
\pakoVspaceSmall

\notes. IMPORTANT:

{\em

\swf\ is 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, and they will be
seen in \swf\ spectra taken toward astronomical sources with a low
Doppler shift. The mesospheric lines will appear at a frequency and
velocity that corresponds to the rest frame of the athmosphere, i. e.,
the observatory. Care must be taken that they are not confused with
the lines from the astronomical source.  (Information computed by the
ASTRO software can help with this decision).

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 doubt, consult the special memo on \swf\ or ask an experienced
\swf\ observer!

}