4MOST completes Operations Rehearsal 4, its full end-to-end rehearsal

4MOST has completed Operations Rehearsal 4 (OpR4), described as a full "dress rehearsal" for the survey. The rehearsal tested the whole operational system, from planning observations through data processing to feeding the results back into later observing decisions.

During the exercise the nightly operations ran on their own from start to finish. The system generated and scheduled Observing Blocks (OBs), chose targets and adjusted the plan to the conditions at Paranal on the night. For ESO, which runs some of the world's leading observatories, a remote scheduler that works without intervention is an important step.

Cambridge DMS team confirms the system works end to end

Nic Walton and David Murphy, who lead the 4MOST Data Management System (DMS) team at the Cambridge Astronomical Survey Unit (CASU), confirmed that OpR4 showed the processing system working from start to finish and turning observations into science-ready products.

CASU leads the core 4MOST DMS and hosts the 4MOST Data Processing Centre (4DPC) in Cambridge. It carries out all Level 1 processing, brings in the Galactic and extragalactic Level 2 pipelines, and delivers validated science products to the ESO Science Archive through Phase 3.

Next: Science Program Verification

With OpR4 finished, the project has moved on to Science Program Verification (SPV), the last stage before routine survey operations. Full science operations are expected to start in the second quarter of 2026. From then on the nightly DMS data flow will run continuously, and every night of 4MOST observations on VISTA will end up as archived spectra available to the public.

Jakob Walcher, 4MOST Operations Manager, said: "For me the most impressive thing to see was the enthusiasm, competence, and hard work of everyone involved."

Source: Reflections on OpR4, 4MOST project website, 2 March 2026