Reading FITS Tapes

FITSIN reads FITS files stored on tape. For efficiency, you should select the `no-rewind' device for the particular tape drive, for example /dev/nrmt0h on OSF/1 and /dev/rmt/1n on Solaris.

We ask for the second file on the tape, and the headers are displayed so we can decide whether this is the file we want. It is so we supply a name of an NDF to receive the FITS file. If it wasn't we would enter ! to the OUT prompt. The FMTCNV parameter asks whether the data are to be converted to _REAL, using the FITS keywords BSCALE and BZERO, if present. If you are wondering why there is (1) after the file number, that's present because FITS files can have sub-files, stored as FITS extensions.

     % fitsin
     MT - Tape deck /@/dev/nrmt0h/ >
     The tape is currently positioned at file 1.
     FILES - Give a list of the numbers of the files to be processed > 2
     File # 2(1)  Descriptors follow:
     SIMPLE  =                    T
     BITPIX  =                   16
     NAXIS   =                    2
     NAXIS1  =                  400
     NAXIS2  =                  590
     DATE    = '03/07/88'                    /Date tape file created
     ORIGIN  = 'ING     '                    /Tape writing institution
     OBSERVER= 'CL      '                    /Name of the Observer
     TELESCOP= 'JKT     '                    /Name of the Telescope
     INSTRUME= 'AGBX    '                    /Instrument configuration
     OBJECT  = 'SYS:ARCCL.002'               /Name of the Object
     BSCALE  =                  1.0          /Multiplier for pixel values
     BZERO   =                  0.0          /Offset for pixel values
     BUNIT   = 'ADU     '                    /Physical units of data array
     BLANK   =                    0          /Value indicating undefined pixel
                 :                :                :
                 :                :                :
                 :                :                :
     END
     FMTCNV - Convert data? /NO/ >
     OUT - Output image > ff1
     Completed processing of tape file 2 to ff1.
     MORE - Any more files? /NO/ >

We can trace the structure to reveal the 2-byte integer CCD image. Notice that the FITS headers are stored verbatim in a component .MORE.FITS. This is the FITS extension. The extension contents can be listed with FITSLIST. There is more on this NDF extension and its purpose in the FITS Airlock.

     % hdstrace ff1
     FF1  <NDF>

        DATA_ARRAY(400,590)  <_WORD>   216,204,220,221,202,222,220,206,218,221,
                                       ... 216,218,218,204,221,218,219,222,221,218
        TITLE          <_CHAR*13>      'SYS:ARCCL.002'
        UNITS          <_CHAR*3>       'ADU'
        MORE           <EXT>           {structure}
           FITS(84)       <_CHAR*80>      'SIMPLE  =                    T','BI...'
                                          ... '   ...','         ING PACKEND','END'

     End of Trace.

If you have many FITS files to read there is a quick method for extracting all files or a selection. In automatic mode the output files are generated without manual intervention and the headers aren't reported for efficiency. Should you want to see the headers, write them to a text file via the LOGFILE parameter. The cost of automation is a restriction on the names of the output files, but if you have over a hundred files on a tape are you really going to name them individually?

The following example extracts the fourth to sixth, and eighth files. Note that the [ ] are needed because the value for Parameter FILES is a character array.

     % fitsin auto
     MT - Tape deck /@/dev/nrmt0h/ >
     FMTCNV - Convert data? /NO/ > y
     PREFIX - Prefix for the NDF file names? /'FITS'/ > JKT
     FILES - Give a list of the numbers of the files to be processed > [4-6,8]
     Completed processing of tape file 4 to JKT4.
     Completed processing of tape file 5 to JKT5.
     Completed processing of tape file 6 to JKT6.
     Completed processing of tape file 8 to JKT8.
     MORE - Any more files? /NO/ >

You can list selected FITS headers from a FITS tape without attempting to read in the data into NDFs by using FITSHEAD. You can redirect its output to a file to browse at your leisure, and identify the files you want to convert. So for instance,

     % fitshead /dev/nrmt1h > headers.lis

lists all the FITS headers from a FITS tape on device /dev/nrmt1h to file headers.lis.

After running FITSIN you may notice a file USRDEVDATASET.sdf in the current directory. This HDS  file records the current position of the tape, so you can use FITSIN to read a few files, and then run it again a little later, and FITSIN can carry on from where you left off. In other words FITSIN does not have to rewind to the beginning of the tape to count files. When you're finished you should delete this file.