1. The Observation

Cen X-3 was observed on December 9, 1990 from 13:10:12 to 13:38:20 UT yielding about 1600 s of usable data. At the time of the observation, the pulsar was at orbital phase $\sim$ + 0.27, measured from mid-X-ray eclipse. Because this was a daytime shuttle observation the star cameras were switched off and an aspect solution was not available. However, the position of the source relative to the center of the A0 pixel was deduced by comparing the ratios of counting rates in the central and outer pixels with those predicted by a ray tracing model. It was found that the source was 1.4 arcmin off axis in A0 and 2.2 arcmin off axis in B0. Thus, in the B telescope, much of the flux fell on the mask separating the pixels.