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