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<H3><A NAME="SECTION00843200000000000000">
SEARCH/ROMAFOT</A>
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<TT>SEARCH/ROMAFOT</TT>
<FONT SIZE="-1"><TT>frame [sky_tab] [cat_tab] [area] [psf_par] [thresh]
[height]</TT></FONT>
<P>
This command performs the actual search of the objects. It examines the
region indicated by the user and detects all the maxima above the
defined threshold (or above a given relative threshold). Note that
the pixel values and maxima are all relative, i.e. with respect to
the sky background. The table produced by <TT>SEARCH/ROMAFOT</TT> is
compatible with the table created by the command <TT>FIND/ROMAFOT</TT>.
Therefore, after this automatic search for objects the user can pass
to the interactive procedure to group the objects with the command
<TT>ANALYSE/ROMAFOT</TT>. This is the first decision point of the two
procedure paths.
<P>
Of course this does not mean that the user is recommended to leave
the automatic route (apart from a few special cases). However, the
interactivity may now be useful to look at the objects found and to
decide if the threshold is appropriate or if the sampling of the
background was sufficient to keep the detection threshold nearly
constant all over the image.
<P>
Since it is expected that the automatic search will be generally followed
by automatic grouping, <TT>SEARCH/ROMAFOT</TT> calculates an additional
parameter with respect to the command <TT>FIND/ROMAFOT</TT>. With this
parameter the output table of <TT>SEARCH/ROMAFOT</TT> can be used as
if they were created by the command <TT>FIND/ROMAFOT</TT>. The inverse
is not possible, the output table created by the command
<TT>FIND/ROMAFOT</TT> cannot be used in the command <TT>GROUP/ROMAFOT</TT>.
This additional parameter is the ``Action Radius'' (hereafter AR)
defined as the distance from the center of a star at which its
photometric contribution drops to a small fraction of the faintest
program star. From this definition it can be inferred that the AR
is a function both of the intensity of the star and of the photometric
limit defined by the user. To calculate the AR the program requires
the PSF parameters.
<P>
How do we define the photometric limit? Suppose one is interested in
studying only stars more luminous than, say, 1000 units. Obviously,
these stars are photometrically disturbed by nearby companions of 900
units. Therefore, in principle the search should be limited to the
``disturbers'' more than to the ``targets''. The grouping module
will then be passed at threshold 1000 and stars in the list fainter
than this limit will be considered only if they can affect photometry
of nearby program stars.
<P>
At this point the function of the AR is clear: it defines an area
surrounding each object within which the object has some influence
in the given context. Photometry of fainter and fainter images requires
larger and larger associated Action Radii, an increasing number of
connections and, ultimately, more computer time.
<P>
After <TT>SEARCH/ROMAFOT</TT> one has obtained a list of candidates
(often thousands). Although a quick inspection with
<TT>ANALYSE/ROMAFOT</TT> is always instructive, the manual
grouping of the objects is a waste of time. This can be avoided
by using the command <TT>GROUP/ROMAFOT</TT>.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
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