MODIFY DOPPLER
Keywords: class

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MODIFY DOPPLER

            LAS\MODIFY DOPPLER [SIGN|*|Value]

        Change or recompute the Doppler factor according to the CLASS convention
        (-V/c).  With no argument, checks whether the header value is consistent
        with the CLASS value computed according to the date and observatory. The
        checking  precision corresponds to 0.6 m/s in V. A logical SIC variable,
        DOPPLER_PB, is set to true if the values disagree.  If  an  argument  is
        present:

          SIGN   change the sign of the Doppler factor but do not recompute it,
          *      compute  and update the Doppler factor according to the observ-
                 ing date-time and the observatory position (this  position  can
                 be  customized  with  SET OBSERVATORY). This is suited when the
                 frequencies in the Class header are described in the observato-
                 ry frame. See also undefined Doppler section below.
          0      force  the  Doppler  factor  to  0, i.e. the frequencies in the
                 Class header are already corrected for the Doppler factor  (de-
                 scribed in the LSR frame).
          Value  set the Doppler factor to <Value>

        Undefined Doppler factor:
            If  the Doppler factor is -1 (no physical meaning), it is considered
          "undefined" by Class. In most of the cases, this happens when  reading
          old Class data files where the Doppler factor was not available in the
          data format. Since then, telescopes are encouraged to use  the  newest
          data  format  and  write  directly a valid Doppler factor value at the
          time the Class files are created.

            At read time (GET or other commands reading a spectrum), Class  con-
          siders  the  value  -1  as  a  request  to recompute automatically the
          Doppler factor from the telescope position on Earth and from observing
          date-time,  assuming  the  frequencies  in the spectrum header are de-
          scribed in the observatory frame. In other words, Class tries to apply
          implicitly the command MODIFY DOPPLER * on the spectrum.

            The above automatic patch works if the observatory can be recognized
          by Class from the telescope name (R%HEAD%GEN%TELES). If not, you  will
          encounter this error:
            E-COMPUTE_DOPPLER,  Telescope not understood from XXX
          In this case, use the command SET OBSERVATORY beforehand.

            Beware  that  the automatic patch makes sense if the frequencies are
          described in the observatory frame. If they are described in  the  LSR
          frame, the Doppler factor must be forced to 0.

            Finally,  if  you have an old Class file without the Doppler factor,
          it is advised to copy it to a newer Class Data Format. This will  save
          the  computed Doppler value directly in the new file instead of recom-
          puting again and again this factor each time the spectrum is read from
          the old file. If MODIDY DOPPLER * is what you want (frequencies in the
          observatory frame), this will do the job:
            FILE IN old.30m
            FIND
            FILE OUT new.30m SINGLE ! Uses newest data format
            COPY                    ! Implicit MODIFY DOPPLER * on each spectrum
          If you need a custom Doppler value (most probably 0), use:
            FILE IN old.30m
            FIND
            FILE OUT new.30m SINGLE ! Uses newest data format
            FOR I 1 TO FOUND
              GET NEXT
              MODIFY DOPPLER 0
              WRITE
            NEXT