Pixelization of the GLAST Sky

1
Pixelization of the GLAST Sky

• Toby Burnett, Bruce Lesnick
• University of Washington

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)

3
Plan B non-projective tesselization 2 schemes

• 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

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 stdfor_each( h.begin(), h.end(),
DrawPixel(hix,piximage))
5
HTM pixel directions
Level3512 pixels
6
HEALPix pixel directions
nside8, 778 pixels
7
Pixelization to/from SkyDir

• The constructor HealpixPixel(const
  astroSkyDir dir, const Healpix hp)creates a
  pixel containing the SkyDir direction dir, for
  the Healpix object hp.
• The methodHealpixPixeloperator astroSkyDir
  ()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)

Behave like
construct
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