To maintain high observing efficiency and to prolong spacecraft sub-systems, it is advantageous to identify thermally-useful science targets (generally to cool particular subsystems) in limited (and time-dependent) areas of the sky. The sky locations of cooling regions vary throughout the year but are more commonly found at lower ecliptic latitudes where targets have a larger range of favorable pitch angles that can maximally cool subsystems. Nominally, Chandra planners arrange observations of non-CCT targets (General Observer (GO) peer review-approved, Guaranteed Time Observer (GTO), Director's Discretionary Time (DDT), and calibration (CAL) targets) in an order that supports cooling of subsystems. However, when such sources are not available at the desired location, we occasionally draw sources from a catalog of Chandra Cool Targets (CCTs).
CCTs were obtained through the inaugural community-wide call for white papers issued in 2018. The call resulted in a catalog of 18697 sources with a broad range of observing configurations. Chandra has observed an average of just over 1 Ms per year of CCTs. As the utility of the existing CCT program reaches its end, we are seeking to replace the CCT with new targets and updated observing configurations.
Policies associated with CCTs can be found in the Call for White Papers and CfP.