Wilkinson Microwave Anisotropy Probe DR2

The data made available through this page has been updated. The most recent version of this data may be accessed through https://lambda.gsfc.nasa.gov/product/wmap/current/

Sky Maps Per Frequency Band

Coordinate System:
Galactic
Projection Type:
0.23°- 0.93° (frequency dependent)
Resolution:
HEALPix, nested, res 9 (Nside=512)
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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).
SPUR_SIGNAL The bandpass mismatch component.
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
Differencing Assembly Frequency Band
Stokes I Stokes Q,U Stokes I Stokes Q,U
K1 1.4391 1.4551 1.4391 1.4551
Ka1 1.4638 1.4831 1.4638 1.4831
Q1 2.2449 2.2693 2.1898 2.2128
Q2 2.1347 2.1563
V1 3.3040 3.3305 3.1249 3.1501
V2 2.9458 2.9696
W1 5.8833 5.9179 6.5112 6.5484
W2 6.5324 6.5707
W3 6.8849 6.9246
W4 6.7441 6.7803

Some files that contain polarization maps contain a second FITS extension containing the #nxn# polarization noise covariance matrices for the pixels in the maps of the first FITS extension. These matrices are used in the map-making process and characterize the noise properties of the polarization maps. See section 3.4 of Jarosik, et.al., 2006 in the set of three year WMAP papers for more details. The columns of this extension are:

N_OBS The effective number of observations as reported in the first extension. This is included here for completeness.
M11 The bandpass mismatch (SS) noise covariance term in units of N_Obs-style counts.
M12 The SQ noise covariance term in units of N_Obs-style counts.
M13 The SU noise covariance term in units of N_Obs-style counts.
M22 The QQ noise covariance term in units of N_Obs-style counts.
M23 The QU noise covariance term in units of N_Obs-style counts.
M33 The UU noise covariance term in units of N_Obs-style counts.

The covariance matrix of a pixel may be constructed by assembling the row elements into a 3x3 or 2x2 symmetric matrix with the form:
M11M12M13
M12M22M23
M13M23M33

QQQU
UQUU

The size of the matrix depends upon whether or not the spurious signal component is included.

Additional Information

A service of the HEASARC and of the Astrophysics Science Division at NASA/GSFC
Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Alan P. Smale
LAMBDA Director: Dr. Thomas M. Essinger-Hileman
NASA Official: Dr. Thomas M. Essinger-Hileman
Web Curator: Mr. Michael R. Greason