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<H3><A NAME="SECTION00843300000000000000">
GROUP/ROMAFOT</A>
</H3>
<TT>GROUP/ROMAFOT</TT>
<FONT SIZE="-1"><TT>frame [area] [cat_tab] [int_tab] [thres] [wnd_max]
[end_rad,sta_rad] [wnd_perc]</TT></FONT>
This command groups the objects automatically. The catalogue table
created by the command <TT>SEARCH/ROMAFOT</TT> is required as input.
The command produces an intermediate table identical to the one
created by <TT>ANALYSE/ROMAFOT</TT>.
<P>
The command works as follows: it starts examining the first object
in the catalogue table. If the AR of this object does not intercept
any other AR, the program continues to establish the window for the
fit. Contrarily, if the AR does intercept the AR of another star
the command examines whether a third intersection exists and so forth.
When no more intersections exist, the program goes on to define the
associate subframe to fit.
<P>
Going to faint photometric limits the intersections grow because
of the higher number of stars and because the AR are larger. The
maximum number per window technically acceptable to ROMAFOT is 36,
but this figure is normally kept lower (typically lower than 20).
The program, in the case of crowded fields, may refuse to group
certain objects. This situation will be discussed later.
<P>
To establish the subframe to fit, a balance of opposite requirements
must be achieved. For instance, since the diffraction creates images
with ``wide'' wings one is tempted to integrate over ``large'' windows,
causing the undesirable inclusion of many objects. On the other hand,
considering that the central pixels of the image are those with
higher signal to noise ratios, ``small'' windows could be preferable.
However, this necessitates the sky background to be known ``a priori'',
a heavy requirement indeed in case of crowded fields!
<P>
These considerations and the idea that the sky background should
be computed together with the star because the two data are naturally
coupled, led to the choice of window-sizes as wide as 9 times the FWHM
in the case of isolated objects. If the command is faced with a multiple
configuration, the window is determined by a frame, 3 times the FWHM width,
surrounding the rectangulus circumscribing the Action Radii.
<P>
Often new Action Radii, not intersecting the one under examination,
fall into the window. These will be ignored during the fit. To
visualise this, a ``hole'', as wide as the relative AR, will be
created at the positions of these objects. In this operation some
pixels are lost for the fitting, this is the reason for the quite
conservative choice of the window size.
<P>
After <TT>GROUP/ROMAFOT</TT> has finished, a histogram of the result
(groups, objects which failed to group and so on) is prepared printed
in the user terminal and in the logfile.
<P>
With the default values the user groups a percentage of all the
objects in the list. This fraction varies a lot and depends
on the crowding, on the seeing, and on the AR. The latter depends
again on the photometric limit requested. Typically, somewhere
between 70% and 100% of the catalogue list will turn out
to be grouped by <TT>GROUP/ROMAFOT</TT>.
<P>
In case of the default values for AR the command starts to group
the objects holding their original AR. Thereafter, if a group
exceeds the maximum number of objects (e.g. 15) GROUP/ROMAFOT
tries to prepare an acceptable window
<!-- MATH: ${\rm AR(new)} = 0.90 \times {\rm AR(original)}$ -->
<IMG
WIDTH="264" HEIGHT="42" ALIGN="MIDDLE" BORDER="0"
SRC="img225.gif"
ALT="${\rm AR(new)} = 0.90 \times {\rm AR(original)}$">.
This limits the intersections and the groups can turn out to have
an acceptable number of components. It is important to note that
the size of holes is not affected by this reduction and its original
value is conserved.
<P>
To group remaining objects, one can execute the command once more
with the same catalogue and intermediate table, but with an increased
maximum number of objects per window and a reduced AR (in the sense
just explained). Here, normally AR reductions exceeding 70% of the
original value will produce windows with too many holes, and the final
window to fit could contain too few pixels. In this case it is wise
to assign a value <IMG
WIDTH="1" HEIGHT="24" ALIGN="BOTTOM" BORDER="0"
SRC="img226.gif"
ALT="$\> 100$">
to the parameter [wnd_perc] in order to
provide the fit with enough pixels to compute the sky background.
<P>
A situation where a large fraction of the objects are not grouped,
even after the AR has been reduced to 70%, can be caused by the
following.
<DL COMPACT>
<DT>1.
<DD>The photometry is extremely difficult (!).
<BR>
If this is the case, the user can only continue to group these
objects interactively. To do so he/she should execute the command
<TT>ANALYSE/ROMAFOT</TT> using the command ``5'' followed by ``S''
and ``R'' or in some cases by ``C''. In case of the latter command
he/she is asked for a threshold. Starting from this point and using
the command ``M'' the objects (possibly above the given threshold)
which <TT>GROUP/ROMAFOT</TT> failed to group are presented to the user.
Obviously, objects grouped interactively can be added to the same
intermediate table generated by <TT>GROUP/ROMAFOT</TT>.
<DT>2.
<DD>The user is trying to group noise peaks.
<BR>
This case can be visualised immediately with the same commands as
indicated for case 1. One should remember that, unless the object
in question is at the frame edge, it is located at the center of
the window presented by <TT>ANALYSE/ROMAFOT</TT>. The solution, in
this case, is to start again with <TT>SEARCH/ROMAFOT</TT> and with an
increased threshold to get a new catalogue list.
<DT>3.
<DD>The AR are too big.
<BR>
The same solution (execute <TT>SEARCH/ROMAFOT</TT> again) can be used
in this case. However, the minimum height must now be increased,
while the photometric threshold can remain unchanged.
</DL>
<P>
Following these considerations one should still be aware that there are
advantages with interactive processing. If, for instance,
<TT>GROUP/ROMAFOT</TT> was successful in grouping 95% of the program
objects, it may be worthwhile to look at the remaining 5%. This
operation to obtain (as a first approximation) complete photometry
can take 10 to 20 minutes. Of course, if one is not interested in
completeness, these last objects can be dropped, since their photometry
will be poor in comparison with the others.
<P>
After having grouped the objects, the user tries to fit the subframes
by executing the command <TT>FIT/ROMAFOT</TT>.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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