For WEIGHTED case uses method given in Weisskopf, Kahn, and Sutherland ( Ap.J. 199, L147 (1975)). This is corrected for the extra statistical noise contribution at zero lag, and can result in estimated ACF values > 1. The unweighted case uses a similar method, omitting the weights.
UNBIASSED results are normalized by * 1 / (NPTS - LAG) BIASSED results are normalized by * 1 / NPTS (suppressing large lags).