SWFREQUENCY
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SWFREQUENCY

        SWFREQUENCY fOffset1 [fOffset2]

        Select and set up frequency switching (FS).

        Real      :: fOffset1          ! 1st frequency offset [MHz]
        Real      :: fOffset2          ! 2nd frequency offset [MHz]

        fOffset2 should be set to be = -fOffset1 (symmetric FS).
        This is done by default, if only fOffset1 is specified.
        (Note that then ABS(fOffset1) = FS amplitude = 1/2 FS throw).
        FS  with fOffset2 not equal -fOffset1 is experimental.

        /RECEIVER allows to set fOffset1 and fOffset2 for each connected
        receiver differently.

        Parameters of other options are always the same for all receivers.

        Limits for fOffset1 and fOffset2 are:

          -9 to  +9   with the 3 mm receivers /bands
         -18 to +18   with the 2 mm receivers /bands
         -27 to +27   with the 1 mm receivers /bands

        IMPORTANT NOTES about use of Frequency Switching (FS)

        Frequency Switching (FS) can be very powerful and efficient for some
        projects, e. g., mapping of narrow spectral lines in cold dark clouds
        outside the plane of the Milky Way.  However, before deciding to use
        frequency switching one should consider some potential drawbacks:

        The target lines should be narrow enough so that line signals from the
        2 phases of the switching cycle are well separated.

        The spectral baseline will generally be less flat than in other
        switching modes.

        Some spectral lines are also emitted in the earth's mesosphere, e.g.,
        the mesopheric lines from (12)CO are rather strong, typically several
        [K], and they will be seen in FS spectra taken toward astronomical
        sources with a low Doppler shift. The mesospheric lines will appear at
        a frequency and velocity that correspond to the rest frame of the
        athmosphere, i. e., to good approximation, the observatory.  If,
        e. g., you observe using Doppler corrections for the LSR scale, the
        mesospheric lines will appear in the spectra at -1 * the velocity of
        the Local Standard of Rest relative to the observatory.

        Care must be taken that mesospheric lines are not confused with the
        lines from the astronomical source, which will appear in the spectrum
        at the velocity of the source relative to the LSR.  The ASTRO software
        can calculate the velocity of the LSR relative to the observatory for
        any source and time.  During observations, this velocity is also
        displayed on one of the NCS monitoring windows.

        When observing sources near the plane of the Milky Way, line emission
        from clouds at other velocities than the target source, e. g., other
        spiral arms, can cause confusion.

        In case of any doubt, there is a special report on FS that gives more
        advice!

Subsections

- SWFREQUENCY /DEFAULTS
- SWFREQUENCY /RECEIVER
- SWFREQUENCY /TPHASE
- SWFREQUENCY EXAMPLES