ISSA Explanatory Supplement
V. FORMATS FOR THE IRAS SKY SURVEY ATLAS (ISSA)

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The IRAS Sky Survey Atlas, ISSA, and the ISSA Reject Set are machine-readable images in Flexible Image Transport System (FITS) (Wells, D.C. et al. 1981) image format. Each 500 × 500-pixel image covers a 12.5° × 12.5° field of sky with pixel size of 1.5'. For each field and wavelength, intensity images were produced for separate HCONs plus the co-add of all HCONs. Coverage and standard deviation images were also produced and can be obtained from IPAC by special request. [Note: These are now available from the ADF; send requests by e-mail to request@nssdca.gsfc.nasa.gov]

FITS image format restrictions require that image samples be either 16 or 32 bits. The dynamic range of the intensity values in each ISSA image drove the choice of sample size. The resolution is 5% of median pixel noise. Intensity images are either 0.5 or 1.0 Mbytes. Sample FITS headers for intensity images are shown in Tables V.1 and V.2. Table V.3 provides a brief description of some FITS header keywords.

Table V.1 A Sample FITS Header for Intensity Images:
(4 byte format) 
SIMPLE  =                    T        / STANDARD FITS FORMAT                    
BITPIX  =                   32        / 4 byte twos-compl int                   
NAXIS   =                    3        / # OF AXES                               
NAXIS1  =                  500        / # SAMPLES PER LINE                      
NAXIS2  =                  500        / # LINES IN IMAGE                        
NAXIS3  =                    1        / # WAVELENGTHS                           
BSCALE  =      2.540014267E-03        / TRUE=TAPE*BSCALE+BZERO                  
BZERO   =      3.795739345E+02        /                                         
BUNIT   = 'MJY/SR  '                  / INTENSITY                               
BLANK   =          -2000000000        / TAPE VALUE FOR EMPTY PIXEL              
CRVAL1  =      1.560000000E+02        / RA AT ORIGIN (DEGREES)                  
CRPIX1  =      2.500000000E+02        /  SAMPLE AXIS ORIGIN (PIXEL)             
CTYPE1  = 'RA---TAN'                  /  DECREASES IN VALUE AS SAMPLE           
                                      /  INDEX INCREASES (GNOMONIC)             
CDELT1  =     -2.500000000E-02        /  COORD VALUE INCR DEG/PIXEL             
                                      /  AT ORIGIN ON SAMPLE AXIS               
CRVAL2  =      7.000000000E+01        /  DEC AT ORIGIN (DEGREES)                
CRPIX2  =      2.500000000E+02        /  LINE AXIS ORIGIN (PIXEL)               
CTYPE2  = 'DEC--TAN'                  /  DECREASES IN VALUE AS LINE             
                                      /  INDEX INCREASES (GNOMONIC)             
CDELT2  =      2.500000000E-02        /  COORD VALUE INCR DEG/PIXEL             
                                      /  AT ORIGIN ON LINE AXIS                 
CRVAL3  =      6.000000000E-05        /  WAVELENGTH IN METERS                   
CRPIX3  =      1.000000000E+00        /                                         
CTYPE3  = 'LAMBDA  '                  /                                         
CDELT3  =      0.000000000E+00        /                                         
DATAMAX =      7.604873693E+02        /  MJY/SR (TRUE VALUE)                    
DATAMIN =     -1.339500327E+00        /  MJY/SR (TRUE VALUE)                    
EPOCH   =      1.950000000E+03        /  EME50                                  
DATE-MAP= '91/03/11'                  /  MAP RELEASE DATE (YY/MM/DD)            
DATE    = '91/10/28'                  /  DATE TAPE WRITTEN(YY/MM/DD)            
ORIGIN  = 'JPL-IPAC'                  /  INSTITUTION                            
TELESCOP= 'IRAS    '                  /                                         
INSTRUME= 'ISSA-FLD'                  /  IRAS SKY SURVEY ATLAS                  
OBJECT  = 'f414h003'                  /  FIELD NUMBER-HCON                      
PROJTYPE= 'GNOMONIC'                  /  PROJECTION TYPE                        
EDITED  =                    T        /  SCANS EDITED                           
APPCAL  =                    T        /  CALIBRATION CORRECTION 25 MICRON       
DE-ZODY =                    T        /  DE-ZODIED IMAGE                        
GLBL-D  =                    T        /  APPLIED GLOBAL PARAMETERS              
LOCAL-D =                    T        /  LOCAL DESTRIPER                        
ASBLANK =                    T        /  ASTEROID BLANKING                      
COMMENT                               /                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
END

Table V.2 A Sample FITS Header for Intensity Images:
(2 byte format) 
SIMPLE  =                    T        / STANDARD FITS FORMAT                    
BITPIX  =                   16        / 2 byte twos-compl int                   
NAXIS   =                    3        / # OF AXES                               
NAXIS1  =                  500        / # SAMPLES PER LINE                      
NAXIS2  =                  500        / # LINES IN IMAGE                        
NAXIS3  =                    1        / # WAVELENGTHS                           
BSCALE  =      8.622583455E-05        / TRUE=TAPE*BSCALE+BZERO                  
BZERO   =      2.175757902E+00        /                                         
BUNIT   = 'MJY/SR  '                  / INTENSITY                               
BLANK   =               -32768        / TAPE VALUE FOR EMPTY PIXEL              
CRVAL1  =      2.730000000E+02        / RA AT ORIGIN (DEGREES)                  
CRPIX1  =      2.500000000E+02        /  SAMPLE AXIS ORIGIN (PIXEL)             
CTYPE1  = 'RA---TAN'                  /  DECREASES IN VALUE AS SAMPLE           
                                      /  INDEX INCREASES (GNOMONIC)             
CDELT1  =     -2.500000000E-02        /  COORD VALUE INCR DEG/PIXEL             
                                      /  AT ORIGIN ON SAMPLE AXIS               
CRVAL2  =      4.000000000E+01        /  DEC AT ORIGIN (DEGREES)                
CRPIX2  =      2.500000000E+02        /  LINE AXIS ORIGIN (PIXEL)               
CTYPE2  = 'DEC--TAN'                  /  DECREASES IN VALUE AS LINE             
                                      /  INDEX INCREASES (GNOMONIC)             
CDELT2  =      2.500000000E-02        /  COORD VALUE INCR DEG/PIXEL             
                                      /  AT ORIGIN ON LINE AXIS                 
CRVAL3  =      6.000000000E-05        /  WAVELENGTH IN METERS                   
CRPIX3  =      1.000000000E+00        /                                         
CTYPE3  = 'LAMBDA  '                  /                                         
CDELT3  =      0.000000000E+00        /                                         
DATAMAX =      4.978097525E+00        /  MJY/SR (TRUE VALUE)                    
DATAMIN =     -6.265817208E-01        /  MJY/SR (TRUE VALUE)                    
EPOCH   =      1.950000000E+03        /  EME50                                  
DATE-MAP= '91/05/15'                  /  MAP RELEASE DATE (YY/MM/DD)            
DATE    = '91/10/28'                  /  DATE TAPE WRITTEN(YY/MM/DD)            
ORIGIN  = 'JPL-IPAC'                  /  INSTITUTION                            
TELESCOP= 'IRAS    '                  /                                         
INSTRUME= 'ISSA-FLD'                  /  IRAS SKY SURVEY ATLAS                  
OBJECT  = 'f358h001'                  /  FIELD NUMBER-HCON                      
PROJTYPE= 'GNOMONIC'                  /  PROJECTION TYPE                        
EDITED  =                    T        /  SCANS EDITED                           
APPCAL  =                    T        /  CALIBRATION CORRECTION 25 MICRON       
DE-ZODY =                    T        /  DE-ZODIED IMAGE                        
GLBL-D  =                    T        /  APPLIED GLOBAL PARAMETERS              
LOCAL-D =                    T        /  LOCAL DESTRIPER                        
ASBLANK =                    T        /  ASTEROID BLANKING                      
COMMENT                               /                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
COMMENT                                                                         
END

Table V.3 FITS Keywords
KeywordDescription
BITPIX Indicates number of bits to represent the sample in two's complement. Intensity maps are either 16 or 32 bits per sample. Coverage and standard deviation images are 16 bits per sample.
BUNIT Intensity and standard deviation of the mean are in antom{MJy/sr} MJy sr-1. Coverage images are dimensionless.
BLANK Value assigned to empty pixels.
DATE Date the image was made at IPAC.
INSTRUME Indication that image is an IRAS Sky Survey Atlas or ISSA Reject product. ISSA-I or ISSA-FLD indicates the ISSA image was processed for the first release in 1991, the || > 50° sky. ISSA-II indicates the ISSA image was processed for the second release, the || < 50° sky. ISSA-REJ indicates the image is part of the ISSA Reject Set.
OBJECT Gives field and HCON number as fxxxhnnn where xxx is a three-digit field number and nnn is either 001,002 or 003 for individual HCON or 000 for the co-add.
CRVAL3 Wavelength in meters.
True/false indicators:
EDITED If TRUE, the field had some initial scans edited based on a list of known anomalies.
APPCAL If TRUE, calibration corrections were applied to the 25 µm detectors.
DE-ZODY If TRUE, the image has had the zodiacal foreground removed.
GLBL-D If TRUE, the image has been globally destriped.
LOCAL-D If TRUE, the image has been locally destriped.
ASBLANK If TRUE, the image has had asteroids removed from the co-added image

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