  With the Thomson scattering (TS) system in KSTAR, temporal evolution of
electron temperature ($T_e$) is estimated using a weighted look-up table method
with fast sampling ($1.25$ or $2.5$ GS/s) digitizers during the 2014 KSTAR
campaign. Background noise level is used as a weighting parameter without
considering the photon noise due to the absence of information on absolute
photon counts detected by the TS system. Estimated electron temperature during
a relatively quiescent discharge are scattered, i.e., $15$\% variation on $T_e$
with respect to its mean value. We find that this $15$\% variation on $T_e$
cannot be explained solely by the background noise level which leads us to
include photon noise effects in our analysis. Using synthetic data, we have
estimated the required photon noise level consistent with the observation and
determined the dominant noise source in KSTAR TS system.
