PAUCam has a built-in auto-guiding system to allow precise tracking of long exposures. When guiding is enabled, 16 detectors integrate light for the scientific exposure while the two outermost detectors are dedicated for guiding. The guiding detectors (G12 and G17) integrate ~1 second and make a fast readout over the Region Of Interest (ROI). Depending on the stellar density of the field 3 to 15 guide stars are analyzed to measure the centroid correction. The error signal is sent to the TCS every 6 to 8 seconds.
The following figure demonstrates how the guider corrects the telescope drift when guiding in closed loop:
The WHT tracks very well for exposures shorter than 3 minutes. The telescope in open loop begins to drift after 5 minutes and the guide star is lost after 10 minutes, while in closed loop the guiding stays aligned after 15 minutes of exposure.