  Over the past 40 years, observational surveys have established the existence
of a tight relationship between a star's age, rotation period, and magnetic
activity. This age-rotation-activity relation documents the interplay between a
star's magnetic dynamo and angular momentum evolution, and provides a valuable
age estimator for isolated field stars. While the age-rotation-activity
relation has been studied extensively in clusters younger than 500 Myr,
empirically measured rotation periods are scarce for older ages. Using the
Palomar Transient Factory (PTF), we have begun a survey of stellar rotation to
map out the late-stage evolution of the age-rotation-activity relation: the
Columbia/Cornell/Caltech PTF (CCCP) survey of open clusters. The first CCCP
target is the nearby ~600 Myr Hyades-analog Praesepe, where PTF has produced
light curves spanning more than 3 months and containing >150 measurements for
~650 cluster members. Analyzing these light curves, we have measured rotation
periods for 40 K & M cluster members, filling the gap between the periods
previously reported for solar-type Hyads (Radick et al. 1987, Prosser et al.
1995) and for a handful of low-mass Praesepe members (Scholz et al. 2007). Our
measurements indicate that Praesepe's period-color relation undergoes at
transition at a characteristic spectral type of ~M1 --- from a well-defined
singular relation at higher mass, to a more scattered distribution of both fast
and slow-rotators at lower masses. The location of this transition is broadly
consistent with expectations based on observations of younger clusters and the
assumption that stellar-spin down is the dominant mechanism influencing angular
momentum evolution at ~600 Myr. In addition to presenting the results of our
photometric monitoring of Praesepe, we summarize the status and future of the
CCCP survey.
