The stereo secchi extreme ultraviolet imager
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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 secchi extreme ultraviolet imager

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

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  • High performance Image Stabilization System (ISS) has been replaced by the simpler and lower cost Fine Pointing System (FPS)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

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

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 System Overview


Optical design prescription

Optical Design: Prescription

Effective focal length:1750 mm

Distance Primary - Secondary:460 mm

Distance Secondary - focus:635 mm


Optical design ray trace results

Optical Design: Ray Trace Results


Wavelength selection coatings

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

Effective Area


Temperature response

Temperature Response


Sensitivity comparison with trace

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

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

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

    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

    Instrument Cross Section


    3 d view

    3-D View


    Primary and secondary mirror mounts

    Primary and Secondary Mirror Mounts


    Pointing stability

    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 jitter performance simulated from trace image

    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


    S c jitter and psf

    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

    • 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

    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

    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

    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

    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

    Preliminary Alignment Error Budget

    DecenterTip/Tilt

    Primary mirror0.25 mm1 arcmin

    Secondary mirror0.25 mm3 arcmin

    CCD0.35 mm6 arcmin


    Optics fabrication flow

    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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