HERA Transverse Polarimeter Operation in 1996

In 1996 HERA is expected to deliver polarized positron beam routinely for the HERMES experiment. The transverse beam polarization is measured with a Compton polarimeter. A pair of spin rotators provides longitudinal spin polarization at the experiment. In 1995, the value of the polarization during HERMES data taking varied between 45% and 70%, with typical values near 53%. The beam polarization was measured every minute with a statistical error of 2% to 4%. The precision of the polarization measurement is of great importance for the HERMES experiment since the error propagates directly into the physics result of interest. Measurements of the polarization build-up time, in combination with Monte Carlo studies and the offline analysis of the data acquired, indicated that the relative systematic error of the polarization was approximately 5% in 1995. For the 1996 run, both statistical and systematic uncertainties are expected to improve.