OPC) during the entire review process of the Observing Time Proposals.

These tools are decision support systems: all of SCHED provides a high degree of flexibility, in order to allow for true decision support. In all cases the operator will have the possibility to ignore a tool's suggestions and impose his/her own view. In particular, this will apply for example to the special case of Targets of Opportunity for both Long Term Scheduling and Short Term Scheduling.

SCHED has three main components: LTS, the Long Term Scheduling tool; MTS, the Medium Term Scheduling tool; and STS, the Short Term Scheduling tool.

The LTS processes only Phase I information (proposals) provided by The Proposal Handling and Reporting System (PHRS). It schedules classical mode programmes over a six month period, while reserving blocks of nights for service mode programmes, and outputs the period's long term schedule - including service mode observing runs. The LTS operates on all ESO telescopes, and handles inter telescope dependencies; for instance, observing runs that need to be performed simultaneously on different telescopes.

The MTS operates on Phase I information, on long term schedule information provided by the LTS, and on Phase II information (observation blocks) provided by the P2PP. It generates ranked lists of observation blocks for one or more observing nights; rank is based on the blocks dynamic priority, while lists may take different parameters into account, like expected seeing and sky brightness. The lists can be used by an ESO staff to prepare for an observing night , and can be further manipulated with the tool's user interface.

The STS operates on observation blocks and on the lists generated by the MTS; it schedules single observation blocks over a short time span, generally a few hours. Main output of the tool is the short term schedule; basically, it generates a different ordering of the input lists.

MTS works as a filter for the STS: it allows the short term scheduling algorithm to operate on a selected set of observation blocks, and it provides priority computation. It is up to the user to built the ranked lists of observation blocks according to the available criteria.