The AllWISE Atlas Images are in the Flexible Image Transport System (FITS) format and follow the World Coordinate System (WCS) standard for astronomical image data. There are three image products per WISE band (see description), with generic filenames as follows:
BAND = 1, 2, 3, or 4, and COADDID is a unique identification string pertaining to the sky footprint. These are variables that match the values of keywords with the same name in FITS headers (see below).
Except for the BUNIT and FILETYPE keyword values (see descriptions below), FITS headers are essentially identical for the intensity, depth-of-coverage, and uncertainty image products for a given band. Keywords that will differ between bands for the same footprint on the sky are indicated below as "band dependent." An example of a typical Atlas Image FITS header in the AllWISE release is here. Below we expand on some of the non-trivial keywords and provide links to where you can find further information.
_________________________________________________________________________________________________________
KEYWORD EXAMPLE VALUE UNITS DESCRIPTION
_________________________________________________________________________________________________________
BITPIX -32 - Number of bits per data pixel. -32 => 32 bit
(4-byte) IEEE floating point. Same for "int",
"cov", and "unc" images.
NAXIS 2 - Number of data axes. Always = 2 for
two-dimensional images
NAXIS1 4095 pixels Length of data axis 1 or number of columns
in image.
NAXIS2 4095 pixels Length of data axis 2 or number of rows
in image.
CRVAL1 271.305017 degrees Right Ascension at CRPIX1,CRPIX2 for EQUINOX.
CRVAL2 -30.288889 degrees Declination at CRPIX1,CRPIX2 for EQUINOX.
EQUINOX 2000.0 year Equinox of the World Coordinate System (WCS).
CTYPE1 'RA---SIN' - Projection type for axis 1. Always set
to use the SIN (orthographic) projection;
for definition, see Calabretta & Greisen, 2002
CTYPE2 'DEC--SIN' - Projection type for axis 2. Always set
to use the SIN (orthographic) projection;
for definition, see Calabretta & Greisen, 2002
CRPIX1 2048.000000 pixels Axis 1 reference pixel at CRVAL1,CRVAL2
= 0.5*(NAXIS1+1)
CRPIX2 2048.000000 pixels Axis 2 reference pixel at CRVAL1,CRVAL2
= 0.5*(NAXIS2+1)
CDELT1 -0.0003819444391411 degrees/pix Axis 1 pixel scale at CRPIX1,CRPIX2.
See PXSCAL1 for arcsec equivalent.
CDELT2 0.0003819444391411 degrees/pix Axis 2 pixel scale at CRPIX1,CRPIX2.
See PXSCAL2 for arcsec equivalent.
CROTA2 0.000000 degrees Image twist: +axis2 W of N at EQUINOX;
In the opposite direction to the
conventional Position Angle, where
P.A. = -CROTA2.
BITMASK 405273647 - Fatal bitstring mask template used to
reject input frame pixels. Bits are
defined here. Band dependent.
WRELEASE 'release-v6.0' - WISE Science Data System (WSDS)
software release tag
FILETYPE 'intensity image' - Image product description. Can be either:
"intensity image" (int),
"depth-of-coverage image" (cov), or
"1-sigma uncertainty image" (unc).
BUNIT 'DN' - Image pixel units: = Data Number (DN) for
int and unc FILETYPEs; = "effective pixels"
for the cov FILETYPE. Image types are
described here.
ORIGIN 'WISE Science Data Center, IPAC, Caltech' Origin of image data.
FCREATE '2013-07-06T20:07:53.000Z' - WSDC file creation date/time (UTC);
format is: yyyy-mm-ddThr:min:sec
DATEOBS1 '2010-03-20T22:30:33.913' - Observation midpoint date/time of earliest
input frame used (UTC); format is:
yyyy-mm-ddThr:min:sec
DATEOBS2 '2010-09-21T14:11:43.369' - Observation midpoint date/time of latest
input frame used (UTC); format is:
yyyy-mm-ddThr:min:sec
MIDOBS '2010-09-19T03:27:36.043' - Observation midpoint date/time of median
time-ordered input frame used (UTC);
format is: yyyy-mm-ddThr:min:sec
TELESCOP 'WISE' - Telescope used to acquire data.
BAND 1 - Wavelength band number, = 1,2,3 or 4.
WAVELEN 3.368 μm Effective wavelength of band. Corresponds
to a spectrum having νfν=constant as
derived here.
COADDID '2713m303_ac51' - Atlas-image tile identifier (see description)
Unique for each sky footprint tile.
NUMFRMS 223 - Final number of frames touching footprint
after all filtering is performed for
given band. The same frames are used
for multiframe photometry. Band dependent.
MOONREJ 3 - Number of frames rejected due to moon
glow by coadd moon rejection algorithm.
Band dependent.
MOONINP 3 - Initial number of frames with suspect moon-glow
as determined by moon-mask prior to any
filtering by coadd moon rejection algorithm.
Usually MOONINP >= MOONREJ. Band dependent.
MAGZP 20.5 magnitudes Relative coadd photometric zero point.
Band dependent.
MAGZPUNC 0.006 magnitudes One-sigma uncertainty in coadd photometric
zero point. Band dependent.
SIZEX 1.564444 degrees Linear dimension of orthographically projected
tile on sky along axis 1.
SIZEY 1.564444 degrees Linear dimension of orthographically projected
tile on sky along axis 2.
PXSCAL1 1.375 arcsec/pix Pixel scale at CRPIX1,CRPIX2 for axis1.
PXSCAL2 1.375 arcsec/pix Pixel scale at CRPIX1,CRPIX2 for axis2.
MEDINT 73.876953125 DN Median of intensity pixels. Band dependent.
MEDCOV 23.248046875 pixels Median depth-of-coverage in units of
number of effective overlapping pixels.
Band dependent.
MINCOV 0 pixels Minimum depth-of-coverage in units of
number of effective overlapping pixels.
Band dependent.
MAXCOV 29.009765625 pixels Maximum depth-of-coverage in units of
number of effective overlapping pixels.
Band dependent.
LOWCOVPC 0.0483749055177627 % Percentage of area in coadd tile with
depth-of-coverage <= 4 pixels - depths at
which no temporal outlier rejection
is performed due to unreliability of
process. Band dependent.
LOWCOPC 0.0806367692814579 % Percentage of area in coadd tile with
depth-of-coverage <= 6 pixels. Band dependent.
NOMCOVPC 0.124050146028168 % Percentage of area in coadd tile with
depth-of-coverage <= 8 pixels - the depth
where some mission requirements are defined.
Band dependent.
PCTMINC 4.6E-05 % Percentage of area of internal working
tile used for outlier-detection with
depth-of-coverage <= 4 pixels. Band dependent.
ROBSIG 4.66322662740479 DN Robust spatial sigma of intensity pixels whose
depths fall within: MEDCOV +/- 0.25. The
sigma is computed from a trimmed standard-
deviation from the mode using only the lower-
tail of the intensity pixel distribution.
Band dependent.
PIXCHIS1 26.4871919096551 - Pseudo reduced-χ2 measure #1 computed from the
ratio: ROBSIG/(median of unc pixels) for depths-
of-coverage falling within: MEDCOV +/- 0.25.
See also section II.3.e.i. Band dependent.
PIXCHIS2 30.0087608261587 - Pseudo reduced-χ2 measure #2 computed from the
ratio: ROBSIG/(mode of unc pixels) for depths-
of-coverage falling within: MEDCOV +/- 0.25.
See also section II.3.e.i. Band dependent.
MEDNMSK 44512 pixels Median number of pixels masked per frame
(over all NUMFRMS) due to bad pixels, saturation,
spatial outliers (e.g., cosmic-ray spikes),
and temporal outliers (e.g., fast moving objects,
non-uniform moon glow, or other inconsistencies
between overlapping pixels). Band dependent.
RANDSEED -999 - Random number seed to assist with reprocessing.
Pertains to frame-randomization in outlier-detection
step of the multiframe pipeline [-999 => not used].
ELON 271.134962303537 degrees Ecliptic longitude (λ) at image center.
ELAT -6.85473907646948 degrees Ecliptic latitude (β) at image center.
GLON 0.975527205832547 degrees Galactic longitude (l) at image center.
GLAT -4.3614880257035 degrees Galactic latitude (b) at image center.
MOONMASK '/wise/fops/ref/coadd/wise-w1-moonmask.fits' Moon image mask used in processing (for
internal tracking). Band dependent.
FLXWTNUM 3874 - number of sources used to compute centroids (see V.3.a.iii.5)
FLXWTDX 0.0291539 arcsec mdex - flux-weighted centroid along X (see V.3.a.iii.5)
FLXWTUDX 0.0301293 arcsec 1-sigma uncertainty in flxwtdx (see V.3.a.iii.5)
FLXWTSDX 0.9676279 - signal-to-noise ratio in flxwtdx (see V.3.a.iii.5)
FLXWTDY 0.0756675 arcsec mdex - flux-weighted centroid along Y (see V.3.a.iii.5)
FLXWTUDY 0.0299443 arcsec 1-sigma uncertainty in flxwtdy (see V.3.a.iii.5)
FLXWTSDY 2.5269451 - signal-to-noise ratio in flxwtdy (see V.3.a.iii.5)
Last update: 22 November 2013