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The STEREO-SECCHI Extreme Ultraviolet Imager

The STEREO-SECCHI Extreme Ultraviolet Imager. J-P. Wülser, J.R. Lemen, T.D. Tarbell, C.J. Wolfson (LMSAL) R.A. Howard, J.D. Moses (NRL) J-P. Delaboudinière (IAS) R. Mercier, M-F. Ravez (IOTA). Status Update.

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The STEREO-SECCHI Extreme Ultraviolet Imager

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  1. The STEREO-SECCHIExtreme Ultraviolet Imager J-P. Wülser, J.R. Lemen, T.D. Tarbell, C.J. Wolfson (LMSAL) R.A. Howard, J.D. Moses (NRL) J-P. Delaboudinière (IAS) R. Mercier, M-F. Ravez (IOTA)

  2. Status Update • Wavelength selection finalized: baseline for the “hot” channel is now Fe XV at 28.4 nm • High performance Image Stabilization System (ISS) has been replaced by the simpler and lower cost Fine Pointing System (FPS) • Flight mirror blanks and all parts for the EUVI Structural Model are currently being fabricated • Structural Model tests scheduled for September • Acoustic test to verify entrance filter design • Vibration test to verify integrity of overall design

  3. Science Goals The Extreme Ultraviolet Imager (EUVI) supports the STEREO-SECCHI science goals, including: • Initiation of CMEs • Interactions of flux systems, reconnection • Role of coronal dimming • Physical evolution of CMEs • 3-D structure, CME acceleration • Response of the low corona • 3-D structure of Active Regions

  4. Main Design Features • Normal incidence Ritchey-Crétien telescope • Multilayer coated optics, thin film filters • Heritage: EIT/TRACE • 98 mm aperture, 4 spectral channels, one in each optical quadrant • Fine pointing system with active secondary • 2k x 2k backside illuminated CCD, 1.6” pixels • Circular full sun field of view to ± 1.7 R • Blue LED aliveness source

  5. Optical System Overview

  6. Optical Design: Prescription Effective focal length: 1750 mm Distance Primary - Secondary: 460 mm Distance Secondary - focus: 635 mm

  7. Optical Design: Ray Trace Results

  8. Wavelength Selection, Coatings • He II 30.4 nm: chromosphere, erupting prominences • Fe IX 17.1 nm: high contrast in coronal loops • Fe XII 19.5 nm: “typical” quiet corona • Fe XV 28.4 nm: “hotter”, 2.5 MK corona • Baseline coating materials: MoSi/Si for 17.1, 19.5, and 30.4. Mg2Si/B4C for 28.4 • Calibration: Synchrotron at IAS

  9. Effective Area

  10. Temperature Response

  11. Sensitivity Comparison with TRACE • Element comparison: • Detector: ~ 8 x higher QE than TRACE • Aperture: ~ 27 x smaller area than TRACE • Pixel area (arcsec2): ~ 10 x larger than TRACE • Pixel saturation (phot/pix): 5 x lower than TRACE • Assumes Aluminum-on-mesh entrance filters and TRACE-like multilayer coatings • Exposure times: • 3 x shorter than TRACE for same # photons/pixel • Min. exp. time: 40 ms (15 x shorter than TRACE)

  12. Entrance Filters (1) • Protection during launch • Front door, but no vacuum chamber • Rationale: similar analysis filters survived launch without vacuum chamber (TRACE, SXT) • Acoustic test program before PDR • Two proven design options to mitigate risk: • Baseline: 1500 Å Aluminum on a fine (70 lpi) mesh • TRACE heritage • Maximizes EUV throughput

  13. Entrance Filters (2) • Alternate design: 1500 Å Al + 500 Å Polyimide on coarse support grid (5 mm spacing) • EIT / EIT Calroc heritage • Potentially stronger due to Polyimide support • Reduced diffraction pattern • Lower throughput: 171 195 284 304 T = 64 % 56 % 32 % 26 % • Analysis filters: TRACE design (size adjusted) • All filters manufactured by LUXEL

  14. Mechanical Design: Main Features • Graphite/Cyanate Ester metering structure with Aluminum liner (SXI heritage) • TRACE heritage active secondary mirror (FPS) • Mechanisms: recloseable front door (LASCO), sector shutter, focal plane shutter (SXI), filter wheel (SXI). No focus mechanism • Thin film filters launched at ambient pressure • Primary mirror mount: Invar bi-pods, bonded • Fully baffled

  15. Instrument Cross Section

  16. 3-D View

  17. Primary and Secondary Mirror Mounts

  18. Pointing Stability • The EUVI instrument requires a 0.8-1.2” (3) pointing stability to meet its proposed science objectives. • The S/C is only required to meet a pointing stability of 3.8” (2) • Pointing jitter at or near the 3.8” level would cause severe SECCHI science loss

  19. Effect of S/C JitterPerformance Simulated from TRACE Image No jitter S/C jitter at spec level (without ISS/FPS) Actual EIT image for comparison

  20. Energy in central pixel drops by factor of 8 Point sources that are two pixels apart become indistinguishable S/C Jitter and PSF

  21. The EUVI Fine Pointing System • The EUVI Fine Pointing System (FPS) bridges the gap between the EUVI pointing stability requirement and the S/C jitter specification • Due to its limited scope, the FPS can be built with modest resources compared to the original ISS • Main FPS features: • Improves pointing stability by a factor of 3-5 • No compensation of PZT hysteresis necessary • Limited tilt range allows low voltage drivers • Simple digital control loop

  22. Comparison FPS - ISS FPS ISS Range +/- 7” > +/- 30” Drive Voltage < 15 V > 60 V Accuracy 0.8-1.9” p-p 0.3” p-p Active element PZT - open loop PZT - closed loop Electronics digital, < 1/2 board analog, 2 boards Control software within GT read loop Mass 0.2-0.4 kg 1.2-1.4 kg Power < 0.5 W 1.5 W Cost approx. 1/3 of ISS

  23. Optical Design Drivers • ± 1.7 R FOV, 27.6 mm detector  f = 1.75 m • Symmetric PSF  Ritchey-Crétien • Maximize focus error tolerance  choose low secondary mirror magnification (mag = 2.42) • Minimize solar energetic particle flux on CCD and minimize stray light  system fully baffled • Maximize aperture within cross sectional envelope of heritage filter wheel mechanism • Unvignetted FOV to 1.7 R

  24. Prescription Details SURFACE DATA SUMMARY: Surf Comment Radius Thickness Glass Diameter Conic Cent.Obstr. OBJ Inf Inf 0 0 1 ENTRANCE FILTER Inf 152.8 107.5789 0 54 2 (Z-LOC OF SEC) Inf 122 105.1787 0 3 OUTSIDE BAFFLE2 Inf 335 103.2623 0 59.7 STO APERTURE MASK Inf 0 98 0 65 5 SPIDER MASK Inf 0 98 0 6 SPIDER MASK Inf 3 98 0 7 PRIMARY -1444 -239 MIRR 98.03406 -1.194 8 OUTSIDE BAFFLE1 Inf -99 69.4181 0 39.7 9 INSIDE BAFFLE2 Inf -122 57.52323 0 10 SECONDARY -892 221 MIRR 42.83404 -8.42 11 INSIDE BAFFLE1 Inf 359 37.48188 0 12 FILTER WHEEL Inf 54.91 28.79774 0 IMA CCD Inf 27.5046 0

  25. Preliminary Focus Error Budget Error Source in Mirror Separation in Focus Location Focus setting 0.030 mm Structural stability • Mirror separation 0.007 mm 0.042 mm • Mirror to focus 0.007 mm Thermal effects (+/- 20 C) • Mirror separation 0.003 mm 0.018 mm • Mirror assy. to focus 0.033 mm Total (worst case) 0.130 mm Note: The (geometrical) instrument PSF is smaller than one pixel at all field angles, if the focus errors is 0.130 mm or less.

  26. Preliminary Alignment Error Budget Decenter Tip/Tilt Primary mirror 0.25 mm 1 arcmin Secondary mirror 0.25 mm 3 arcmin CCD 0.35 mm 6 arcmin

  27. Optics Fabrication Flow • Mirror blank fabricated, mirror pads bonded (LMSAL) • Mirror blank shipped to IOTA • Mirror surface ground and polished to sphere (IOTA) • Ion beam aspherization (IOTA) • Deposition of multilayer coatings (IOTA) • Mirror bonded to its mount (at IAS by LMSAL team) • Mirror set calibrated at synchrotron (IAS) • Mirror set shipped to LMSAL • Mirror set integrated into EUVI (LMSAL)

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