New observations show a small difference between the sun's polar and equatorial limb darkening functions. This excess equatorial brightness varies in time and can be of sufficient magnitude to account for the solar oblateness inferred by Dicke and Goldenberg from their measurements. This removes the serious consequence of their work for Einstein's general theory of relativity. The problems of a solar-edge definition and the derivation of a solar mass quadrupole moment are discussed.
An f/100, 12.2-m focal length photoelectric telescope designed specifically for daytime astrometry of objects near the sun is now operative at its Tucson, Arizona, site. The design goal was to achieve accuracies of order 0.001 sec of arc in field position measurements of stars. To accomplish this, many features reducing systematic and random errors are employed, including Schupmann medial telescope optics, compensation for lateral color aberration, apodization for reduction of diffracted light, and use of an accurately measured solar diameter for calibrating the field.
The limitations arising from scattered light from lenses and mirrors in a program of daytime astrometry near the sun are discussed. Measurements were made of the angular distribution of the scattered radiance for (1) a coronagraphic lens, (2) a 2.54-cm thick coronagraph quality window, and (3) flat quartz mirrors. Results are also given for the optical elements after they have been exposed in a vacuum for 48h. The results indicate that the accuracy of a daytime astrometry program would be determined by the shot noise of the scattered light produced by the refracting elements and the sky, each producing equal contributions.
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