Conversion of single spectra to FITS format

To convert to a format readable by CLASS, it is first necessary to generate spectra arrays continuous in velocity/frequency space. Thus one must first have used the das-merge feature of SPECX for all but single-subband data of the DAS. There are two ways of doing this¹⁰.

• First write the spectra to a SPECX output file after applying das-merge and any other processing you want (such as baseline fits, binning etc). Then use the SPECX command

  $>\!>$tofits

  This will ask four questions: (a) which SPECX file number you want to use (hence the file must be open and read-permitted), (b) whether you want to use the original scan numbers or the sequence number in the SPECX file, and (c) the first and (d) the last scan in the input file to be processed by tofits. If you have single-position spectra (each with a distinctive integration number), or averages of the same, then choosing the original scan number option would probably be best. If your data are a sequence comprising a map, say, made with the grid, pattern or raster observing commands, then the scan numbers will be the same, but the subscan numbers will be incremented. In this case either the sequence number or the scan/subscan will do.

  The output filename has the following pattern in case one selects the option to imprint it with the original scan number:

  jcmt_nnnn_xxx.fits

  where nnnn is the scan number and xxx the subscan number. That is, the output file names might look like

       jcmt_0051_001.fits
       jcmt_0051_002.fits
              .
              .
              .
       jcmt_0051_015.fits
  

  in the instance of a 15-position map having been used for observation 51.

  If one chooses to imprint the filenames with the sequence number in the SPECX input data file, the subscan number is omitted: file names will look like jcmt_nnnn.fits.

  In both cases it will be useful to make a print-out of the input file using

  $>\!>$s-l-f f file.list
  $>\!>$in-f 1 l
  $>\!>$s-l-f t

  for example. In this case a complete (`long') listing of the SPECX input file number 1 is produced in file.list.

  To avoid digitization problems when converting FITS I4 integers tofits clips the data intensity values $I$ to be within the range $-500 < I < +500$. The routine now also tells where it sits, so that one can copy and edit the procedure in case one wants to change the thresholds.

• GSD2FITS (or G2F): this routine (adapted from a SPECX script contributed by Christine Wilson) directly converts GSD files to FITS (with the above naming convention), where necessary using concat and das-merge (using default settings). It runs automatically without user intervention once started, and hence for all practical purposes appears as a disk-to-disk conversion. For example:

   >> g2f 
   
   Routine to directly convert GSD DAS/AOSC files to FITS using CONCAT
   and DAS-MERGE if necessary.
   
   *** WARNING *** THIS ROUTINE IS DANGEROUS: data that need CONCAT
   and/or DAS-MERGE should really be inspected after these operations
   and before conversion to FITS, otherwise the DATA QUALITY may be
   severely compromised.
   
   /jcmt_sw/sun4_Solaris/specx/gsd2fits> First scan #? 51
   /jcmt_sw/sun4_Solaris/specx/gsd2fits> Last scan #? 59
  

  converts scans 51 through 59 (and all subscans) to FITS.

  The routine is designed to skip non spectral-line data. However, the user should be warned (as one can see!) against the blind use of this routine for data that needs concat and/or das-merge. I have had mixed results with it, and prefer to avoid it. The routine is usually fine for single-quadrant data, however.