PRELIMINARY-RESULTS OF A BALLOON FLIGHT OF THE SOLAR DISK SEXTANT

This paper reports preliminary results of a balloon flight on 1990 October 11 of the solar disk sextant (SDS) experiment. The SDS is an instrument which measures the solar diameter at different orientations with respect to the solar polar axis. The flight, which lasted 9.5 hr, yielded useful data for approximately 6 hr, which included 26 full rotations of the instrument about its axis in addition to three 20 minute periods at fixed angles. For the two fixed angle data sets reduced, the probable error is of the order of 15 mas per 13 ms measurement, with a small systematic change from one set to the next. Fitting straight lines through these data sets with time as the independent variable yields slopes of (7.1 +/- 1.5) x 10(-3) and (6.7 +/- 1.6) x 10(-3) mas s-1, consistent with the value of 6.47 x 10(-3) mas s-1 expected from the Earth's approach to the Sun due to the orbital motion toward perihelion. Upon rotating the instrument on its axis a sinusoidal component of the diameter measurements was observed in each rotation cycle, with a (variable) amplitude of approximately 150 mas. This variation, which we think is due to inadequacies of the wedge holder to keep the objective wedge flats at a constant angle for all telescope orientations, greatly complicates the determination of the solar oblateness for the present instrument configuration, resulting in a relatively large error. The present result is epsilon = (5.6 +/- 6.3) x 10(-6), approximately 30-degrees offset from the polar-equator position. The absolute diameter obtained by means of the FFT definition is found to be 1919.269" +/- 0.240" or 1919.1.31" +/- 0.240", depending on the orientation mode of the measurement.