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Theia: some input from Gaia

Theia: some input from Gaia. Alcione Mora Aurora Technology BV. Gaia throughput loss. Monitoring of response by comparison to Tycho-2 photometry. First decontamination (FOV2). Second decontamination. C.Fabricius, Gaia commissioning. Micrometeorids and microclanks.

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Theia: some input from Gaia

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  1. Theia: some input from Gaia Alcione Mora Aurora Technology BV

  2. Gaia throughput loss • Monitoring of response by comparison to Tycho-2 photometry First decontamination (FOV2) Second decontamination C.Fabricius, Gaia commissioning

  3. Micrometeorids and microclanks • Micrometeorids: L2 is not perfect vacuum! • Attitude discontinuities  expected and calibrated • Micro clanks: 1-2 mas micro clanks per minute  7.5 nm  20 Si atoms • Calibration in place. What if target precision were 10-100x higher? F. van Leeuwen, Gaia commissioning

  4. Gaia focus evolution • Astrometric focus: Cramér-Rao  PSF sharpness (derivatives) • The focus evolves during the mission. The rate gets progressively smaller • Some hypotheses: glue shrinkage, hysteresis, water contamination • Mitigation: adaptive calibration, occasional refocus FoV1. FoV1 FoV2. FoV2 A. Mora

  5. Focus evolution • The focus evolves througout the mission • At a smaller pace as time goes by • Related to water ice contamination? • Up to 20% performance degradation during commissioning • Refocusing will be needed by Theia • Wavefront sensing: Gaia yes, Euclid no  empirical • Gaia M2MM: 5 dof. Euclid: 3 dof (simplified mechanism) • Intrusive: ~1 day lost synchronise with decontamination?

  6. Astrometric performance • Multiple dependences • Single transit precision, telescope and focal plane calibration, SAA, attitude, electronics, radiation, … • Single transit precision • Faint stars: most important factor • Bright stars: driven by reference stars and calibration Faint stars: single transit precision Bright stars: calibration errors Theia proposal

  7. Astrometric performance • Estimate: Cramér-Rao lower bound • FWHM just orientative • Centroiding precision depends on PSF slopes • Circular and square pupil: similar performance per area • Theia proposal seems too optimistic • PSF sampling is second order  less detectors? • ~4.8x4.8 for 0.8m square pupil. ~5.8x5.8 for 1m square

  8. Figure of merit: filled telescope • λ: wavelength. Degrades PSF • ΔΩsurvey: sky surveyed area • a: spacecraft orbit semi-major axis. Only for parallax • D: optics aperture: more photons and sharper PSF • FOV: field of view linear size • T: system throughput • Fλ: Source spectrum • Δλ: system passband • Δt: survey duration. ~linear for proper motion

  9. Diameter and field of view • Diameter: strongest dependence • Reduces PSF size and increases photon collection • Use the correct value! • Field of view: minor corrections, and expensive (detectors, electronics, computers, downlink)

  10. Thermal stability • CNES optical study: • TMA quality similar for Korsch, Cooke and 5, 10 µm pixels • Key parameter: distortion stability. Target: 1e-5 pix • Critical element: M3, mK stability. Impact of low and high term WFE • Payload thermal stability • Gaia: sub-mK. Passive, constant SAA. SVM-PLM decoupled, • Euclid-Theia: ~3-15 mK in one day • Ensure constant Sun Aspect Angle? • Constant input solar power • SAA 30º/45º  FoR: 50/71% sky • Parallax depends on SAA • Deployment mechanism needed Theia proposal A. Mora Gaia commissioning

  11. ECho-like umbrella sunshield ECho ESA/SRE(2013)2

  12. Euclid-like sliding door sunshield Theia proposal

  13. Gaia: impact of computers • 24 h slow component: related to downlink (transponder + PDHU) • Peaks: follow sky density (galactic plane)  SVM computers thermoelastic efffect • Most SVM perturbations modify basic angle. Rule of thumb: 100 μas/K Reverse scale for temperature Airbus DS & Lennart Lindegren

  14. Payload power consumption • Keep computers as far as possible • Use constant and predictable detector read-out • Gaia: almost constant power, but still affected by stellar density • Theia: difference between exoplanets (HDR) and dark matter (deep field) Theia proposal

  15. System throughput • Gaia saturates at G ~ 12.0  gates  throughput losss • Teledyne Hawaii: fast guide window + normal integration • Slow mode. Low RON. G ~ 2.5 • Fast mode. High RON. G ~ 0.0 • Single window  OK for Gaia FOV covered with Hawaii 2RG • Multiple windows  possible (JWST FGS)  TECH: explore guiding. PEMs?. Compression • G: 9.5 – 12.0. N > 1790000. σ ~ 1/1 – 1/3.2 σGaia • G: 7.0 – 9.5. N ~ 186000. σ ~ 1/3.2 – 1/10 σGaia • G: 4.5 – 7.0. N ~ 13400. σ ~ 1/10 – 1/32 σGaia • G: 2.0 – 4.5. N ~ 780. σ ~ 1/32 – 1/100 σGaia Hodapp

  16. e2V alternatives?

  17. Calibration strategy • Focal plane  laser metrology • Very high precision, but monochromatic, no telescope • PSF and distortion  calibration fields • Globular clusters, Galaxy (e.g. Baade’s window, Sgr I) • Telescope aberrations  wavefront sensors (WFS) • Gaia-like: own detector at focal plane edge • Distortion change between target and calibrator • Do not rely on passive stability alone. • Use high precision thermometers in critical items (e.g. M3) • Use constant Sun aspect angle (sun-shield, if possible) • Empirical correction  WFS + temps. + target field stars

  18. Gaia wavefront sensors • Two. Own CCD. In centre and edge field of view

  19. Theia WFS strawman layout • Real-time aberration monitoring across the focal plane Astrometric focal plane

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