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Pixelization of the GLAST Sky

Pixelization of the GLAST Sky. Toby Burnett, Bruce Lesnick University of Washington. How to subdivide (pixelate) the full Sky? Two schemes. Plan A: a grid in a projection, like Aitoff, Mercator, Carr é e, …

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Pixelization of the GLAST Sky

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  1. Pixelization of the GLAST Sky Toby Burnett, Bruce Lesnick University of Washington Catalog Meeting 1/5/05 - T. Burnett

  2. How to subdivide (pixelate) the full Sky? Two schemes • Plan A: a grid in a projection, like • Aitoff, • Mercator, • Carrée, … Not equal area, or boundaries, or poles(pictures from http://www.progonos.com/furuti/MapProj/Normal/ProjTbl/projTbl.html) Catalog Meeting 1/5/05 - T. Burnett

  3. HTM (Hierarchical Triangular Mesh) Index the sphere (used by GSSC database) Supports data binning – hierarchical for storage Not equal area All triangles, 4 neighbors HEALPix (Hierarchical Equal Area isoLatitude Pixelization) Numerical analysis Topological analysis Equal area pixels 4-sided pixels, 7 or 8 neighbors Adopted by WMAP, PlanckGAIA Plan B: non-projective tesselization: 2 schemes Catalog Meeting 1/5/05 - T. Burnett

  4. HTM and Healpixx classes: STL containers of Pixels • Constructors: set up configuration (number of pixels)HTM needs to actually create the hierarchy • begin() and end() methods: return iterators that evaluate to pixels. • Following code uses STL methods to create MapParameters par(argc, argv);// defines image parameters SkyImage piximage(par); // manage a FITS image Healpix hpix(8); // the HEALPix class std::for_each( h.begin(), h.end(), DrawPixel(hix,piximage)); Catalog Meeting 1/5/05 - T. Burnett

  5. HTM pixel directions Level=3512 pixels Catalog Meeting 1/5/05 - T. Burnett

  6. HEALPix pixel directions nside=8, 778 pixels Catalog Meeting 1/5/05 - T. Burnett

  7. Pixelization: to/from SkyDir Behave like • The constructor Healpix::Pixel(const astro::SkyDir& dir, const Healpix &hp)creates a pixel containing the SkyDir direction dir, for the Healpix object hp. • The methodHealpix::Pixel::operator astro::SkyDir ()constreturns the pixel central direction as a SkyDir: a Pixel “behaves” like a SkyDir. • A pixel “knows” its • direction • Area (solid angle) • Neighboring pixels (7 or 8) construct Catalog Meeting 1/5/05 - T. Burnett

  8. Status, prospects • HTM and Healpix created to do integrals over the sphere. • Also a good way to pixelate exposure functions, and bin the data for sky survey • HTM: • Available now in astro package • Pixelization function not implemented yet • Neighbor code also not ready • Healpix: • Using C code from WMAP, not checked in yet • Code to obtain neighbors almost ready. • Next: interface to special FITS image format • After that: SkyFunction containing Healpix image Catalog Meeting 1/5/05 - T. Burnett

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