The Plate Scale of the SODISM Instrument and the Determination of the Solar Radius at 607.1 nm (original) (raw)
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The space instrument SODISM, a telescope to measure the solar diameter
UV/Optical/IR Space Telescopes and Instruments: Innovative Technologies and Concepts V, 2011
PICARD is a satellite dedicated to the simultaneous measurement of the solar diameter, the solar shape, the solar irradiance and the solar interior. These measurements obtained throughout the mission will allow study of their variations as a function of solar activity. The objectives of the PICARD mission are to improve our knowledge of the functioning of our star through new observations and the influence of the solar activity on the climate of the Earth. PICARD was launched on June 15, 2010 on a Dnepr-1 launcher. SODISM (SOlar Diameter Imager and Surface Mapper), an instrument of the PICARD payload, is a high resolution imaging telescope. It was built on an innovative technological concept. SODISM allows us to measure the solar diameter and shape with an accuracy of a few milliarcseconds, and to perform helioseismologic observations to probe the solar interior. SODISM provides continuous observations of the Sun since mid-July 2010. A brief comparison of measurements of solar diameter since the seventeenth century and solar diameter variability are described. In this article, we present the instrumental concept and design and we give an overview of the thermal stability of the telescope. First results from the SODISM experiment are briefly reported (housekeeping and image). are described in details by . These parameters are essential for the understanding of the physics of the Sun. The PICARD micro-satellite mission will provide two to three years simultaneous measurements of the diameter and the solar asphericity at several wavelengths (215, 393.37, 535.7, 607.1 and 782.2 nm), the limb shape at the same wavelengths, the differential rotation, the Total Solar Irradiance (TSI), the Spectral Solar Irradiance (UV, visible and IR), the solar oscillation (helioseismology) and the variation of these quantities as a function of the solar activity. To achieve these very precise measurements on the Sun, it is necessary to stabilize images of the Sun on the CCD (Charge Coupled Device). Sun pointing accuracy of the Satellite, initially insufficient, was significantly improved thanks to a solar tracking sensor with 36 arcseconds precision. The PICARD/SODISM pointing mechanism accuracy is achieved thanks to a piezoelectric system acting on the telescope primary mirror, which allows the Sun's image to be stabilized on the CCD with an accuracy of 0.2 arcseconds. The dimensional stability of the telescope is fundamental to the mastery of solar diameter measurements. Hire were among the first to measure the diameter of the Sun. The seventeenth century measurements covered the Maunder minimum and some time after. All the data were re-examined and are described by E. Ribes [4] who, after removing the seasonal variation of the solar diameter, obtained the annual means. M. Toulmonde has made an analysis of the diameter of the Sun over the past three centuries. gives a list of seventeenth and eighteenth century measurements and shows that discordant measurements are obtained at different times with different instruments. A lot of measurements were obtained from the ground. Atmospheric turbulence affects the quality of the measurements. References to solar diameter observations (modern measurements) are listed in the . We can see again that there is a lack of consistency in results. The main reasons for these differences are also related to the nature of the instruments and the spectral domain of observation.
Monitoring the scale factor of the PICARD SODISM instrument
Astronomische Nachrichten, 2008
The SODISM Telescope of the PICARD Space mission will perform diameter measurements by directly imaging the Sun on a CCD camera. An internal calibration system allows to follow scale factor variations induced by instrument deformations resulting from temperature fluctuations on orbit or others causes. We present this system calibration in this paper and some simulations to how correct observations.
Astronomy & Astrophysics, 2018
Context. In 2015, the International Astronomical Union (IAU) passed Resolution B3, which defined a set of nominal conversion constants for stellar and planetary astronomy. Resolution B3 defined a new value of the nominal solar radius (R N = 695 700 km) that is different from the canonical value used until now (695 990 km). The nominal solar radius is consistent with helioseismic estimates. Recent results obtained from ground-based instruments, balloon flights, or space-based instruments highlight solar radius values that are significantly different. These results are related to the direct measurements of the photospheric solar radius, which are mainly based on the inflection point position methods. The discrepancy between the seismic radius and the photospheric solar radius can be explained by the difference between the height at disk center and the inflection point of the intensity profile on the solar limb. At 535.7 nm (photosphere), there may be a difference of ∼330 km between the two definitions of the solar radius. Aims. The main objective of this work is to present new results of the solar radius in the near-ultraviolet, the visible, and the nearinfrared from PICARD space-based and ground-based observations. Simulations show the strong influence of atmosphere effects (refraction and turbulence) on ground-based solar radius determinations and highlight the interest of space-based solar radius determinations, particularly during planet transits (Venus or Mercury), in order to obtain more realistic and accurate measurements. Methods. Solar radius observations during the 2012 Venus transit have been made with the SOlar Diameter Imager and Surface Mapper (SODISM) telescope on board the PICARD spacecraft. We used the transit of Venus as an absolute calibration to determine the solar radius accurately at several wavelengths. Our results are based on the determination of the inflection point position of the solar limb-darkening function (the most common solar radius definition). A realistic uncertainty budget is provided for each solar radius obtained with the PICARD space-based telescope during the 2012 Venus transit. The uncertainty budget considers several sources of error (detection of the centers of Venus and Sun in PICARD images, positions of Sun and Venus from ephemeris (planetary theory), PICARD on-board timing, PICARD spacecraft position, and optical distortion correction from PICARD images). Results. We obtain new values of the solar radius from the PICARD mission at several wavelengths and in different solar atmosphere regions. The PICARD spacecraft with its SODISM telescope was used to measure the radius of the Sun during the Venus transit in 2012. At 535.7 nm, the solar radius is equal to 696 134 ± 261 km (combined standard uncertainty based (ξ) on the uncertainty budget). At 607.1 nm, the solar radius is equal to 696 156 ± 145 km (ξ), and the standard deviation of the solar radius mean value is ±22 km. At 782.2 nm, the solar radius is equal to 696 192 ± 247 km (ξ). The PICARD space-based results as well as PICARD ground-based results show that the solar radius wavelength dependence in the visible and the near-infrared is extremely weak. The differences in inflection point position of the solar radius at 607.1 nm, 782.2 nm, and 1025.0 nm from a reference at 535.7 nm are less than 60 km for the different PICARD measurements.
The Astrophysical Journal, 2014
On 5 to 6 June, 2012 the transit of Venus provided a rare opportunity to determine the radius of the Sun using solar imagers observing a well-defined object, namely the planet and its atmosphere, occulting partially the Sun. A new method has been developed to estimate the solar radius during a planetary transit. It is based on the estimation of the spectral solar radiance decrease in a region around the contact between the planet and the Sun at the beginning of the ingress and at the end of the egress. The extrapolation to zero of the radiance decrease versus the Sun-to-Venus apparent angular distance allows estimating the solar radius at the time of 1 st and 4 th contacts. This method presents the advantage of being almost independent on the plate scale, the distortion, the refraction by the planetary atmosphere, and on the point-spread function of the imager. It has been applied to two space solar visible imagers, SODISM/PICARD and HMI/SDO. The found results are mutually consistent, despite their different error budgets: 959.85" ± 0.19" (1 σ) for SODISM at 607.1 nm and 959.90" ± 0.06" (1 σ) for HMI at 617.3 nm.
The space instrument SODISM and the ground instrument SODISM II
Space Telescopes and Instrumentation 2010: Optical, Infrared, and Millimeter Wave, 2010
PICARD is a French space scientific mission. Its objectives are the study of the origin of the solar variability and the study of the relations between the Sun and the Earth's climate. The launch is scheduled for 2010 on a Sun Synchronous Orbit at 725 km altitude. The mission lifetime is two years, however that can be extended to three years. The payload consists of two absolute radiometers measuring the TSI (Total Solar Irradiance) and an imaging telescope to determine the solar diameter, the limb shape and asphericity. SOVAP (SOlar VAriability PICARD) is an absolute radiometer provided by the RMIB (Royal Meteorological Institute of Belgium) to measure the TSI. It also carries a bolometer used for increasing the TSI sampling and ageing control. PREMOS (PREcision MOnitoring Sensor) radiometer is provided by the PMOD/WRC (Physikalisch Meteorologisches Observatorium of Davos / World Radiation Center) to measure the TSI and the Spectral Solar Irradiance. SODISM (SOlar Diameter Imager and Surface Mapper), is an 11-cm Ritchey-Chrétien imaging telescope developed at CNRS (Centre National de la Recherche Scientifique) by LATMOS (Laboratoire, ATmosphere, Milieux, Observations Spatiales) ex Service d'Aéronomie, associated with a 2Kx2K CCD (Charge-Coupled Device), taking solar images at five wavelengths. It carries a four-prism system to ensure a metrological control of the optics magnification. SODISM allows us to measure the solar diameter and shape with an accuracy of a few milliarcseconds, and to perform helioseismologic observations to probe the solar interior. In this article, we describe the space instrument SODISM and its thermo-elastic properties. We also present the PICARD payload data center and the ground instrument SODISM II which will observe together with the space instrument.
Picard SODISM, a Space Telescope to Study the Sun from the Middle Ultraviolet to the Near Infrared
2014
The Solar Diameter Imager and Surface Mapper (SODISM) onboard the PICARD space mission provides wide-field images of the photosphere and chromosphere of the Sun in five narrow pass bands (centered at 215.0, 393.37, 535.7, 607.1, and 782.2 nm). PICARD is a space mission, which was successfully launched on 15 June 2010 into a Sun synchronous dawn-dusk orbit. It represents a European asset aiming at collecting solar observations that can serve to estimate some of the inputs to Earth climate models. The scientific payload consists of the SODISM imager and of two radiometers, SOVAP (SOlar VAriability PICARD) and PREMOS (PREcision MOnitor Sensor), which carry out measurements that allow estimating the Total Solar Irradiance (TSI) and the Solar Spectral Irradiance (SSI) from the middle ultraviolet to the red. The SODISM telescope monitors solar activity continuously. It thus produces images that can also feed SSI reconstruction models. Further, the objectives of SODISM encompass the probing of the interior of the Sun via helioseismic analysis of observations in intensity (on the solar disc and at the limb), and via astrometric investigations at the limb. The latter addresses especially the spectral dependence of the radial limb shape, and the temporal evolution of the solar diameter and asphericity. After a brief review of its original science objectives, this paper presents the detailed design of the SODISM instrument, its expected performance, and the scheme of its flight operations. Some observations with SODISM are presented and discussed.
PICARD SOL, a new ground-based facility for long-term solar radius measurements: first results
Journal of Physics: Conference Series, 2013
PICARD SOL is the ground component of the PICARD mission and is operational since March 2011. A set of instruments including the replica of the space instrument and several atmospheric monitors was set up at Calern observatory in order to compare solar radius measured in space and on ground and to better understand and calibrate atmospheric effects on ground based measurements. SODISM II provides full disk images of the chromosphere and photosphere of the Sun in five narrow pass bands ranging from the near ultraviolet to the near infrared. Our preliminary results show a very good instrumental stability. After plate scale calibration using star doublet observations and corrections for atmospheric refraction, first estimates of the mean solar radius at the five wavelengths (393.37, 535.7, 607.1, 782.2, and 1025.0 nm) are deduced from measurements recorded between
Astronomy & Astrophysics, 2017
Context. Despite the importance of having an accurate measurement of the solar disc radius, there are large uncertainties of its value due to the use of different measurement techniques and instrument calibration. An item of particular importance is to establish whether the value of the solar disc radius correlates with the solar activity level. Aims. The main goal of this work is to measure the solar disc radius in the near-UV, visible, and near-IR regions of the solar spectrum. Methods. Three instruments on board the PICARD spacecraft, namely the Bolometric Oscillations Sensor (BOS), the PREcision MOnitoring Sensor (PREMOS), and a solar sensor (SES), are used to derive the solar disc radius using the light curves produced when the Sun is occulted by the Moon. Nine eclipses, from 2010 to 2013, resulted in 17 occultations as viewed from the moving satellite. The calculation of the solar disc radius uses a simulation of the light curve taking into account the center-to-limb variation provided by the Non-local thermodynamic Equilibrium Spectral SYnthesis (NESSY) code. Results. We derive individual values for the solar disc radius for each viewed eclipse. Tests for a systematic variation of the radius with the progression of the solar cycle yield no significant results during the three years of measurements within the uncertainty of our measurements. Therefore, we derive a more precise radius value by averaging these values. At one astronomical unit, we obtain 959.79 arcseconds (arcsec) from the bolometric experiment; from PREMOS measurements, we obtain 959.78 arcsec at 782 nm and 959.76 arcsec at 535 nm. We found 960.07 arcsec at 210 nm, which is a higher value than the other determinations given the photons at this wavelength originate from the upper photosphere and lower chromosphere. We also give a detailed comparison of our results with those previously published using measurements from space-based and ground-based instruments using the Moon angular radius reference, and different methods. Conclusions. Our results, which use the Moon as an absolute calibration, clearly show the dependence of the solar disc radius with wavelength in UV, visible and near-IR. Beyond the metrological results, solar disc radius measurements will allow the accuracy of models of the solar atmosphere to be tested. Proposed systematic variations of the solar disc radius during the time of observation would be smaller than the uncertainty of our measurement, which amounts to less than 26 milliarcseconds.