In addition to measuring gravitational-wave strain, the LIGO detectors record about 200,000 other data channels per detector, referred to as auxiliary channels. These channels measure the internal state of the instrument and the physical environment around the detectors. Auxiliary channels are used to check data quality. In some cases, an auxiliary channel will record a source of noise, like scattered light or the mains power frequency, allowing LIGO to clean up the strain data by subtracting out this noise. In other cases, an auxiliary channel can act as a witness to a glitch in the detector, identifying when it is misbehaving so that analysts know not to trust that part of the data (called “vetoing”). However, this method can be flawed if auxiliary channels also pick up disturbances in the gravitational wave channel, i.e., an astrophysical signal can appear in both the gravitational wave channel and an auxiliary channel. Channels that might record excess power originating in the gravitational wave channel are considered "unsafe" for vetoes, while those that are not coupled in this way are classified as "safe." Only "safe" channels are utilized when vetoing gravitational wave candidates. Studying the connections between auxiliary channels and glitches can ultimately guide improvements to the detector to eliminate the noise.
Each auxiliary channel in Gravity Spy 2.0 has a page on this wiki to describe it. The channels are named systematically: the first part of the name is the subsystem of the detector that is being measured and subsequent parts describe the location and type of sensor and the kind of processing. That naming convention is used to organize the pages on the Wiki. To see the individual channels, please browse the Channels folder and subfolders.
The subsystems are as follows: