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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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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)


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  • High performance Image Stabilization System (ISS) has been replaced by the simpler and lower cost Fine Pointing System (FPS)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


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


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


Optical System Overview


Optical Design: Prescription

Effective focal length:1750 mm

Distance Primary - Secondary:460 mm

Distance Secondary - focus:635 mm


Optical Design: Ray Trace Results


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


Effective Area


Temperature Response


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)


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


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:

      171195284304

      T =64 %56 %32 %26 %

  • Analysis filters: TRACE design (size adjusted)

  • All filters manufactured by LUXEL


  • 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


    Instrument Cross Section


    3-D View


    Primary and Secondary Mirror Mounts


    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


    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


    Energy in central pixel drops by factor of 8

    Point sources that are two pixels apart become indistinguishable

    S/C Jitter and PSF


    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


    Comparison FPS - ISS

    FPSISS

    Range+/- 7”> +/- 30”

    Drive Voltage< 15 V> 60 V

    Accuracy0.8-1.9” p-p0.3” p-p

    Active elementPZT - open loopPZT - closed loop

    Electronicsdigital, < 1/2 boardanalog, 2 boards

    Control softwarewithin GT read loop

    Mass0.2-0.4 kg1.2-1.4 kg

    Power< 0.5 W1.5 W

    Costapprox. 1/3 of ISS


    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


    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


    Preliminary Focus Error Budget

    Error Sourcein Mirror Separationin Focus Location

    Focus setting0.030 mm

    Structural stability

    • Mirror separation0.007 mm0.042 mm

    • Mirror to focus0.007 mm

      Thermal effects (+/- 20 C)

    • Mirror separation0.003 mm0.018 mm

    • Mirror assy. to focus0.033 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.


    Preliminary Alignment Error Budget

    DecenterTip/Tilt

    Primary mirror0.25 mm1 arcmin

    Secondary mirror0.25 mm3 arcmin

    CCD0.35 mm6 arcmin


    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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