Regularly scheduled observations during MPIA blocks are subject to interrupts from Target of Opportunity Observations (ToOs)  of Gamma Ray Bursts (GRBs) and X-ray transient afterglows using GROND. Exceptions are time-critical observations which are flagged as such and which may not be interrupted by GROND/ToOs. In general, GROND/ToOs are executed automatically and in Rapid Response Mode (RRM).

On average, 15% of the available time per year may be used for GROND/ToOs. Due to the orbital inclincation of the SWIFT satellite, the GRB trigger rate is variable in time. During times of increased trigger rates, up to 25% of the available observing time in a given month may be used for GROND/ToO observations.

Regularly scheduled programmes that suffer from GROND/ToO interrupts are compensated in real time via ToO placeholders embedded in the regular observing schedule. The service observer at the telescope takes a real-time decision about how to fill those placeholders.

In cases where a full compensation for GROND/ToO interrupts is not possible, due to constraints imposed by lunar phase, right ascencion, seeing or weather, the highest-ranked programmes should be served first. The rank of a programme is given in the observing plan.

During periods of lower than average trigger rates, the placeholders may also be used to compensate for weather losses. Again, the highest-ranked programmes should be served first, unless this is not possible from the various constriants such as lunar phase, seeing, etc.

Time accounting is based on the REMEDY ticket database containing all respective trigger information for the GROND/ToO observations. A final balancing reflecting actual losses incurred by GROND/ToO will be made after each year of operation between MPIA and MPE, based on detailed time accounting provided by the REMEDY database.

The use of twilight for GROND, WFI, and FEROS calibrations follows the rules outlined above. On average, 15% of the time available for twilight calibrations will be made available for calibrations required for the GROND/ToO programme.