Basic Information 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 

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