Measurement of pulsar parallaxes by VLBI.

Results of a program of astrometric observations of pulsars by very long baseline interferometry are presented, including two measurements of annual parallaxes. The absence of relative motion of two quasar pairs is also observed. Observations were made at 1.66 GHz over a period of 18 months. Each program source was observed relative to a nearby reference source, and then the observed phases of each were corrected for nutation, precession, polar motion, and the effects of the troposphere and ionosphere. For pulsars PSR 0823 + 16 and PSR 0950 + 08, parallaxes of 2.8 + or - 0.6 and 7.9 + or - 0.8 mass were obtained, respectively. In a combination with published measurements of pulse dispersion, the results yield average free-electron densities of 0.054 + or = 0.011 and 0.023 + or 0.002 cu cm along the lines of sight to these pulsars.