Cosmic Background Explorer/DMR
4-year Sky Maps, HEALPix tessellation
HEALPix, nested, res 5 (Nside=32)
These maps were made by re-processing the DMR 4-year, time-ordered, differential data into sky maps using the HEALPix tessellation. Although not an official product of the COBE project, they are being served by LAMBDA with the hope that it will foster continued use of the DMR sky maps. We also provide the 'custom' Galaxy cut, used by the COBE/DMR team, in the HEALPix tessellation.
There are six sky maps, corresponding to the two independent channels (designated A and B) and three frequencies (31.5, 53, and 90 GHz) of the COBE Differential Microwave Radiometers (DMR). Each map was produced by Tony Banday and Gary Hinshaw in 1998 as follows. The final 4-year time-ordered differential data archive was converted to a calibrated archive in FITS format (the DMR "CIO" data). The WMAP prototype map-making software was adapted to the DMR experiment and used to generate these maps using the Healpix tessellation of the sky. As a check, the same archive and code were used to generate maps in the original COBE quad-cube pixelization. Except for the special case of the 31B map, these latter maps agreed with the officially released COBE-DMR maps to numerical precision. For the 31B channel (only) the maps differ slightly from the officially released COBE-DMR maps because of a difference in how the radiometer's magnetic susceptibility correction was applied. In the original pipeline, the correction involved three separate fit coefficients for three separate time periods of usable data. For technical reasons, the CIO-based pipeline used one fit coefficient for the correction of all three time periods. Since the 31B channel has the least weight of the 6 DMR channels, this difference should be of no significant consequence.