In weather conditions that the observatory staff deem unsuitable for the scheduled observation, the observing time reverts from the scheduled observation to Director's Time. The observatory staff will make an effort to involve the scheduled observer in this decision. Experience suggests that it is generally clear to everyone when weather conditions are not suitable, and that no one wants the telescope collecting useless data.
The basic criteria that will be used to determine if the weather is unsuitable is if the observation cannot reasonably address any of it aims. In particular, for observations that are noise-limited, the weather is deemed to be unsuitable if the sensitivity of the observation is halved. This halving may be because:
- The system temperature has doubled.
- Phase decorrelation within 5 minutes would lead to a halving of sensitivity.
- Wind-induced pointing errors would reduce the sensitivity by a factor of 2.
Once overridden, the rules for allocating Directors Time are followed in determining how best to use of this time.
Observers, particularly those working at 3mm, are encouraged to plan for the possibility that their observing run could be "rained" out. These observers are encouraged to come prepared with a back-up observation to make use of the Directors Time. Preferrable this back-up should do similar science at a longer wavelength.
Original: Bob Sault (28-Jan-2006)