The U.S. Naval Observatory program on trigonometric stellar parallaxes with the 61-in. astrometric reflector has been in progress since 1964. To date 201 definitive and 8 preliminary negligible parallaxes have been published, including the UBV photometry of these stars. Data for an additional 35 stars are still unpublished.
The mean error in position for an image of unit weight is 1.2 μ or 0″.016, of which 0.8 μ originates from the measuring error of the automatic measuring machine.
An error of the parallax of 0″.004 can usually be obtained with 40 or fewer plates and with an average parallax factor of 0.7.
A statistical investigation of the derived parallaxes shows that they are free from significant internal systematic errors.