For astrometry, 30 and 45 fixed angles reflecting prisms, with zerodur faces are used. The zerodur prisms maintain a very high angular stability in the long term, whereas in the case of the solar diameter measurements a good stability is required just for the few minutes of the Sun's transit through the almucantar. Therefore, for all the observations presented here a front prism formed by two zerodur plates making an adjustable dihedron had been used. The Variable Prism is the 1986 CERGA prototype, working by a system of coils compensation. The functional zenith distances interval so defined extends from about 27 to 58.
The observation consists in taking 46 frames of the twin images of the portion of the solar limb crossing the almucantar. Each frame lasts 20 ms and the observations are taken with regard to the upper and lower limbs. The images are acquired by a COHU 4710 CCD camera. The camera is IR sensitive (4000 - 10000 Å). The effective wavelength is 6400 Å and the bandpass is 3000 Å. Every other vertical line is digitized, producing 256 lines and 512 columns. The vertical pixel size corresponds to 05 and thus about 15 of the solar semi-diameter is imaged.
Following the Observatoire de Paris digital acquisition system software (Sinceac et al. 1997), the solar limb edge, at each CCD line, is represented by the inflection point of the luminosity curve along the line. The observed solar limb can be adequately represented by a parabola that fits the set of inflection points. In each frame, the characteristic parabola's summit admits a horizontal tangent parallel to the CCD columns and is recorded as (), where X and Y are the abscissa and the ordinate while t is the corresponding UTC. The two series of summits, from the direct and reflected twin images, are adjusted by least squares to two crossing lines in the () plane. The location of the point of intersection of these lines gives the instant of crossing of the almucantar by the observed limb. The observation of the crossing of the same almucantar by the second border enables the determination of the Sun's diameter from the difference between the two instants.
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