Preliminary results of a balloon flight of the solar disk sextant

Preliminary results of a balloon flight on October 11, 1991, of the solar disk sextant (SDS) experiment are reported. The SDS is an instrument which measures the solar diameter at different orientations with respect to the solar polar axis. Fitting straight lines through two fixed-angle data sets with time as the independent variable yields slopes of (7.1 +/ - 1.5) x 10 exp -3 and (6.7 +/- 1.6) x 10 exp -3/mas s, consistent with the value of 6.47 x 10 exp -3/mas s expected from the earth's approach to the sun due to the orbital motion toward perihelion. Upon the instrument's rotation on its axis a sinusoidal component of the diameter measurement was observed in each rotation cycle, with a variable amplitude of about 150 mas. The present result is epsilon of (5.6 +/- 6.3) x 10 exp -6, about 30 deg offset from the polar-equator position. The absolute diameter obtained by means of the FFT definition is found to be 1919.269 +/- 0.240 arcsec or 1919.131 +/- 0.240 arcsec, depending on the orientation mode of the measurement.