The Plate Scale of the SODISM Instrument and the Determination of the Solar Radius at 607.1 nm

Abstract

International audienceKnowledge of the Solar Diameter Imager and Surface Mapper (SODISM) plate scale is a fundamental parameter for obtaining the solar radius. We have determined the plate scale of the telescope on the ground and in flight onboard the Picard spacecraft. The results show significant differences; the main reason is that the conditions of observation are not the same. In addition, the space environment has an impact on the performance of a metrology instrument. Therefore, calibration in space and under the same conditions of observation is crucial. The transit of Venus allowed us to determine the plate scale of the SODISM telescope and hence the absolute value of the solar radius. The transit was observed from space by the Picard spacecraft on 5 - 6 June 2012. We exploited the data recorded by SODISM to determine the plate scale of the instrument, which depends on the characteristics of optical elements (mirrors, filters, or front window). The mean plate scale at 607.1 nm is found to be 1.0643 arcseconds pixel−1 with 3×10−4 RMS. The solar radius at 607.1 nm from 1 AU is found to be equal to 959.86 arcseconds

Solar irradiance observations with PREMOS filter radiometers on the PICARD mission: In-flight performance and data release

“…The UV filters are of primary importance to ozone and molecular oxygen chemistry of planetary atmospheres. The filter design of the UV channels at 215 nm, and the visible-light (VIS) and NIR (NIR) channels are similar to that of SODISM (Meftah et al 2014b). The filter designs were chosen to measure the continuum, by minimizing interference from spectral lines.…”

“…2. The SOlar DIameter and Surface Mapper (SODISM) instrument was designed to measure the solar diameter, to monitor the solar asphericity, and to perform helioseismologic observations (Meftah et al 2014b). The SOlar VAriability PICARD (SOVAP) instrument was devoted to observation of the TSI (Conscience et al 2011;Meftah et al 2014a).…”

Ground-based measurements of the solar diameter during the rising phase of solar cycle 24

“…The SODISM II ground-based instrument and solar radius measurements For the space mission, two identical units were developed. One unit named SODISM (Meftah et al 2014b) was launched on 15 June 2010, and the second was installed some months later at Calern Observatory (N 43 • 44 53 latitude, E 6 • 55 36 longitude and altitude of 1271 m). This ground-based unit, named SODISM II, is placed in a vacuum tank (Fig.…”

“…Another way for estimating SODISM II plate-scale is to use the solar radius obtained by the SODISM space instrument during the Venus transit as reference (Meftah et al 2014a). The synthesis associated with these three approaches is given in Table 3.…”

“…The SODISM II instrument is thermally controlled and appears to be robust against changes on the order of few microns or a few degrees of temperature (Meftah et al 2014b). Temperatures of the instrument (housekeeping) are measured, and their variations (thermal effects) introduce an uncertainty smaller than 10 mas on the determination of the solar radius (for each wavelength).…”