Under the CSO weather multiplexing policy, programs that require excellent weather conditions, typically 350 μm observations, are scheduled in parallel with programs that are not so demanding, typically 230 GHz observations. Each afternoon, the CSO Manager, or a designated staff member, will decide which program has priority. The decision is based on the sky transparency, other meteorological data, and weather forecasts. The nominal threshold for the zenith optical depth τ225 GHz = 0.06. Any questions about the priority decision should be directed to the CSO Manager.
CSO safety rules require the presence of at least two observers at the telescope at any time. For weather multiplexed programs, therefore, each program must provide at least one observer for all scheduled time. (Non multiplexed programs must provide two observers.) If the priority program has only one observer, an observer from the other program must go to the telescope as well. Observing groups are encouraged to be collegial. The two observer rule will be enforced for all observing programs.