The Chandra X-ray Observatory (CXO) is in a highly elliptical orbit that enables high observing efficiency and flexible pointing (avoiding the Sun, Moon, Earth and anti-sun direction). However, there are significant restrictions in the durations of observations at many solar pitch angles (i.e., angles between viewing directions and the direction to the sun). These restrictions are necessary to avoid overheating (or excessive cooling) of various observatory sub-systems.
In October 2018 the CXC issued a white paper call for lists of astrophysical objects (Cool Attitude Targets, CATs) that can be included in the schedule to provide spacecraft cooling. CATs are intended to produce scientifically useful data while the observatory is cooling (i.e. the observatory will never be pointed at blank sky to cool off). The mission planning team started using CATs for spacecraft cooling in December 2018. CAT data does not have associated proprietary time.
Policies associated with CATs can be found in the CfP.