The WISE 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 preliminary 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 62.999995 degrees Right Ascension at CRPIX1,CRPIX2 for EQUINOX. CRVAL2 -48.462222 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 136838191 - Fatal bitstring mask template used to reject input frame pixels. Bits are defined here. Band dependent. WRELEASE 'release-v3.5.1' - 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 '2010-10-19T00:10:10.000Z' - WSDC file creation date/time (UTC); format is: yyyy-mm-ddThr:min:sec DATEOBS1 '2010-01-20T12:59:19.768' - Observation midpoint date/time of earliest input frame used (UTC); format is: yyyy-mm-ddThr:min:sec DATEOBS2 '2010-01-28T16:19:18.187' - Observation midpoint date/time of latest 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 '0629m485_aa11' - Atlas-image tile identifier (see description) Unique for each sky footprint tile. NUMFRMS 369 - 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 0 - Number of frames rejected due to moon glow by co-add moon rejection algorithm. Band dependent. MOONINP 0 - Initial number of frames with suspect moon-glow as determined by moon-mask prior to any filtering by co-add moon rejection algorithm. Usually MOONINP >= MOONREJ. Band dependent. MAGZP 20.5 magnitudes Relative co-add photometric zero point. Band dependent. MAGZPUNC 0.0002 magnitudes One-sigma uncertainty in co-add 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 2.00513887405396 DN Median of intensity pixels. Band dependent. MEDCOV 36.796875 pixels Median depth-of-coverage in units of number of effective overlapping pixels. Band dependent. MINCOV 5.724609375 pixels Minimum depth-of-coverage in units of number of effective overlapping pixels. Band dependent. MAXCOV 53.7890625 pixels Maximum depth-of-coverage in units of number of effective overlapping pixels. Band dependent. LOWCOVPC 4.106051312E-07 % Percentage of area in co-add tile with depth-of-coverage <= 5 pixels - depths at which temporal outlier rejection could be unreliable. Band dependent. NOMCOVPC 7.156051112E-05 % Percentage of area in co-add tile with depth-of-coverage <= 8 pixels - the depth where some mission requirements are defined. Band dependent. ROBSIG 0.033795153767903 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 0.904601390490027 - 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 0.913015147327707 - 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 12610 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. ELON 39.0639507312699 degrees Ecliptic longitude (λ) at image center. ELAT -67.1869042616238 degrees Ecliptic latitude (β) at image center. GLON 255.794081986761 degrees Galactic longitude (l) at image center. GLAT -46.0174380744293 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. _________________________________________________________________________________________________________
Last update: 2011 April 26