Appendix G  THE COBE SKYCUBE

COBE data is pixelized using a variation of a quadrilateralized
spherical projection that was originally developed for Earth science
databases.  In this scheme, the sky is projected onto a cube (also
called a skycube), with faces numbered 0 through 5.  Face 0 contains
the North Ecliptic Pole and face 5 contains the South Ecliptic Pole.
The Ecliptic plane spans the remaining faces.  (See Figures G-1 and
G-2.)  The reference frame for the cube is defined by axes normal to
faces 1 and 0.  In Geocentric Ecliptic coordinates, these directions
correspond to the vernal equinox and the North Ecliptic Pole.
Additional details of the quad-sphere projection, including coordinate
transformations, are found in the references.

The terminology used to indicate the level of resolution is resolution
n, where n-1 is the number of subdivisions that were made to the face.
For instance, a face with resolution 6 was subdivided 5 times, has 2^5
= 32 pixels on a side, and has 4^5 = 1024 total pixels.

The standard DIRBE pixel resolution is 256 x 256 per cube face
(resolution 9), with a pixel size of approximately 0.32 x 0.32
degrees.  For DMR and FIRAS, the standard resolution is 32 x 32 per
cube face (resolution 6), corresponding to a pixel size of about 2.6 x
2.6 degrees.  Figures G-3 and G-4 illustrate the pixel numbering for
both DIRBE and FIRAS/DMR resolutions.


References

Chan, F. K. and O'Neill, E. M.  Feasibility Study of a Quadrilateralized
Spherical Cube Earth Data Base, EPRF Technical Report 2-75 (CSC), March 1975.

O'Neill, E. M. and Laubscher, R. E.  Extended Studies of a Quadrilateralized
Spherical Cube Earth Data Base, NEPRF Technical Report 3-76 (CSC), May 1976.

White, R. A. and Stemwedel, S. W. 1992, ``The Quadrilateralized Spherical Cube
and Quad-Tree for All Sky Data,'' in Astronomical Data Analysis Software and
Systems I, eds. D. M. Worrall, C. Biemesderfer, and J. Barnes (San Francisco:
ASP), pp. 379-381.


        Figure G-1.  COBE skycube, ecliptic coordinates.


        Figure G-2.  COBE skycube, galactic coordinates.


        Figure G-3.  COBE skycube, resolution 6 pixel numbering.


        Figure G-4.  COBE skycube, resolution 9 pixel numbering.