NOAO R-band FITS Image Header Updates

In the process of organizing the NOAO R-band data for public distribution using Altas, IRSA found several FITS-compliance issues with the FITS image headers, in both the FLS NOAO and FLS NOAO ELAIS datasets.

IRSA repaired the image headers using the IRAF; the format of the headers has been changed to bring them into compliance with the FITS standard; the technical content of the headers is unaltered. Please note that the data distributed from NOAO and the SSC ftp area DO NOT contain these corrections.

The problem with the headers were as follows:

  • The "DATE-OBS" header entry (keyword name, value and comment) was longer than the standard allowed 80 characters. The comment line spilled over to a new line in all the NOAO R-band FITS headers. The entry looked like this:
    DATE-OBS= '2000-05-06T09:56:52.0'         / UT Date of Earliest Obs. Used In Com
    bined Image
    Since the comment was too long to fit on one line, it appeared to the FITS library as two separate keywords. The part that did not fit on the first line ("bined Image") was therefore treated as a separate FITS keyword, "bined". To fix this problem, the "DATE-OBS" keyword comment has been shortened as follows:
    DATE-OBS= '2000-05-06T09:56:52.0' / UT Date of Earliest Obs. Used In Combined Im
    The last word in the comment, "Image", was shorted to "Im" in order to fit within the 80 character limit.

  • Directly after the "TIME-OBS" header keyword entry, there was a blank line before the next header keyword entry. Our suspicion is that the "TIME-OBS" header keyword entry was 81 characters and so the last character (blank space or newline) created a blank line in the header. We deleted this blank line from all the FITS headers.

  • Each header contained the keyword "SKY"; all the values for this keyword were set to 21 asterisks (********************). This value was not a number, nor was it quoted to be a string. Since IRSA had no knowledge of the value of "SKY" for these data, we set the "SKY" value to a string of asterisks ('********************'). By putting quotes around the value "SKY", it became a legal FITS keyword value. Please note that we do not know the correct value for the "SKY" keyword; we have simply made this change in order for the header to be FITS-compliant.

  • Similar to the problem listed above, a number of other header keyword values were set to 21 asterisks (********************). We placed quotes around each of these strings, making them FITS-compliant keyword values. Here is a list of the FITS file names and header keywords which had this problem:
    FLS_fullfield_KPNO_R_v01.fits:EFFGAIN = ******************** / Conversion ...
    FLS_fullfield_KPNO_R_v01.fits:SEEING  = ******************** / Seeing ...
    FLS_fullfield_KPNO_R_v01.fits:SKY     = ******************** / Sky background 
    FLS_fullfield_KPNO_R_v01.fits:SKYSIG  = ******************** / 1 sigma ...
    FLS_fullfield_KPNO_R_v01.fits:DEPTH   = ******************** / Approximate 
    FLS_fullfield_KPNO_R_v01.fits:MAGZERO = ******************** / R Kron-C ...
    ELAIS_fullfield_KPNO_R_v01.fits:EFFGAIN = ******************** / Conversion ...
    ELAIS_fullfield_KPNO_R_v01.fits:SEEING  = ******************** / Seeing FWHM 
    ELAIS_fullfield_KPNO_R_v01.fits:SKY     = ******************** / Sky background 
    ELAIS_fullfield_KPNO_R_v01.fits:SKYSIG  = ******************** / 1 sigma ...
    ELAIS_fullfield_KPNO_R_v01.fits:DEPTH   = ******************** / Approximate 
    ELAIS_fullfield_KPNO_R_v01.fits:PROPOSAL= '******************' / Proposal 
    ELAIS_fullfield_KPNO_R_v01.fits:MAGZERO = ******************** / R Kron-C ...

We also noticed that there were extra blank lines between the last COMMENT keyword ("COMMENT 2004, A.J., submitted.") and the "END" of each header. This is not "wrong" in terms of FITS compliance, but we wanted to make a note of it here for completeness. We did not remove these blank lines.