The USNO stellar parallax program

K. Aa. Strand

Although I have been retired from the U. S. Naval Observatory for nearly 18 years, I still follow closely the 61-inch trigonometric parallax program at the Flagstaff station. I am proud, that the program is still functioning, so I want to thank the people at the U. S. Naval Observatory, who are still keeping it going. I am sorry, that Dr. R. S. Harrington, who was an important part of the program, died so suddenly in January 1993.

From its beginning early in 1964, the photographic program, limited to stars in the brightness of 1016, has produced parallaxes for 908 stars.

Changes in the program over the years, have led to a typical parallax precision for a completed series, which has improved by a factor of two from +/- 4 mas to +/- 2 mas.

The program was extended to stars as bright as Sirius, by the use of neutral-density magnitude compensating filters. There are in the most recent list 34 stars, with formal relative parallax uncertinaties between +1.2 and +2.0 mas. A comparison with Hipparcos parallaxes show no systematic differences.

The Xth list in the program will include 105 stars and bring the total program to 1013 stars. It will also be the end of the photographic program and the retirement of the SAMM measuring engine.

The program will still be continued, with the parallaxes determined by the CCD, which have already shown accuracies of +/- 1.0 mas for a completed series.