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WMAP

WMAP Sky Map File Format Description

WMAP maps are stored in FITS binary table extensions. The maps are stored in the first extension in a file; the number of columns in this table depends upon the maps 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. Each row in the table represents a single pixel. The number of columns depends upon which maps are included; these columns may be:

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 for I, Q, U, and N_Obs maps 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. The Q_POLARISATION and U_POLARISATION exist in files containing polarization maps; the names were selected to be compatible with existing HEALPix software. The SPUR_SIGNAL columns exists only in polarization map files that contain an 'S' map.

The WMAP skymaps are supplied using the HEALPix nested pixel ordering scheme in Galactic coordinates. For the convenience of users who do not have HEALPix software installed, we provide a series of FITS sky map files that give the sky coordinates of each pixel center for various resolutions.

Some files that contain polarization maps contain a second FITS extension containing the 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 Jarosik, et.al., 2007 and Hinshaw, et.al., 2008 for more details. There are two forms of this matrix, depending upon whether or not a bandpass mismatch term (S map) has been included in the file.

If the S map is supplied then 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 for a pixel may then be constructed by assembling the row elements into a 3x3 symmetric matrix with the form:
M11M12M13
M12M22M23
M13M23M33

If, on the other hand, the S map is not supplied then the columns of this extension are:

QQ The QQ noise covariance term in units of N_Obs-style counts.
QU The QU noise covariance term in units of N_Obs-style counts.
UU 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 2x2 symmetric matrix with the form:
QQQU
QUUU

Additional Information

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Goddard Space Flight Center, National Aeronautics and Space Administration
HEASARC Director: Dr. Alan P. Smale
LAMBDA Director: Dr. Eric R. Switzer
NASA Official: Dr. Eric R. Switzer
Web Curator: Mr. Michael R. Greason