commensal mode

The phrase commensal mode has grown common for a telescope mode of operation such that two different observing goals are achieved simultaneously. One common current use is for adaptation and use of radio telescopes to simultaneously carry out a program to survey the sky for static information (e.g., intensity mapping) and for transients that happen to be in the same location. For sensitivity, long observations are needed for static maps of the sky, but for short transients, short observations need to be recorded. Since radio telescopes can produce enormous streams of data, more than can be saved in its entirety, the commensal mode strategy means streaming the data into two different mechanisms to process it for recording: one that integrates long observations, and another that triggers on interesting short-term transients (e.g., fast radio bursts, or pulsar pulses, or search for extraterrestrial intelligence monitoring) and records them. Having entire radio telescopes (and arrays) with narrow beams necessary to identify sources, sitting waiting for transients is very inefficient. The term piggyback is sometimes used as an alternate term to explain the meaning of commensal mode.

The notion can be used outside radio astronomy and in principle, outside survey work: for example, simultaneously surveying the data stream for transients during individual targeted observations. Sensors such as CCDs potentially can offer data streams that can be broken down using multiple time intervals, recording the data in multiple ways for multiple goals. Any major research telescope has some motivation to consider this, perhaps the highest motivation being for space observatories with limited lifespans. Slew surveys, surveys collecting data while the telescope is slewed could be classified as a kind of commensal mode.