  C-Band Mueller matrices for the Green Bank Telescope are presented here which
enable on-sky Stokes parameters for point sources at the beam center to be
determined. Standard calibrators, 3C138 and 3C286, were observed using the
Spider program to steer the telescope across a broad range of Right Ascensions
on both sides of the zenith transit. For this analysis, only the observations
at the peak of the Spider pattern were used rather than the full sweep of the
runs. Therefore, the results presented here only apply to point sources at the
beam center. The Mueller matrices are shown to vary with frequency and with use
of the Hi-Cal or Lo-Cal noise diodes, due to the relative calibration gain
between the X and Y components of the feed.
  However, the relative calibration gain can be determined from observations of
a source with known polarization. Correcting the data for the relative
calibration gain prior to data analysis allows for use of a frequency
independent Mueller matrix. This generic Mueller matrix is shown to provide
reliable C-Band polarization measurements.
