Astronomy and Astrophysics – Astrophysics
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000dda....31.1403r&link_type=abstract
American Astronomical Society, DDA Meeting #31, #14.03; Bulletin of the American Astronomical Society, Vol. 32, p.870
Astronomy and Astrophysics
Astrophysics
Scientific paper
The deflection of light by the Sun provides one of the classical tests of general relativity. The deflection is closely related to the Shapiro time delay and, in the context of the PPN formalism, each has an overall coefficient of (1 + γ ) / 2. The current published set of strong determinations of the PPN parameter γ are based on Viking radio delay (Reasenberg et al., 1979, ApJL: 1.000 +- 0.002), VLBI data intended for geodesy (Robertson et al., 1991, Nature: 1.0002 +- 0.002), VLBI data for relativity (Lebach et al., 1995, Phys. Rev. Lett: 0.9996 +- 0.0016), and Hipparcos (Froeschle et al., 1997, ESA SP-402: 0.997 +- 0.003). FAME will observe 40 million stars about 1000 times each during a 2.5 year mission, and a 2.5 year extended mission is planned. For V-mag = 9, its nominal measurement accuracy is 1.1 mas along the great circle defined by the spin direction, which remains about 45 deg from the Sun direction as it precesses around the Sun. Although FAME observations are made as close as 45 deg from the Sun, those have zero sensitivity because they are made in the wrong direction. At 90 deg from the Sun, the deflection seen from Earth is 4.1 mas, which would yield a SNR of 3.7 for FAME, were it not for the ``cosine term." The most sensitive measurements are made when the spacecraft has rotated the view port 45 deg from the observations closest to the Sun -- Sun-star angle of 60 deg -- yielding a single-measurement SNR of 4. A preliminary propagation-of-error analysis shows that a 2.5 year FAME mission has the sensitivity to measure γ with an uncertainty of 3.4 10-5, using all stars from V-mag 5 to 15. However, as was the case for Hipparcos, correlations and systematic errors will result in a larger uncertainty, perhaps 3 to 10 fold larger. This work has been supported by the Smithsonian Astrophysical Observatory. FAME is a NASA MIDEX mission with the PI at the U.S. Naval Observatory.
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