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The RHESSI Imaging Concept

The RHESSI Imaging Concept. Rick Pernak Laboratory for Solar and Space Physics Goddard Space Flight Center And The Catholic University of America. RHESSI. NASA SMEX mission Designed to observe solar flares in X-rays and Gamma Rays Unprecedented Resolution Spatial Spectral Temporal

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The RHESSI Imaging Concept

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  1. The RHESSI Imaging Concept Rick Pernak Laboratory for Solar and Space Physics Goddard Space Flight Center And The Catholic University of America

  2. RHESSI • NASA SMEX mission • Designed to observe solar flares in X-rays and Gamma Rays • Unprecedented Resolution • Spatial • Spectral • Temporal • Launch: February 5, 2002

  3. Instrument • Rotating Modulation Collimator (RMC) • 9 Subcollimators, Grids, and Detectors • 2 sets of grids

  4. X-ray Imaging • Difficult to focus X-rays (or Gamma Rays) because of such a short wavelength • Focusing Optics not feasible • Need to block X-rays and cast shadows • Use collimator-based Fourier-transform imaging

  5. Detector and Incident Photons

  6. Modulation Profiles

  7. Imaging Software • From data (modulation profiles and/or visibilities), have the desired information to construct an image • Current algorithms: Back Projection, Clean, Pixon, MEM, Forward Fit

  8. Back Projection • Workhorse of RHESSI imaging • “Simplest” image reconstruction procedure • Constructs “probability maps” of incident photons

  9. How Back Projection Works

  10. Clean • First used in Radio Imaging • An extension of Back-Projection • Most-published • Iterative algorithm • Treats extended sources as superposition of point sources • Picks out brightest pixels • Convolves with Clean beam • Adds residuals

  11. Pixon • Most reliable algorithm (theoretically) in terms of photometry • Arranges pixels in map, creates its own modulation profile and tries to match the data (χ2 check) • The tradeoff is efficiency

  12. Visibilities • Radio-based concept • Essentially an amplitude and a phase • Created from modulation profiles • Analogous to radio interferometer visibilities

  13. Mathematical Approach to Visibilities • Each detector has a pitch, k, which is used to construct visibilities • UV points based on pitch and phase angle • Visibilities based on UV points and spatial coordinates

  14. MEM • Maximum Entropy Method • Another Radio-based imaging method • RHESSI MEM programs: MEM_VIS, MEM_SATO, MEM_NJIT • Entropy term: log of the map flux • Bayesian: S = H - (λ*χ2) • Cornwell/Evans: S = H – β*χ2 - α*F • H = Σ Fij*log(Fij)

  15. MEM_NJIT • MEM_SATO, MEM_VIS not successful • Both used counts/modulation profiles • MEM_NJIT is visibility-based, designed by New Jersey Institute of Technology • Starting to be used more and more by RHESSI researchers

  16. Forward Fitting • “Guess” at parameters of an event • Generate map that is consistent with data • Useable with modulation profiles, but new software uses visibilities

  17. Qualitative Comparison

  18. Scientific Studies • Using MEM_NJIT and Forward Fitting: • Asymmetries in Flux and Source Size • Energy dependence in the asymmetries

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