II. User's Guide to the 2MASS All-Sky Data Release


4. Atlas Images

a. Atlas Image Header Description

The 2MASS Atlas and Quicklook Images are in FITS format.

The 2MASS Atlas and Quicklook Images are in the same format but the Quicklook Images are lossy-compressed. With the exception of the "bitpix" keyword, headers are identical.

An actual 2MASS Atlas Image header may be found here.


Click on the underlined Field Names for more detail on those parameters.

Field Name Format Units null Description  
Primary Fits Information
 bitpix %20d --- no -32 => 32 bit IEEE floating point data (Atlas Images)
  16 => 16 bit integer data (lossy compressed Quicklook Images)
 
 naxis %20d --- no (always= 2)  2 dimensional image  
 naxis1 %20d --- no  Number of columns in image  
 naxis2 %20d --- no  Number of lines in image  
Observatory Data Identifiers
 ordate %20s --- no Observation reference date in the YYMMDD. This is the UT day value at the start of operations for the night on which the source was observed. Because the UT day incremented during the night at CTIO, the actual UT observation date for a source can differ from this value by one day.  
 daynum  %20d day no Sequential survey day number, where day 1 is defined to be 1 March 1997 UT. Observations were started at Mt. Hokpins on daynum=109 (7 June 1997 UT) and at CTIO on daynum=384 (19 March 1998 UT).  
 fn_prfx  %20s ---  no  Observatory Filename Prefix   
  type  %20s --- no Scan type: dar (dark), flt (flat), sci (survey), cal (calibration), or tst (test)  
 scanno  %20d ---  no  Nightly scan number   
 scandir  %20s ---  no  Scan Direction: n (north), s (south)    
 strip_id  %20d ---  no  2MASS Tile number.    
 positnid  %20s ---  no  Observatory Scan Position Identifier (Hddhhmm -> H = hemisphere, dd = dec at tile center, hhmm = hours and minutes at tile center)    
 origin  %20s ---  no  2MASS Survey Camera   
Projection and Location Information
 ctype1  %20s ---  no  Orthographic Projection   
 ctype2  %20s ---  no  Orthographic Projection   
 crpix1  %20f pix  no  Column Coordinate at Image Center (crval1/crval2)   
 crpix2  %20d pix  no  Line Coordinate at Image Center (crval1/crval2)   
 crval1  %20f deg  no  Right Ascension at Image Center (J2000)   
 crval2  %20f deg  no  Declination at Image Center (J2000)   
 crota2  %20f deg  no  Twist about Image Center, +AXIS2 WofN (J2000)   
 cdelt1  %20f deg  no  Axis1 Pixel Size (=1 arcsec)   
 cdelt2  %20f deg  no  Axis2 Pixel Size (=1 arcsec)   
Calibration and Auxillary Image Information
 usxref  %20f asec  no  2MASS U-scan X Coordinate at Image Center   
 usyref  %20f asec  no  2MASS U-scan Y Coordinate at Image Center   
 skyval  %20f dn  no  Modal Sky Estimate for Image (image dn)    
 skysig  %20f dn  no  Point Source Filtered Noise Estimate for Image (image dn)    
 magzp  %20f mag  no  Calibrated Zero Point for Image (magnitudes)    
 calid  %20s ---  no  Calibration Identifiers for Image    
 seesh  %20f ---  no  Seeing Shape Parameter for Image (image dn)    
 dxfs  %20f asec  no  Cross-scan Frame Step for Image    
 dyfs  %20f asec  no  In-scan Frame Step for Image    
 pixnam  %20s ---  no  PIXPHOT Processing Namelist Configuration Identifier    
 QT0227  %20f ---  no  Image Histogram 2.27 Percentile (Image DN)    
 QT0668  %20f ---  no  Image Histogram 6.68 Percentile (Image DN)    
 QT1586  %20f ---  no  Image Histogram 15.86 Percentile (Image DN)    
 QT3085  %20f ---  no  Image Histogram 30.85 Percentile (Image DN)    
 QT5000  %20f ---  no  Image Histogram 50.00 Percentile (Image DN)    
 QT6914  %20f ---  no  Image Histogram 69.14 Percentile (Image DN)    
 QT8413  %20f ---  no  Image Histogram 84.13 Percentile (Image DN)    
 QT9331  %20f ---  no  Image Histogram 93.31 Percentile (Image DN)    
 QT9772  %20f ---  no  Image Histogram 97.72 Percentile (Image DN)    
Auxillary Observatory Information
  base  %20d ms  no  Pixel Dwell Time (milliseconds)    
  wavef_p  %20s ---  no  Secondary Waveform Data File    
 timer0  %20d ms  no  Read1 Time (milliseconds)    
 timer1  %20d ms  no  Read2-Read1 Time (milliseconds)    
 timer2  %20d ms  no  Secondary Settling Time (milliseconds)    
 icsversn  %20s ---  no  ICS Script Version    
 schedver  %20s ---  no  Scan Scheduler Version    
 scriptfn  %20s ---  no  Script Template Filename    
 scrptver  %20s ---  no  Script Template Version    
 ut_off1  %20f sec  no  UT Offset - SOS to Frame 1   
 xs_off1  %20f asec  no  X-scan Offset - SOS to Frame 1   
 is_off1  %20f asec  no  In-scan Offset - SOS to Frame 1   
 xs_off2  %20f asec  no  X-scan Offset - EOS to End Frame   
 is_off2  %20f asec  no  In-scan Offset - EOS to End Frame   
 focus  %20d ---  no  Telescope Focus Setting   
 numfrms  %20d ---  no  Number of Frames in Scan   
 skiprdos  %20d ---  no  Number of Frames Skipped at Start of Scan   
 telname  %20s ---  no  Telescope Location - Hopkins or CTIO   
 filter  %20s ---  no  Wavelength Band - j, h, or k (J, H, or Ks)   
 ut_date  %20s ---  no  UT Date of Scan (YYMMDD)   
 ut  %20s ---  no  UT Time of Scan (HH:MM:SS.SSS)   
 amass_fc  %20s ---  no  Approximate Airmass at Image Center   
 ut_date  %20s ---  no  UT Date of Scan (YYMMDD)   
Auxillary Observatory Information at Start Of Scan
 date_sos  %20s ---  no  UT Date at Start of Scan (YYMMDD)   
 equinox  %20s ---  no  Equinox at Start of Scan   
 am_sos  %20f ---  no  Airmass at Start of Scan   
 zd_sos  %20f deg  no  Zenith Distance at Start of Scan    
 ha_sos  %20s ---  no  Hour Angle at Start of Scan (HH:MM:SS.SS)   
 ut_sos  %20s ---  no  UT at Start of Scan (HH:MM:SS.SS)   
 st_sos  %20s ---  no  Sidereal Time at Start of Scan (HH:MM:SS.SS)   
 ra_sos  %20f ---  no  Scan Reference RA J2000 (HH:MM:SS.S)   
 dec_sos  %20f ---  no  Scan Reference Declination J2000 (DD:MM:SS.S)   
 wspd_sos  %20d ---  no  Wind Speed at Start of Scan (not implemented)   
 wdir_sos  %20d ---  no  Wind Direction at Start of Scan (not implemented)   
 see_sos  %20d ---  no  Seeing at Start of Scan (not implemented)   
 bar_sos  %20d in Hg  no  Barometric Pressure at Start of Scan   
 airt_sos  %20f deg C  no  Air Temperature at Start of Scan   
 telt_sos  %20f deg C  no  Telescope Temperature at Start of Scan   
Auxillary Observatory Information at End Of Scan
 date_eos  %20s ---  no  UT Date at End of Scan (YYMMDD)   
 am_eos  %20f ---  no  Airmass at End of Scan   
 zd_eos  %20f deg  no  Zenith Distance at End of Scan    
 ha_eos  %20s ---  no  Hour Angle at End of Scan (HH:MM:SS.SS)   
 ut_eos  %20s ---  no  UT at End of Scan (HH:MM:SS.SS)   
 st_eos  %20s ---  no  Sidereal Time at End of Scan (HH:MM:SS.SS)   
 ra_eos  %20f ---  no  Scan Reference RA J2000 (HH:MM:SS.S)   
 dec_eos  %20f ---  no  Scan Reference Declination J2000 (DD:MM:SS.S)   
 wspd_eos  %20d ---  no  Wind Speed at End of Scan (not implemented)   
 wdir_eos  %20d ---  no  Wind Direction at End of Scan (not implemented)   
 see_eos  %20d ---  no  Seeing at End of Scan (not implemented)   
 bar_eos  %20d in Hg  no  Barometric Pressure at End of Scan   
 airt_eos  %20f deg C  no  Air Temperature at End of Scan   
 telt_eos  %20f deg C  no  Telescope Temperature at End of Scan   
 aryt_eos  %20f deg C  no  Array Temperature at End of Scan   

[Last Updated: 2003 March 14, by Gene Kopan & R. Cutri]



Return to Section II.4.