The Space instrument SODISMand the Ground instrument SODISM II

M. Meftaha,*, M. Meissonniera, A. Irbaha, S. Abbakia, P. Assusb, E. Bertrana, J.P. Duboisc,E. Ducourta, C. Dufoura, J.P. Marcovicia, G. Poieta, A.J. Vieaua and G. Thuilliera

a CNRS/INSU, LATMOS-IPSL, Universite Versailles St-Quentin, Guyancourt, Franceb CNRS/OCA, Parc Valrose, Bat. H. Fizeau, Nice, France

c CNRS/IAS, Universite Paris XI, Orsay, France

ABSTRACT

PICARD is a French space scientific mission. Its objectives are the study of the origin of the solar variabilityand the study of the relations between the Sun and the Earth’s climate. The launch is scheduled for 2010 ona Sun Synchronous Orbit at 725 km altitude. The mission lifetime is two years, however that can be extendedto 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 (SOlarVAriability PICARD) is an absolute radiometer provided by the RMIB (Royal Meteorological Institute of Bel-gium) 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 Meteorolo-gisches 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-Chretien imaging telescope devel-oped at CNRS (Centre National de la Recherche Scientifique) by LATMOS (Laboratoire, ATmosphere, Milieux,Observations Spatiales) ex Service d’Aeronomie, associated with a 2Kx2K CCD (Charge-Coupled Device), takingsolar images at five wavelengths. It carries a four-prism system to ensure a metrological control of the opticsmagnification. SODISM allows us to measure the solar diameter and shape with an accuracy of a few milliarc-seconds, and to perform helioseismologic observations to probe the solar interior. In this article, we describe thespace instrument SODISM and its thermo-elastic properties. We also present the PICARD payload data centerand the ground instrument SODISM II which will observe together with the space instrument.

Keywords: Solar Astrometry, Microsatellite, Telescope, Sun, Diameter

1. INTRODUCTION

The solar mission PICARD will simultaneously measure several key parameters of the Sun. These parametersare essential for the understanding of the physics of the Sun. They are: the diameter and the solar asphericityat several wavelengths (215, 393.37, 535.7, 607.1 and 782.2 nm), the limb shape at the same wavelengths, thedifferential rotation, the Total Solar Irradiance (TSI), the Spectral Solar Irradiance (UV, visible, IR), the solaroscillation (helioseismology) and the variation of these quantities as a function of the solar activity.

The solar diameter is measured since 3.5 centuries and its variation with the solar activity has been thesubject of many researches. Data has shown than the diameter variation is in phase, anti-phase, or withoutany correlation with the solar activity. As the data were gathered from ground, the atmosphere was claimedto be the cause of the above inconsistencies and discussed in Thuillier et al. [1]. Very few observations outsidethe atmosphere exist showing variation from 30 to 400 milliarcseconds (mas). One of the primary objectivesof the mission is to measure the solar diameter and shape with an accuracy of a few mas in orbit to avoid allatmospheric influence. These measurements as listed above are key parameters to validate models of the Sun, i.e. the functioning of our star. This is why, PICARD mission has also a modeling activity concerning the physicsof the Sun with application to climate physics. The scientific objectives of the PICARD mission are describedin details by [2].

* Corresponding authorE-mail address: Mustapha.Meftah@latmos.ipsl.fr

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Given that the scientific objectives require measurement as a function of solar activity, it is imperative thatthe SODISM instrument maintains a very high level of stability over the duration of the mission.

A typical structure stability requirement is about a few microns for a telescope dimension of 0.6 meter. CNRShas designed, manufactured and tested a very high dimensional stability telescope using advanced materials ofhigh stability and structural performance.

Among these materials, the CC (Carbon/Carbon) composite was selected due to its high technical perfor-mance (coefficient of thermal expansion less than -7.10-7 m/m/K), its long-term stability, and its industrialmaturity. CC with invar will constitute the basic structure of the SODISM telescope. Optics will make use ofsilica and zerodur.

2. THE MISSION CONSTRAINTS

The PICARD mission will be operated during the rising phase of the solar cycle 24 allowing us to study therelationship between all the measurements simultaneously gathered, in particular between the diameter and thesolar luminosity. The orbit is chosen to allow periods of Sun visibility as long as possible for the helioseismologicmeasurements (internal structure of the Sun) and to provide the best conditions for the instrument thermalstability.

A SSO (Sun Synchronous Orbit) with an ascending node at 06h00, an altitude of 725 km (period of 98minutes) and an inclination of 98 degrees was selected. The duration of eclipses will not exceed 20 minutes withthese orbit parameters, in particular in December.

The mission lifetime is two years; however a longer mission is expected especially given the late start of cycle24. The first operations and measurements with SODISM will be performed as soon as possible after the launch,allowing us to adjust the parameters of the thermal regulation of the instrument.

3. THE SPACE INSTRUMENT SODISM

SODISM is an 11-cm diameter Ritchey-Chretien telescope associated with a 2048x2048 pixels CCD detector.The Ritchey-Chretien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolicprimary). It is free of coma and spherical aberration at a flat focal plane. The instrument field of view andits angular resolution are respectively about 35 arcminutes and 1.06 arcseconds. Given that the solar shapein the photospheric continuum is nearly perfectly circular, the image on the detector provides 4000 diametermeasurements registered at different positions within each pixel. Using this set of measurements, the pixellisationeffect is averaged and the diameter can be obtained with a precision of a few mas, with however limitations dueto the noise, flatfield and optical distortion, which have to be independently calibrated. SODISM observes theSun in five wavelength domains by using interference filters.

Table 1. SODISM main characteristics.

Telescope type Ritchey-ChretienFocal length 2626 mmMain entrance pupil 90 mmVolume 670 (d) x 308 (w) x 300 (h) mm3

Weight 27.7 kgField of view 35 arcminutesAngular resolution 1.06 arcseconds/pixelPower consumption 43.5 W nominalData rate 2.2 Gbits per day

The SODISM optical system (shown in Figure 1) consists of an entrance window (curved window in silica backcoated with a density filter), a primary mirror M1, a secondary mirror M2 both in zerodur, several interference

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filters and a CCD. The Sun image is stabilized on the detector by a three-level optical system, i) stars are used toachieve a prepointing within a few degrees, ii) a four-quadrant system achieves a pointing within ±36 arcsecondswith a maximum drift of ±5 arcseconds per second, iii) a part of the image provided by each prism is usedthrough a four-diode system whose signals allow centering the Sun image on the CCD with ±0.2 arcsecondsprecision. The optical system includes four optical prisms that will create four auxiliary images located in eachcorner of the CCD. The reference instrument scale factor [3] is provided by the prisms at few mas precision.The four prisms’ temperature must be stable to better than 0.5oC. The absolute angular reference is providedby using stars having an angular distance comparable to the Sun diameter. These observations are made everythree months by rotating the spacecraft toward couples of bright stars (9 couples are selected). Using 20 images,an accuracy of few mas can be achieved.

Figure 1. SODISM optical scheme. Figure 2. SODISM exploded view.

3.1 The telescope characteristics

The PICARD payload is operated by a global on-board electronics named the PGCU (PICARD GestionCharge Utile). Table 1 summarizes the main characteristics of the SODISM instrument. Solar images arerecorded every minute with SODISM and processed on-board. They are compressed before being transmitted.The SODISM experiment package is shown in the Figure 2.

3.2 The telescope structure design

SODISM consists of a monolithic structure in which three invar optical plates (M1 mirror, M2 mirror and CCDplates) are linked up by a CC tube. A titanium plate holds the two filters wheels. M1 mirror plate and M2 areassembled together with the CC tube using some glue. The monolithic structure provides the following functions:support of the telescope mirrors, support of the mechanisms and interface with the platform. A cylindrical baffleprotects the main telescope beam from straight light effects. Two circular diaphragms are located in the maintelescope beam; their internal diameter is sharpened for stray light purposes. A disc diaphragm is located nearthe mirror M2 for central obstruction purposes. The secondary structures include the following assemblies:covers, thermal radiators and support devices for the MLI (Multi Layer Insulation) blankets. CCD proximityelectronics are located on the secondary structure of the telescope.

3.3 The telescope thermal design

SODISM requires a very high level of thermo-mechanical stability to obtain the desired precision on diametermeasurements. The telescope relies on a combination of passive and active means of thermal control. A complexthermal control is implemented on the telescope and consists of i) an efficient thermal protection (MLI) of theoverall telescope, ii) a protection from solar illumination, iii) the implementation of a complex heating systemmonitored by thermal sensors.

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20 heating lines are used to thermally stabilize the instrument at 20oC ±1oC. The CCD will be thermallyregulated at -13oC ±0.2oC to have no area deformations and low noise in images. The temperature of theinterference filters will be regulated at 20oC ±5oC to avoid shift of their central wavelength and bandwidth. Thetemperature of the entrance window will be kept between 0 and 40oC (during operating mode). The entrancewindow is thermally insulated from the mechanical structure to avoid optical lens effects due to its deformationsby thermal constraints. Temperature gradients from center to edge and inside the window are also minimized.These gradients will be less than 2oC and 1oC respectively. The temperature of the prisms will be kept between10oC and 35oC. However, the thermal stability of each prism will be better than 0.05oC per minute while thethermal gradient will be less than 1oC. The temperature of each prism is measured within 0.3oC accuracy. Theresulting temperature range in orbit is foreseen to be close to -35oC and 50oC in non-operating mode, avoidingany degradation of structure, electronics and optical parts.

A thermal model has been developed allowing us to foresee the temperature variations and the gradients,which may be experienced by the telescope in some cases. The thermal load cases are then implemented in thestructural model to analyze the thermo-elastic behavior of the telescope. This model is also used for the thermalbalance test with optical performance.

The thermal control includes the hardware necessary to control the temperature of all components of thetelescope, including the structure, the mirrors, the mechanisms and also the temperature regulation algorithm.The temperature control provides several independent temperature controlled areas located over the structure.Each area consists of a set of heaters, mounted in the same electrical circuit, over which the heating poweris adjusted based on the temperature measured through a temperature sensor located in the vicinity of theheaters. Principal temperature controlled areas are redounded. The temperature set point of each area shall bereprogrammable and will be adjusted during the commissioning phase.

The temperature sensors are used for the temperature regulation process, as well as for temperature house-keeping over the telescope. Analog temperature sensors have been used (for high accuracy and high stability),from the following type: AD590MF/883B (Flat pack). They have been calibrated in order to offer an accuracyof ±0.1oC. There are 45 temperature sensors on the entire telescope and 3 for the electronics units.

The thermal radiators have been oriented in the direction of the space (avoiding solar flux) in order to offer thebest heat rejection capability. The thermal radiators’ external surfaces have been coated with SG120FD whitepaint and Second Surface Mirror coating. The mechanical parts on the optical path are black anodized accordingto ESA PSS 01/703. A lot of thermal shunt has been utilized at certain interfaces in order to provide a highconductive thermal coupling with a reduced structural stiffness. Thermal shunt in copper has been developedby CNRS.

3.4 The mechanisms

SODISM uses several mobile mechanisms such as i) a system to unlock the door at the entrance of theinstrument, ii) a system to open/closed the door using a stepper motor, iii) two filters wheels using a steppermotor and a mechanical shutter.

For the fine pointing, SODISM uses three piezoelectric devices acting on the primary mirror M1. Pointingerror signal given by the photodiodes (X axis, Y axis) is transformed into a signal for the three piezoelectricactuators which moves the primary mirror around its summit.

3.4.1 System to unlock the door

The system of unlocking keeps the SODISM door during the launch. This mechanism (Titanium Nickel pinpuller use for rearmament) retracts the engagement pin with 111 N of force and 9.5 mm of stroke. The openingtime is less than 24 milliseconds.

3.4.2 System to open/closed the door

The system to open/closed the door using a stepper motor is a cover of the entrance windows in order toprotect the telescope from molecular contamination. The opening time is less than 70 seconds.

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3.4.3 Two filter wheels

Two filter wheels operated by a stepper motor are used to choose the spectral domain of observation of thetelescope. Each filter wheel has 5 positions (filter diameter is 34 mm). Table 2 summarizes the main configurationof the filter wheels. The precision of the positioning of one filter is ±40 arcminutes. The positioning time is lessthan 5.6 seconds.

Table 2. SODISM filters position.

Phase Configuration Filter wheel 1 Filter wheel 2

Helioseismology Position 11: Hole Position 21: 535.7 nmDiameter Position 12: 535.7 nm Position 22: HoleDiameter Position 13: 607.1 nm Position 22: HoleDiameter Position 14: 782.2 nm Position 22: HoleActivity Position 15: 215 nm Position 22: HoleActivity Position 11: Hole Position 24: 393.37 nmFlatfield 535.7 nm Position 12: 535.7 nm Position 25: lensFlatfield 607.1 nm Position 13: 607.1 nm Position 25: lensFlatfield 782.2 nm Position 14: 782.2 nm Position 25: lensFlatfield 215 nm Position 15: 215 nm Position 25: lensStellar calibration Position 11: Hole Position 23: DiopterDark signal – –

3.4.4 The shutter

The mechanical shutter lets you choose the exposure time between 0.5 second to 16 seconds. We have twodifferent coatings on the blade of the shutter. The blade’s surfaces facing the Sun are in AlMgF2 coating. Theblade’s surfaces facing the CCD are coated with black Teflon. The main characteristics of the shutter are: a 35mm diameter aperture, an opening time of the shutter less than 21 milliseconds and an accuracy of the exposuretime of 90 microseconds or better.

3.4.5 The fine pointing

SODISM uses three piezoelectrics devices acting on the primary mirror M1. The parallel pre-stressed actuatoris a preloaded stack of low voltage piezoelectric ceramics. Piezoelectric displacement is 50 µm peak for 170 Vpeak. The capacitance of the piezoelectric is 2.7 µF. Strain gauges are used for each piezoelectric (repeatabilityon the positioning of a few nm).

SODISM uses four photodiodes. Any unbalanced signal between pairs of detectors indicates a modificationof the pointing. Photodiodes are semiconductors that generate a current or voltage when illuminated by light.A sapphire window is used. Sapphire is a very rugged radiation resistant optical material with a very broadtransmission range.

The main characteristics of the fine pointing are: the ability to have an offset alignment four-quadrantoptical path / SODISM optical path of ±10 arcseconds, a pointing accuracy of ±0.2 arcseconds, a stability of thesummit of the primary mirror in Z (optical axis) of ±0.2 µm and an ability to image offset of ±20 arcseconds.It is possible to fix the error signal photodiodes by offset (equivalent to a sliding): ±60 arcseconds. The finepointing is operational two seconds before taking the image on the CCD and the functioning of the shutter.

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3.4.6 Environment tests

Environment tests for the qualification of the mechanisms are: sine vibration, random vibration, shock,thermal cycling and life test (100 cycles minimum for the system to unlock the door, 1 200 cycles minimum forthe system to open/closed the door, 1 329 560 cycles minimum for the shutter and 20 106 cycles minimum forpiezoelectric). All tests were successful.

3.5 Spectral domain of observation

Table 3 summarizes the main characteristics of the interferences filters. In order to perform diameter mea-surements, three domains in the photospheric continuum (535.7, 607.1 and 782.2 nm) have been chosen [2].

Table 3. SODISM interferences filters characteristics.

Wavelength λ in nm ∆λ in nm Equivalent Transmission Function

215 ±0.5 7 Near 25% Sun activity, O3, diameter393.37 ±0.1 0.7 6.2 at 10% Active regions observation535.7 ±0.05 0.5 26.3 at 32.8% Oscillations, Helioseismology535.7 ±0.05 0.5 29.5 at 35.1% Diameter607.1 ±0.1 0.7 33.5 at 42.8% Diameter782.2 ±0.2 1.6 29.7 at 37.6% Diameter

4. THE SODISM MODELS

Models were needed to design the telescope.

4.1 The mechanical analysis

The mechanical design is analyzed with a detailed mathematical model in order to verify stiffness requirements,general and local strength, to be conform to quasi statics loads, random loads but also to establish a stabilitybudget of the optical path. It is a very complex model that requires some verification criteria. The telescopefirst eigen mode (given by the finite element model) is found equal to 163 Hz which is greater than a givenspecification of 150 Hz. A eigen mode of SODISM is shown in Figure 3.

Figure 3. Eigen mode of the telescope. Figure 4. Internal SODISM GMM.

4.2 The thermal analysis

A thermal mathematical model was developed to study temperature variations and thermal gradients affectingthe telescope in the two typical cases in orbit. The Geometrical Mathematical Model (GMM) is given in Figure4. The thermal mathematical model consists of 1407 nodes. The SODISM thermal control is based on aProportional Integral (PI) regulation. The instrument stability will strongly depend upon precise regulation ofall defined parameters. They are first deduced from the model and thermal balance test and will be adjusted

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during the flight. For all SODISM heating lines, we will use the first satellite eclipses by the Earth as temperatureechelon response to estimate the instrument relaxation time which is an important parameter for the regulation.The regulation temperature of each sensitive parameter will be also adjusted during the flight. The onboardelectronic system will continuously calculate the optimized electrical power needed to achieve the instrumentregulation. Temperature and stability conditions will be optimized in orbit during the commissioning period.The cold and hot cases implemented in the structural model allowed us to analyze the thermo-elastic behaviorof the telescope structure and the consequences on the optical path.

The thermal regulation of the CCD is very important in the PICARD experiment. It allows minimizing thedistortion of the pixels lines and columns by operating the detector at low temperature. An optimization oftemperature and stability conditions will be studied in orbit during the commissioning phase. We have however,provided preliminary results obtained from the thermal model. They are given in Table 4.

Table 4. Temperatures and Gradients results obtained by analysis.

Optical component and structure Temperatures Gradients

Entrance Window -9oC < T < 37oC 6oCGreen Filter 22oC < T < 26oC 1.3oCPrisms 22oC < T < 24oC 1oCMirror M2 29oC < T < 34oC 1.18oCMirror M1 23oC < T < 25oC 0.22oCMirror M3 18oC < T < 22oC < 0.2oCPhotodiodes 10oC < T < 30oC < 0.5oCInterferences Filter 18oC < T < 24oC 0.1oCCCD -13oC ±0.2oC 2.5oCMirror M1 plate 20oC ±5oC 3oCMirror M2 plate 20oC ±5oC 7.9oCTitanium plate 20oC ±5oC 1.5oCCCD plate 20oC ±5oC 9.3oCCC Tube 20oC ±5oC 1.1oCCC Plate 20oC ±5oC 0.8oC

The interference filters have their central bandpass, which is temperature dependent. Typical values are 0.015nm/oC at 500 nm and 0.020 nm/oC at 900 nm. A maximum shift of 0.09 nm is expected for the duration ofthe mission according to the given temperature range. This is compatible with scientific specifications where weneed to observe outside solar Fraunhofer lines. The calculation has been carried out to analyze thermal behaviorof some optical elements of SODISM using cold and hot cases.

The thermal control was designed using dedicated software (Esarad and Esatan from Alstom). Softwareis very often based on Monte Carlo ray tracing computes the energetic exchanges between the surfaces whichcompose the geometrical model, as well as the external heat fluxes on the instrument, and thermal radiation tothe cold deep space. The cold deep space is modeled by a black body at 4K, taking into account fossil radiationand stars. The thermal method implies a decomposition of the model in isothermal elements called thermalnodes. For this reason, the emissive power is constant across each node. The thermal properties of each nodeare concentrated in one point which can be the geometrical center of the node. The nodal decomposition alsoimplies that the thermo-optical properties are constant across each thermal node.

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5. TESTS AND RESULTS

Environments for the qualification and acceptance of SODISM telescope are: units tests, ARIANE sine,DNEPR random, shock (Solar array release with pyrosofts nuts and separation of the Microsatellite with pyrodevices) and thermal environment of SSO at 725 km. The qualification of the instrument SODISM was accom-plished on MTS satellite (Mechanical and Thermal Structure). Vibration levels measured on PICARD MTS havebeen used for the acceptance testing of the flight model instrument. A thermal balance test was accomplishedon SODISM qualification and flight model telescope alone. A thermal balance test was accomplished on flightmodel satellite. During all the tests, one of the objectives was to not have sliding of the optical path.

5.1 The units tests

Unit tests were achieved to validate the design of the telescope. They consisted in: characterization of CC,characterization of invar and characterization of the link CC/invar.

5.1.1 The characterization of CC

High stability requirements of the telescope particularly depend of the use of CC material. We have thencarefully characterized this element. The thermal expansion was measured using a dilatometer. The coefficientof thermal expansion is less than -7.10-7 m/m/K at 23oC.

5.1.2 The characterization of invar

The thermal expansion was measured using a dilatometer. The coefficient of thermal expansion is less than1.10-6 m/m/K at 23oC. As this parameter is important for the stability of the structure, a special thermalprocess was applied on invar plate.

5.1.3 The characterization of the link CC/invar

Two CC/invar samples have been glued with HYSOL 9321EA. Sheared surface was around 250 mm2 andtensile test was realized with 1mm/minute speed. Results were: Sample No1 broke at 14.11 MPa (Glue thicknessis about 0.1 mm, calibrated with a wire) and Sample No2 broke at 13.85 MPa (Glue thickness is about 0.2 mm,calibrated with a wire).

Both samples broke with a delaminating of carbon fibers.

5.2 Sinus low level

The telescope first eigen mode found with the sine test, is 182 Hz. The Finite Element Model is representative,since it has given 163 Hz for the first eigen mode.

5.3 Random vibration

A random vibration has been performed on SODISM flight model with the levels given in the Table 5. Thistest was performed successfully. The table shows the ASD (Acceleration Spectral Density in g2/Hz) as a functionof f (frequency in Hz). Practically any quantity squared and divided by frequency is a spectral density, commonlycalled the PSD (Power Spectral Density).

5.4 Shock test

A shock test was performed on MTS satellite for a solar array release with pyrotechnics nuts and separationof the microsatellite with pyrotechnics devices. This test allowed qualifying the SODISM instrument.

5.5 Thermal cycling

During the thermal cycling, the telescope was subjected to hot and cold environment conditions with tem-peratures between 50oC and -35oC for non-operational conditions. For operational conditions, the telescopethermal cycling was between 40oC and 5oC. This test was performed successfully.

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Table 5. Flight model level input.

X axis Y axis Z axis

f in Hz ASD in g2/Hz f in Hz ASD in g2/Hz f in Hz ASD in g2/Hz

20 0.001 20 0.001 20 0.001100 0.001 150 0.001 100 0.001150 0.01 200 0.01 300 0.012000 0.01 2000 0.01 2000 0.01

60 seconds 4.33 grms 60 seconds 4.28 grms 60 seconds 4.24 grms

5.6 Thermal balance

A thermal balance was achieved with SODISM qualification and flight models. This test allowed qualifyingthe thermal control of the instrument. A thermal balance test was also achieved with the flight model placed onthe PICARD satellite. The results presented in Table 6, show that the instrument behavior is very close to thepredictions given by the models. However, the final fine tuning of the thermal regulation will be achieved duringthe commissioning phase in orbit. Tests reveal that the regulation of the CCD is the most critical. It will benecessary to choose between stability and its coldest possible temperature.

Table 6. Temperatures results obtains by analysis and test.

Optics Thermal model prediction Results of the thermal balance

Entrance Window 28oC < T < 35oC Performance will be obtained only by calculationGreen Filter 21oC < T < 22oC Performance will be obtained only by calculationPrisms 19oC < T < 20oC Performance will be obtained only by calculationMirror M2 30oC < T < 31oC 30.0oC ±0.26oCMirror M1 25oC < T < 26oC 25.1oC ±0.27oCMirror M3 18oC < T < 22oC Performance will be obtained only by calculationPhotodiodes 10oC < T < 30oC Performance will be obtained only by calculationInterferences Filter 18oC < T < 24oC Performance will be obtained only by calculationCCD -10.8oC ±0.2oC -10.8oC

Structures Thermal model prediction Results of the thermal balance

Mirror M1 plate 20.4oC < T < 20.5oC 19.4oC < T < 19.7oCMirror M2 plate 19.0oC < T < 22.0oC 18.0oC < T < 20.0oCTitanium plate 21.1oC < T < 21.2oC 19.5oC < T < 19.6oCCCD plate 15.0oC < T < 17.0oC 15.9oC < T < 16.9oCCC Tube 19.1oC < T < 21.3oC 18.7oC < T < 20.3oCCC Plate 19.6oC < T < 20.5oC 18.6oC < T < 19.2oC

The thermal regulation of the CCD is very difficult. The strategy on the adjustment of this parameter dependsupon many parameters (aging of the coating, orbital effect). During the thermal balance of the satellite, we trieddifferent ranges of regulation and we found different variations: -10.8oC ±0.01oC, -12.1oC ±0.02oC and -15.0o

C ±0.08oC. The fine tuning of the thermal regulation of the CCD will be achieved during the commissioningphase (in orbit).

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6. THE PICARD PAYLOAD DATA CENTER

The PPDC (PICARD Payload Data Center) collects, process and distributes the PICARD data [4]. TheBUSOC (Belgian User Support and Operation Centre) is in charge of operating the PPDC.

6.1 The SODISM data

There are several kinds of data (Figure 5).

a− Full solar images of size 2048x2048 and 256x256 pixels: the first kind of images are recorded at thewavelengths 215, 393.37, 535.7, 607.1 and 782.2 nm with a cadence of two of them per orbit while the others arerecorded each minute at 535.7 nm.

b− Solar and dark signal limb images of size 2048x2048 pixels: they correspond to 22 and 40 pixels widelimbs recorded respectively at the wavelength 535.7 nm each two minutes and at those mentioned above witha rate of two per orbit per wavelength. Dark signal limb images are 40 pixels wide and are not related to anywavelength. There are also several markers (pieces of image) extracted from the full SODISM CCD image and4 auxiliary ones, which are present with the limbs. The auxiliary images exist only in limb images recorded at535.7 nm.

c− Other data: they consist of doublet stars, dark signal and flat field images. One image of respectively adark signal and flat field at a given wavelength will be recorded every day while doublet stars images every fewmonths.

6.2 The SODISM data products

There are three levels of PPDC products.

a−Level 0 (L0) and associated quick-look L0’: the first step to create L0 products is to assemble the datapackets incoming from the satellite to obtain a binary stream. The second step is to decompress if needed andto process it with the appropriate software components to build the L0 product containing the PICARD data.All markers and auxiliary images present with the solar limbs are extracted to form daily L0 products. A quickanalysis data is made on daily marker products and dark signal full image. It is to compute the mean andstandard deviation of intensity pixels but also to count and to locate the bad pixels in the image. All L0 andL0’ SODISM products are fits format.

b−Level 1 (L1) and quick-look L1’: the Level 1 processing sequences consist of L0 products calibration (sci-entific and housekeeping data) and adding new information in their header to create self-consistent L1 products.They consist also to create daily products containing the auxiliary images present in the limb images. These prod-ucts are used to compute instrument scale factors. For helioseismology objectives, other processing sequences areimplemented to monitor limb intensity and macro-pixel image photometry and to compute YLM masks neededfor l- -diagram diagnostics. One of quick looks made on L1 products is the calculation every 15 days over a periodof 15 days of the power spectrum of temporal fluctuations of 22 pixels wide limb and macro-pixel image intensitycorrected from Satellite-Sun distance. The temporal intensities of macro-pixel images projected over sphericalharmonics basis (YLM masks) are also computed as quick look. Sum of their power spectrums over m-modelead to l- -diagram. This kind of l- -diagram will be computed every 3 days using SODISM macro-pixel imagesrecorded over 3 days. The last quick look is an intensity analysis of full images recorded at several wavelengths.The obtained solar spectral irradiance is compared with data recorded with the onboard radiometers. All L1and L1’ products are fits format.

c−Level 2A: the level 2 processing sequences are to compute mean solar radius value from each limb imageand its daily variations. The accuracy of measurements and noise that affects them, are also estimated. L2Aproducts are unique for the mission and enriched every day. L2A products are also fits format.

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7. SODISM II

The mission consists of observations carried out from orbit and from ground in order to separate the contri-butions of the atmosphere and the Sun. The SODISM qualification model named SODISM II will be used forthis goal. It will be installed at Calern Observatory (France) since the longer series of solar diameter measure-ments was obtained there using a solar astrolabe [5]. The solar seeing monitor MISOLFA (Moniteur d’ImagesSolaires Franco Algerien) [6] will observe together with SODISM II. MISOLFA will give all atmospheric turbu-lence parameters at the moments when SODISM II will record the solar images. The PICARD ground segmentwill allow then [7]: a comparison of diameters acquired in space and on the ground to understand the influenceof the atmosphere on the solar diameter; to link space and ground measurements with the help of atmosphericparameters given by MISOLFA; to compare diameter measurements obtained with SODISM II and ground basedinstruments in order to identify possible biases and correct historical series; to continue diameter measurementswith ground based instrument at the end of the PICARD mission with the results obtained from simultaneousobservations with the space; to analyze possible anomalies noticed in orbit with the SODISM instrument; todeepen of instrumental performances; to validate once of Flight Software. The couple SODISM and SODISM IIwill operate in the same way and at the same moment. SODISM II will observe the Sun in the same wavelengthdomain except in the UV domain where the 215 nm filter will be replaced by another one (1025 nm filter).SODISM II will be operational in 2011.

Figure 5. All kinds of L0 SODISM products.

8. CONCLUSION

The SODISM instrument has been developed for solar astrometry and helioseismology observations. Withthe TSI, and helioseismologic oscillations, the solar diameter, the limb shape and the solar asphericity, are keyparameters for characterizing the physics of the Sun. They constitute fundamental quantities to validate the

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Sun modeling, i. e. the functioning of our star. To achieve these key measurements, radiometers will be used tomeasure the TSI. For the diameter, limb shape and asphericity, a metrological instrument has been built. It isan imaging telescope working at several wavelengths. Its main characteristics consist of its dimension stabilitybased on the use of very stable materials, a precise thermal regulation and the use of an angular referenceallowing to take into account any change of the internal instrument angular scale, this scale being ultimatelycalibrated against star angular distances. The instrument (see Figure 6) has been characterized in detail, andspace qualified.

The first month in orbit will be dedicated to the outgassing of the instrument and different mechanical actions(unlocking, check of mechanisms, sequencing and instrument parameters adjustments). The fine tuning of thethermal regulation will be achieved during the commissioning phase in orbit and after analysis orbital fluctuationeffects. The space instrument SODISM and the ground instrument SODISM II are two identical instrumentsdedicated to the measurement of the solar diameter, the first being placed in orbit, and the second operated onthe ground.

Figure 6. The SODISM experiment partly assembled.

ACKNOWLEDGMENTSThe SODISM instrument has been built by CNRS - LATMOS. We thank CNES and CNRS for their support

as well as the PICARD Team and all participants or industrials having devoted their expertise to this project.

REFERENCES[1] G. Thuillier, S. Sofia, M. Haberreiter and the PICARD team, Past, Present and Future Measurements of the

Solar Diameter, Adv. Space Res. 35, 329-340, 2005.[2] G. Thuillier, S. Dewitte, W. Schmutz and the PICARD team, Simultaneous Measurements of the Total Solar

Irradiance and Solar Diameter by the PICARD mission, Adv. Space Res. 1792-1806, 2006.[3] P. Assus, A. Irbah, P. Bourget, T. Corbard and the PICARD team, Monitoring the scale factor of the

PICARD SODISM instrument, Astron. Nachr. 329, No. 5, 517 - 520, 2008[4] G. Pradels, T. Guinle, G. Thuillier, A. Irbah et al., The PICARD payload data center, SPACEOPS 2008

conference, American Institute of Aeronautics and Astronautics, 2008[5] F. Laclare, J.P. Coin, C. Delmas and A. Irbah, Measurements and long-term changes of the solar, Solar

Physics, 1996, 166, 211-229[6] A. Irbah, M. Chibani, L. Lakhal, A. Berdja, J. Borgnino, F. Martin and P. Assus, MISOLFA: a generalized

solar seeing monitor, SF2A, meeting held in Lyon, France, May 28-June 1st, 2001, Eds.: F. Combes, D. Barret,F. Thevenin, EDP Sciences, Conference Series, p. 59

[7] C. Delmas, F. Morand, F. Laclare, A. Irbah, G. Thuillier and P. Bourget, Ground solar radius survey in viewof microsatellite missions, Adv. Space Res., 37, 1564-1568, 2006.

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