7/12/20011 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)

7/12/20012 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finalized: baseline for the “hot” channel is now Fe XV at 28.4 nm High performance Image Stabilization System (ISS) has been replaced by the simpler and lower cost Fine Pointing System (FPS) Flight mirror blanks and all parts for the EUVI Structural Model are currently being fabricated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

7/12/20013 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

7/12/20014 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

7/12/20015 Optical System Overview

7/12/20016 Optical Design: Prescription Effective focal length:1750 mm Distance Primary - Secondary:460 mm Distance Secondary - focus:635 mm

7/12/20017 Optical Design: Ray Trace Results

7/12/20018 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 Mg 2 Si/B 4 C for 28.4 Calibration: Synchrotron at IAS

7/12/20019 Effective Area

7/12/ Temperature Response

7/12/ Sensitivity Comparison with TRACE Element comparison: –Detector: ~ 8 x higher QE than TRACE –Aperture: ~ 27 x smaller area than TRACE –Pixel area (arcsec 2 ): ~ 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)

7/12/ 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

7/12/ Entrance Filters (2) –Alternate design: 1500 Å Al Å Polyimide on coarse support grid (5 mm spacing) EIT / EIT Calroc heritage Potentially stronger due to Polyimide support Reduced diffraction pattern Lower throughput: T =64 %56 %32 %26 % Analysis filters: TRACE design (size adjusted) All filters manufactured by LUXEL

7/12/ 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

7/12/ Instrument Cross Section

7/12/ D View

7/12/ Primary and Secondary Mirror Mounts

7/12/ Pointing Stability The EUVI instrument requires a ” (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

7/12/ Effect of S/C Jitter Performance Simulated from TRACE Image No jitter S/C jitter at spec level (without ISS/FPS) Actual EIT image for comparison

7/12/ S/C Jitter and PSF Energy in central pixel drops by factor of 8 Point sources that are two pixels apart become indistinguishable

7/12/ 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

7/12/ Comparison FPS - ISS FPSISS Range+/- 7”> +/- 30” Drive Voltage 60 V Accuracy ” p-p0.3” p-p Active elementPZT - open loopPZT - closed loop Electronicsdigital, < 1/2 boardanalog, 2 boards Control softwarewithin GT read loop Mass kg kg Power< 0.5 W1.5 W Costapprox. 1/3 of ISS

7/12/ 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 

7/12/ Prescription Details SURFACE DATA SUMMARY: Surf Comment Radius Thickness Glass Diameter Conic Cent.Obstr. OBJ Inf Inf ENTRANCE FILTER Inf (Z-LOC OF SEC) Inf OUTSIDE BAFFLE2 Inf STO APERTURE MASK Inf SPIDER MASK Inf SPIDER MASK Inf PRIMARY MIRR OUTSIDE BAFFLE1 Inf INSIDE BAFFLE2 Inf SECONDARY MIRR INSIDE BAFFLE1 Inf FILTER WHEEL Inf IMA CCD Inf

7/12/ 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 mm or less.

7/12/ Preliminary Alignment Error Budget DecenterTip/Tilt Primary mirror0.25 mm1 arcmin Secondary mirror0.25 mm3 arcmin CCD0.35 mm6 arcmin

7/12/ 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)