Documentation on imsad

Task: imsad
Purpose: Image search and destroy
Categories: image analysis

       IMSAD fits a gaussian to the image histogram to determine the true
       image rms noise, then searches for islands of pixels above some
       cutoff, and attempts to fit gaussian components to the islands. The
       fitting is borrowed from the miriad imfit task and the island
       detection from the AIPS SAD task. 

       Pixels above the clip level are marked in yellow.  Then a square
        region centred on each island is coloured in. 

       When fitting Gaussians, a cross is plotted at the midpoint of
       the island.  Each island is displayed separately in the top
        left of the display as it is fit.  The centroid of the fit is
        plotted.

        Information about the fit for each island is given.

Key: in
        The name of the input image data set.

Key: region
        The region of interest within the image (rectangular only).

Key: range
        Image intensity range to display. Default is min to max for the
        current plane being displayed.

Key: clip
        Clip  level. For input images of intensity, any pixels below the
        clip level are excluded from the fitting process. For other sorts
        of images (e.g. Stokes Q, U or V) pixels whose absolute values are
        below the clip level are excluded from the fit.

       The clip level can be specified as a multiple of the true image 
       rms (using the hist option) or as an absolute pixel value. No
       default.

Key: box
       The minimum extents for island boundaries. This would usually be
       some multiple of the beam extents in x and y (2,2 is the default).
       For images with no beam characteristics there is no default, the
       units in this case are pixels.

Key: max
        Sets the maximum number of boxes to return, if more then max
        are detected then the max boxes with largest peak flux / pixel
       are returned. 

Key: rad
       Will only report those islands detected within some angular radius
       of the specified coordinate pair.  The default units are absolute
       pixels, eg. 16,18,10.  If units other than the default are used
       you must specify these after the radius, eg. 10:34:45.3,-45:54:02,
       0.005,hms,dms,degrees. 

Key: options
        Extra processing options. Possible values are:

       hist ....... compute the image pixel histogram and compute the
                    true image rms
       gauss ...... fit a gaussian model to each component
       fixed ...... the FWHM is fixed (circular cross-section)
       point ...... a gaussian with the characteristics of the
                    point-spread-function is fit
       box ........ return the island box extents
       arcsec ..... output box BLC/TRC units are arcsec offset from reference,
                    default is absolute pixels
       fiddle ..... interactively adjust the display lookup table (LUT),
                    and alter the transfer function
       nofit ...... do not perform any fitting
       noplt ...... disable the plotting features
        nodet ....... skip island detection and fit the box given by region 
        	     - not implemented

Key: device
        PGPLOT plot device

Key: out
        Dataset to write fitted source parameters.  Each line of this file
        summarises the result of the Gaussian fit for one island.
        Each line contains:

        Island name, island number, ra and dec of island centroid, 
        peak flux density, integrated flux density, deconvolved (from
        beam) major axis FWHM (arcsec), deconvolved minor axis (arcsec),
        deconvolved position angle (degrees), FLAG, DFLAG, FFLAG

        FLAG:   IFAIL value (an integer) means Gaussian fit did not converge
                F means failed to find covariance matrix for Gaussian fit
                N means it was not possible to attempt a Gaussian fit
                C converged OK

       DFLAG:  D  deconvolution of size from beam OK
        	P  deconvolution gave result close to point source
        	?  No synthesised beam so could not deconvolve
                F  deconvolution failed

       FFLAG:  F  Integrated flux   peak flux (caused by beam   source size)
               If blank then ok.

Key: log
       If specified, output is written to the file given by log= instead
       of to the terminal.

Key: label
       Special purpose label