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We present time series measurements of chromospheric activity for more than2600 main sequence and subgiant stars on the California Planet Search (CPS)program with spectral types ranging from about F5V to M4V for main sequencestars and from G0IV to about K5IV for subgiants. The large data set of morethan 44,000 spectra allows us to identify an empirical baseline floor forchromospheric activity as a function of color and height above the mainsequence. We define $\Delta S$ as an excess in emission in the Ca II H\&K linesabove the baseline activity floor and define radial velocity jitter as afunction of $\Delta S$ and \bv\ for main sequence and subgiant stars. Althoughthe jitter for any individual star can always exceed the baseline level, wefind that K dwarfs have the lowest level of jitter. The lack of correlationbetween observed jitter and chromospheric activity in K dwarfs suggests thatthe observed jitter is dominated by instrumental or analysis errors and notastrophysical noise sources. Thus, given the long-term precision for the CPSprogram, radial velocities are not correlated with astrophysical noise forchromospherically quiet K dwarf stars, making these stars particularlywell-suited for the highest precision Doppler surveys. Chromospherically quietF and G dwarfs and subgiants exhibit higher baseline levels of astrophysicaljitter than K dwarfs. Despite the fact that the \rms\ in Doppler velocities iscorrelated with the mean chromospheric activity, it is rare to see one-to-onecorrelations between the individual time series activity and Dopplermeasurements, diminishing the prospects for correcting activity-inducedvelocity variations.

Chromospheric Activity and Jitter Measurements for 2630 Stars on the California Planet Search