II. User's Guide to the WISE Preliminary Data Release


3. Atlas Images

b. Atlas Image Header Description

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.
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Last update: 2011 April 26


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