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CREOL & FPCE: The College of Optics and Photonics Solar Physics, Space Weather, and Wide-field X-ray Telescopes James E. Harvey Optical Design and Image Analysis Laboratory [email protected] Presented at CREOL Industrial Affiliates Day “Optics & Photonics for Space-Based and Medical Applications” April 21, 2006 Abstract The sun is essentially a giant thermonuclear fusion reactor (105 x the size of the Earth). The detrimental effects of solar storm induced “space weather” ranges from disruption of our space communication systems to astronaut health hazards to power grid overloads and blackouts. The National Oceanic & Atmospheric Administration (NOAA) and NASA are cooperating on a Solar X-ray Imager (SXI) program intended to allow NOAA to monitor and predict space weather. Four flight models and a spare of the SXI telescope have been designed and fabricated. The first is scheduled to be launched on the next-generation Geosynchronous Operational Environmental Satellite (GOES-N) in 2006. After briefly reviewing solar physics and space weather, this talk will discuss a new class of wide-field grazing incidence X-ray telescopes developed at CREOL specifically for the SXI program. This new design differs significantly from the classical Wolter Type I designs previously built and used as X-ray stellar telescopes. Outline • High-Energy Solar Physics • Space Weather and its Detrimental Effects • X-ray Astronomy and Grazing Incidence X-ray Telescopes • The Solar X-ray Imager (SXI) Telescope • Summary and Conclusions Solar Physics: The Photosphere The Visible Sun (Photosphere) ● Sun is a ball of gas ~ 700,000 km radius. ● Photosphere is the sun’s visible surface. ● Actually a layer about 100 km thick ● Easily observable features include: Ο Ο sunspots. “limb darkening” The Sunspot Cycle ● Galileo made the first European observations of Sunspots in 1610. ● Daily observations were started at the Zurich Observatory in 1749. ● Continuous observations since 1849. ● Detailed records indicate a dramatic periodic variation (factor > 100) in the number of sunspots, with a period of about 11 years. ● The nature and causes of the sunspot cycle constitute one of the great mysteries of solar astronomy. Solar Physics: The Solar Corona The White-light Corona ● Corona is the Sun's outer atmosphere. ● Visible during total eclipses of the Sun as a pearly white crown surrounding the Sun. ● Displays a variety of features including streamers, plumes, and loops. ● Coronal gases are super-heated temperatures greater than 1,000,000ºC. to ● Hydrogen and helium are stripped of their electrons, producing spectral emission lines. ● The highest temperature regions of the corona produce intense X-ray emissions. Emission-line Corona (Green Line) The X-ray Corona Solar Physics: The Sun as a Laboratory ● Serves an important role in helping to understand the rest of the astronomical universe. The Sun as a Star Star-birth Clouds (M16) recorded by HST ● Only star close enough to us to reveal details about its surface. ● Sun is thus the key to understanding the birth and evolution of other stars. The Solar Core ● Sun is a giant thermonuclear fusion reactor (105 X size of Earth). ● Sun produces its energy by nuclear fusion of four hydrogen nuclei to form single helium nuclei deep within the Sun's core. ● This energy diffuses outward by radiation and by convective fluid flows. Magnetism: the Key to Understanding the Sun ● Magnetic fields are produced on the Sun by the flow of electrically charged ions and electrons. Magnetic Loops and Solar Flares ● Sunspots are places where very intense magnetic lines of force break through the Sun's surface. ● Streamers, flares, and loops seen in the corona are shaped by magnetic fields. Coronal Mass Ejections July 1, 2002 Soft X-ray Image Solar Environment ● Solar Wind: low energy charged particles created by magnetic storms propagate from the sun (v < c/1000). ● Sunspots (X-ray hot spots). ● Solar Flares (solar-magnetic storms) Solar and Heliospheric Observatiory (SOHO) ● Coronal mass ejections (CME’s) are explosions in the sun’s corona that spew out tons of high-energy charged particles (v < c/100). Outline • High-Energy Solar Physics • Space Weather and its Detrimental Effects • X-ray Astronomy and Grazing Incidence X-ray Telescopes • The Solar X-ray Imager (SXI) Telescope • Summary and Conclusions Solar Storm Induced “Space Weather” ● Earth’s magnetic field provides protection from the brunt of solar flares and CME’s. Sun-Earth Electromagnetic Connection ● However, the Sun-Earth electromagnetic connection serves to funnel residual charged particles into the Van Allen belts where they become trapped and initiate various geomagnetic instabilities. ● These geomagnetic instabilities constitute a type of space weather that can result in a variety of detrimental effects. Sun-Earth Electromagnetic Connection ● If the Earth’s position happens to coincide with one of these plasma “hurricanes” caused by a coronal mass ejection, it becomes a detrimental geomagnetic storm. ● Solar storm induced “space weather” erodes our space communications technological infrastructure. ● Although auroras are a pleasing side-affect of this extra-terrestrial phenomenon, they are outweighed many times over by the dangerous and costly consequences. Detrimental Effects of “Space Weather” ● Communication satellites and scientific spacecraft are vulnerable to the ravages of severe space weather which can jeopardize Ο Cell phone service Ο GPS systems Ο Scientific data links Communication Satellite Failure Synchronous Orbit Low earth Orbit ● Also, fluctuating electro-jet currents high in the atmosphere can produce magneticallycoupled current overloads in terrestrial power lines resulting in electrical blackouts. Power Grid Overloads Detrimental Effects ● Geomagnetic Storms (compass / GPS malfunction) ● Power Grid Overloads (blackouts) ● Ionospheric Expansion (satellite deorbit) ● Electromagnetic Interference (communications failure) ● Radiation Overdose (astronaut health hazard) ● Single Event Upsets (satellite electronics malfunction) Outline • High-Energy Solar Physics • Space Weather and its Detrimental Effects • X-ray Astronomy and Grazing Incidence X-ray Telescopes • The Solar X-ray Imager (SXI) Telescope • Summary and Conclusions Deterrents to the Development of X-ray Astronomy ● Adequate X-ray detectors have not always been available. ● No suitable refractive material exists for fabricating X-ray lenses. ● Normal incidence mirrors exhibit extremely low reflectance at X-ray wavelengths, thus the development of grazing incidence optical designs. ● Grazing incidence optical designs are cumbersome and difficult to fabricate and align. ● Surface scatter effects (even from our smoothest mirrors) severely degrade X-ray image quality. ● Atmospheric absorption requires a spaced-based telescope. X-ray Astronomy is a Relatively New Science ● 1948: First 2-D X-ray imager (Kirkpatrick and Grazing Incidence Mirrors Required Baez). ● 1952: X-ray Telescope Designs (Wolter). ● 1962: First X-ray telescope launched into Kirkpatrick-Baez space (no mirrors); discovered Sco-X1 in the constellation Scorpius (Giacconi). ● 1970: First satellite, Uhuru, dedicated to the discovery of X-ray (Giacconi, Tananbaum). cosmic sources ● 1978: First X-ray telescope with mirrors, the Einstein Observatory (Giacconi, Tananbaum). Barrel, not Dish Shaped Mirrors Einstein Observatory (Wolter Type I) 2002 Nobel Prize in Physics Riccardo Giacconi For pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources. X-ray Astronomy and High-energy Solar Physics Grazing Incidence Imaging Systems • • • • • • • The Einstein Observatory (HEAO B) 1978 The European ROSAT Telescope 1990 Japanese Yohkoh Solar Telescope 1991 UC Berkeley EUVE Telescope 1992 The Chandra Observatory (AXAF) 1999 ESA X-ray Spectrum Mission (XMM) 2000 The Solar X-ray Imager (SXI) Solar Cycle 200 Sunspot Number 2006 150 100 50 0 1987 1991 1995 1999 2003 2007 August 1999 Classical Wolter Type I X-ray Telescope Design (Ideal for Stellar Telescopes) rp2 = 2 Rp ( z − zp ) 2 2 Hyperboloid: (z − 2zh ) − rh2 = 1 ● Three independent parameters: Rp, a, b. Paraboloid: a r ● Equalizing grazing angle: two remain. b ● ro and f ’ completely defines system. Lh Lp Gap Paraboloid Hyperboloid ro z = 0 z1 . . Rp x2 a(ε -1) f’ z 2aε ● Co-aligned and confocal grazing incidence paraboloid and hyperboloid mirror. ● Corrected for spherical aberration. ● Can only balance defocus against field curvature by displacing the focal plane. ● Ideal for small-field stellar telescopes, not for wide-field staring solar telescopes. Outline • High-Energy Solar Physics • Space Weather and its Detrimental Effects • X-ray Astronomy and Grazing Incidence X-ray Telescopes • The Solar X-ray Imager (SXI) Telescope • Summary and Conclusions National Oceanic & Atmospheric Administration (NOAA) • Operating unit of the U.S. Department of Commerce. – National Weather Service – National Satellite and Information Service • NOAA’s charter has historically been to monitor and predict terrestrial and atmospheric weather and climatic changes. • NOAA operates the Geosynchronous Operational Environmental Satellites (GOES) whose down-looking telescopes produce the weather maps we see on TV each night. • NOAA’s charter has recently been changed to include monitoring and predicting of Space Weather. • The Solar X-ray Imager (SXI) Instrument is intended to help NOAA accomplish that goal. The Solar X-ray Imager (SXI) Contract ● NASA/GSFS was the contract monitor for the SXI program. ● Lockheed Martin’s Solar and Astrophysics Laboratory (LMSAL) was the prime contractor responsible for designing and building the SXI instrument. ● Raytheon Optical Systems, Inc. of Danbury, CT was the subcontractor responsible for fabricating the precision grazing incidence X-ray mirrors. ● CREOL also had a small subcontract from LM on the SXI program (primarily because of our surface scatter expertise). CREOL’s Role in the SXI Program (Perform a Complete Systems Engineering Analysis of Image Quality) ● Include Surface Scatter Effects, and ● and Misc. Errors from Manufacturer’s Error Budget Tree * * Aperture Diffraction PSF * Geometrical PSF Surface Scatter PSF Residual Misc. Error PSF Intensity 1.0 Hsystem = Hdiffraction Haberrations Hscattering Hmisc 0.5 0.0 F -20 Az i m -10 ut ha lA n F -20 gl 0 e (a r -10 c 0 10 se c) 10 20 20 di Ra ng al A ar le ( cs e c) PSFsys = PSFdiff PSFaberr PSFscat PSFmisc Resulting System PSF (or Aerial Image) of SXI Telescope (Six arc min Field Angle, λ = 44.7 Å) * * * Determined the Need for an Alternative Optical Design for SXI ● Kick-off meeting in Palo Alto with NOAA, NASA, Lockheed Martin, Raytheon, and CREOL representatives present. ● Bi-monthly optical fabrication progress reviews at Raytheon Optical Systems, Inc. in Danbury, CT. ● Graduate student Patrick Thompson and I started developing our image analysis MatLab code. ● Quickly came to realize that the top level image quality requirement was inappropriate for a staring solar telescope, and the classical Wolter Type I was a non-optimum design. ● Tried to raise a question concerning the optical design at a general SXI meeting (with customers present). Nobody wanted to hear it! ● Back at CREOL Patrick and I continued to evaluate other designs. ● Finally, I got LM to give me 30 min on the agenda at one of the bi-monthly optical fabrication meetings (without NOAA and NASA customer present). Image Quality Criterion for Wide-field Applications (Desire Fine Detail in Extended Object) ● SXI is a wide-field staring X-ray telescope pointed at the center of the sun. ● The total information content in a given image is maximized if we minimize the field weighted average resolution element as degraded by all error sources. HPR fwa = 1 AT where Solar X-ray Imager (SXI) Wide-field X-ray Imaging Applications θy dθ θ dθ dϕ dϕ θx OFOV ∫ HPR(θ ) 2πθ dθ , θ =0 AT = π (OFOV ) 2 ● Since scattering effects are a significant error source at these wavelengths, the field weighted average half power radius (HPRfwa) of the point spread function (PSF) as degraded by all error sources is an appropriate image quality criterion for the SXI mission. Solar Disc (15 arc min radius) OFOV N = # of Res. Ele. = 2π θ ∫ π HPR (θ ) dθ θ =0 2 Hyperboloid-Hyperboloid Grazing Incidence X-ray Telescope Design ● Five parameters (Rvp, εp, Rvs, εs, and svv) are required to completely characterize the hyperboloid-hyperboloid optical prescription. ● In addition, the primary and secondary mirror lengths and the gap separating them must be specified (Lp, Ls , and gap). y Ls Lp Primary Gap Secondary rj f’ z = 0 z1 zj svv z•sf1 zsv1 zf Δf zcs 2asεs Δps x zsv2z•sf 2 zpf 1 zpv1 z Extra design variables allow us to balance various aberrations over a given OFOV. Geometrical Performance of a Family of Optimal Hyperboloid-Hyperboloid Designs 12 θB = 1.5 θB = 3.8 θB = 7.9 θB = 10.5 θB = 12.1 θB = 14.3 θB = 16.1 θB = 17.2 θB = 20.0 RMS Image Radius (arc sec) 10 8 6 4 2 Linear Component (coma) Cannot be Corrected! 0 0 3 6 9 12 15 18 21 Field Angle (arc min) Defocus, field curvature, SA3, astigmatism, and oblique spherical aberration are all balanced at a different field angle for each design! This leaves only a residual linear coma aberration at that unique field angle. Optical Design Methodology ● Choose the desired OFOV, and plot the field-weighted average rms image size vs. optical design parameter θB . This yields optimum parameter θB. ● Go to ZEMAX Optical Design Code with that parameter to obtain the optimum optical prescription of the telescope. ● Perform exhaustive ray trace analysis at various field angles (geometrical PSF). Finding Optimum Design Parameter Field-weighted average HPR vs OFOV 14 Field-Weighted-Average RMS Image Radius (arc sec) ● 2-D integration of previous curves yield the field-weighted average rms image size versus the OFOV. θB θB θB θB θB θB θB θB θB 12 10 = 1.5 = 3.8 = 7.9 = 10.5 = 12.1 = 14.3 = 16.1 = 17.2 = 20.0 = 22.5 8 6 4 2 0 0 3 6 9 12 15 18 21 24 27 H-T#17 Optical Design Prescription Field-Weighted-Average RMS Image Radius (arc sec) 5.0 OFOV = 18 arc min 4.5 SXI Telescope System Parameters 4.0 3.5 3.0 2.5 0 2 4 6 8 10 12 14 16 Angle at which rms Image size is Minimized (θB in arc min) 18 30 Operational Field Of View (arc min) Complete Systems Engineering Analysis of Image Quality 6 arc min Field Angle, λ = 44.7Å * * Aperture Diffraction PSF * Geometrical PSF Surface Scatter PSF Residual Misc. Error PSF Intensity 1.0 Hsystem = Hdiffraction Haberrations Hscattering Hmisc 0.5 0.0 F -20 Az i m -10 ut ha lA n -20 gl 0 e (a r -10 c 0 10 se c) 10 20 20 d ia Ra ng lA ar le ( c ) se c F PSFsys = PSFdiff PSFaberr PSFscat PSFmisc Resulting System PSF (or Aerial Image) of SXI Telescope * * * Image Evaluation of the Optimum Design ● Insight concerning the relative effect of the various image degradation mechanisms obtained by plotting the fractional encircled energy of the aerial image and its four main contributors. Fractional Encircled Energy of on-axis PSF HT#17 Fractional Encircled Energy ● Detailed systems engineering analysis of image quality is very computationally intensive. Scatter Effects Dominate Geo. Aberrations for Small Field Angles θ = 0.0 arc min λ = 13.3Å ADPSF GPSF SSPSF HPR ● Plot HPR of aerial image vs. field angle for different wavelengths (or X-ray energies). RMEPSF Aerial Image Radius of Circle (arc sec) Fractional Encircled Energy of off-axis PSF Graphical Display of Systems Performance 9 Wavelength = 44.7 A Geo. Aberrations Dominate Scatter Effects For Large Field Angles HT#17 θ = 20.0 arc min λ = 13.3Å ADPSF HPR GPSF SSPSF RMEPSF Aerial Image Wavelength = 8.34 A 7 Half Power Radius (arc sec) Fractional Encircled Energy 8 6 5 4 3 2 1 0 0 Radius of Circle (arc sec) 3 6 9 12 Field Angle (arc min) 15 18 21 H-T#17 Design yields an 80% Increase in the Number of Spatial Resolution Elements over the Solar Disc Percent increase of the H-T#17 design (over NOAA baseline design) in the number of spatial resolution elements versus the radius of the operational field-of-view. % Increase in # of Spatial Res. Ele. 100 90 87% Increase 80 76% Increase 70 60 50 18 arc min radius OFOV 40 30 Wavelength = 44.7 A 20 OFOV N = # of Res. Ele. = 2π 10 Wavelength = 8.34 A θ ∫ π HPR (θ ) dθ θ =0 2 0 12 13 14 15 16 17 18 19 Radius of Operational Field-of-View (arc min) 20 21 CREOL X-ray Telescope Design Adopted by NOAA for GOES Series N-P Missions ● Solar X-ray Imager (SXI) is a complimentary, add-on instrument designed primarily for use on the GOES next generation satellites. SXI Engineering Model ● Modular design is suitable for installation on many other spacecraft platforms. ● Four flight models and a spare of the grazing incidence X-ray mirrors were fabricated to the CREOL HT#17 design by Goodrich Optical & Space Systems, Inc. ● The Engineering Model underwent vibration and electrical testing at LMSAL in Palo Alto. Fabricating Grazing Incidence Mirrors CREOL’s Role in GOES/SXI ● Developed a systems engineering analysis capability to include the effects of surface scatter and other manufacturing errors. ● Continued to provide general technical support by modeling the “as manufactured” mirrors and optimizing final detector plane position. ● Re-defined image quality requirem’t appropriate for wide-field X-ray imaging applications. ● Developed new X-ray telescope design to yield an 80% increase in performance over the NOAA Baseline Design. Conclusion ● GOES-N is the first of the next generation weather satellites that will include a Solar X-ray Imager (SXI) instrument to provide full solar disc images of the sun at X-ray wavelengths. ● The first model was incorporated into the GOES-N satellite and scheduled for launch in May 2005 (launch was scrubbed due to computer glitch). ● GOES-N was re-scheduled for launch in June, and again in July. It was cancelled each time, and is currently awaiting the installation of new batteries before being re-scheduled for launch. GOES-N