Wilkinson Microwave Anisotropy Probe

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Iterative algorithms are used to create skymaps from the calibrated differential time-ordered WMAP data for each of the ten differencing assemblies. Each pixel in a map represents a sky temperature for the bandpass appropriate to the differencing assembly. The CMB dipole has been removed from the Stokes I maps.

Maps for each year of WMAP data [currently the first, second, and third] were produced using the iterative method described above and have been supplied through LAMBDA. Full three year maps were produced by performing a weighted, pixel-by-pixel, mean of the three single year maps; the N_Obs measurement was used to weight this mean. The zero point of each three year Stokes I map has been set using a Galactic csc|b| model. The N_Obs measurements were added together to produce the final, three year, N_Obs measurement.

This same weighted mean method was then used to combine three year maps of the same frequency band into a single map for each frequency. Two maps were combined to produce the Q and V band maps; four maps were combined to produce the W band map.

WMAP maps are stored in FITS binary table extensions. The maps are stored in the first extension in a file; the format of this table depends upon whether polarization maps have been included in the file. Some files also contain the polarization covariance matrices for the maps; if supplied these matrices are stored in a second binary table extension.

As previously stated, the first FITS extension contains the maps. These maps are in a nested HEALPix format, with each row representing a single pixel. There are either four or two columns depending upon whether or not polarization maps are included; these columns are:

TEMPERATURE	The Stokes I, or temperature, measurement in mK (thermodynamic).
Q_POLARISATION	The Stokes Q polarization measurement in mK (thermodynamic).
U_POLARISATION	The Stokes U polarization measurement in mK (thermodynamic).
N_OBS	The effective number of observations.

The column names were selected to be compatible with existing HEALPix software. The TEMPERATURE and N_OBS columns are the only columns that exist in files that do not contain polarization maps.

Pixel noise in units of mK can be evaluated from N_obs with the expression σ = σ₀ / sqrt(N_Obs) where

	σ0 For Each Frequency Band
	K	Ka	Q	V	W
Stokes I	1.4391	1.4638	2.1898	3.1249	6.5112
Stokes Q,U	1.4551	1.4831	2.2128	3.1501	6.5484

These σ₀'s were computed by averaging together the per-DA numbers.

Additional Information

• Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Beam Profiles, Data Processing, Radiometer Characterization and Systematic Error Limits, N. Jarosik, et.al. (2006)
• WMAP Three Year Explanatory Supplement

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