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Next: GROUP/ROMAFOT Up: The Automatic Path Previous: SKY/ROMAFOT


SEARCH/ROMAFOT frame [sky_tab] [cat_tab] [area] [psf_par] [thresh] [height]

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 SEARCH/ROMAFOT is compatible with the table created by the command FIND/ROMAFOT. Therefore, after this automatic search for objects the user can pass to the interactive procedure to group the objects with the command ANALYSE/ROMAFOT. This is the first decision point of the two procedure paths.

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.

Since it is expected that the automatic search will be generally followed by automatic grouping, SEARCH/ROMAFOT calculates an additional parameter with respect to the command FIND/ROMAFOT. With this parameter the output table of SEARCH/ROMAFOT can be used as if they were created by the command FIND/ROMAFOT. The inverse is not possible, the output table created by the command FIND/ROMAFOT cannot be used in the command GROUP/ROMAFOT. 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.

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.

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.

After SEARCH/ROMAFOT one has obtained a list of candidates (often thousands). Although a quick inspection with ANALYSE/ROMAFOT is always instructive, the manual grouping of the objects is a waste of time. This can be avoided by using the command GROUP/ROMAFOT.

next up previous contents
Next: GROUP/ROMAFOT Up: The Automatic Path Previous: SKY/ROMAFOT
Petra Nass