\secA{Notes and hints on the use of \duchamp}
\label{sec-notes}

In using \duchamp, the user has to make a number of decisions about
the way the program runs. This section is designed to give the user
some idea about what to choose.

The main choice is whether or not to use the wavelet
reconstruction. The main benefits of this are the marked reduction in
the noise level, leading to regularly-shaped detections, and good
reliability for faint sources. The main drawback with its use is the
long execution time: to reconstruct a $170\times160\times1024$
(\hipass) cube often requires three iterations and takes about 20-25
minutes to run completely. Note that this is for the more complete
three-dimensional reconstruction: using \texttt{reconDim=1} makes the
reconstruction quicker (the full program then takes about 6 minutes),
but it is still the largest part of the time.

The searching part of the procedure is much quicker: searching an
un-reconstructed cube leads to execution times of only a couple of
minutes. Alternatively, using the ability to read in previously-saved
reconstructed arrays makes running the reconstruction more than once a
more feasible prospect.

On the positive side, the shape of the detections in a cube that has
been reconstructed will be much more regular and smooth -- the ragged
edges that objects in the raw cube possess are smoothed by the removal
of most of the noise. This enables better determination of the shapes
and characteristics of objects.

A further point to consider when using the reconstruction is that if
the two-dimensional reconstruction is chosen (\texttt{reconDim=2}), it
can be susceptible to edge effects. If the valid area in the cube (\ie
the part that is not BLANK) has non-rectangular edges, the convolution
can produce artefacts in the reconstruction that mimic the edges and
can lead (depending on the selection threshold) to some spurious
sources. Caution is advised with such data -- the user is advised to
check carefully the reconstructed cube for the presence of such
artefacts. Note, however, that the 1- and 3-dimensional
reconstructions are \emph{not} susceptible in the same way, since the
spectral direction does not generally exhibit these BLANK edges, and
so we recommend the use of either of these.

If one chooses the reconstruction method, a further decision is
required on the signal-to-noise cutoff used in determining acceptable
wavelet coefficients. A larger value will remove more noise from the
cube, at the expense of losing fainter sources, while a smaller value
will include more noise, which may produce spurious detections, but
will be more sensitive to faint sources. Values of less than about
$3\sigma$ tend to not reduce the noise a great deal and can lead to
many spurious sources (this depends, of course on the cube itself).

When it comes to searching, the FDR method produces more reliable
results than simple sigma-clipping, particularly in the absence of
reconstruction.  However, it does not work in exactly the way one
would expect for a given value of \texttt{alpha}. For instance,
setting fairly liberal values of \texttt{alpha} (say, 0.1) will often
lead to a much smaller fraction of false detections (\ie much less
than 10\%). This is the effect of the merging algorithms, that combine
the sources after the detection stage, and reject detections not
meeting the minimum pixel or channel requirements.  It is thus better
to aim for larger \texttt{alpha} values than those derived from a
straight conversion of the desired false detection rate.

Finally, as \duchamp\ is still undergoing development, there are some
elements that are not fully developed. In particular, it is not as
clever as I would like at avoiding interference. The ability to place
requirements on the minimum number of channels and pixels partially
circumvents this problem, but work is being done to make \duchamp\
smarter at rejecting signals that are clearly (to a human eye at
least) interference. See the following section for further
improvements that are planned.