Create simulated data containing clumps and noise
The clumps profiles are Gaussian, with elliptical isophotes. The values of each parameter defining the clump shape can be either fixed at a constant value or selected from a given probability distribution. The clump positions can be distributed randomly, or may be on a regular grid (see parameter GRID).
Peak1: The position of the clump peak value on axis 1.
Peak2: The position of the clump peak value on axis 2.
Peak3: The position of the clump peak value on axis 3.
Cen1: The position of the clump centroid on axis 1.
Cen2: The position of the clump centroid on axis 2.
Cen3: The position of the clump centroid on axis 3.
Size1: The size of the clump along pixel axis 1.
Size2: The size of the clump along pixel axis 2.
Size3: The size of the clump along pixel axis 3.
Sum: The total data sum in the clump.
Peak: The peak value in the clump.
Volume: The total number of pixels falling within the clump.
There is also an optional column called "
Shape"
containing an STC-S description of the spatial
coverage of each clump. See parameter SHAPE.
The coordinate system used to describe the peak and centroid positions is determined by the value
supplied for parameter LIKE. If LIKE is null (!), then positions are specified in the pixel coordinate
system. In addition, the clump sizes are specified in units of pixels, and the clump volume is specified
in units of cubic pixels (square pixels for 2D data). If an NDF is supplied for LIKE, then
positions are specified in the current coordinate system of the specified NDF. In addition, the
clump sizes and volumes are specified in WCS units. Note, the sizes are still measured
parallel to the pixel axes, but are recorded in WCS units rather than pixel units. Celestial
coordinate positions are units of degrees, sizes are in units are arc-seconds, and areas in square
arc-seconds. Spectral coordinates are in the units displayed by the KAPPA command "
ndftrace"
.
If the data has less than 3 pixel axes, then the columns describing the missing axes will not be present in the catalogue.
The catalogue inherits any WCS information from the NDF supplied for parameter LIKE.
The "
size"
of the clump on an axis is the RMS deviation of each pixel centre from the clump centroid,
where each pixel is weighted by the correspinding pixel data value. This excludes the instrumental
smoothing specified by BEAMFWHM and VELFWHM.
The KAPPA command "
listshow"
can be used to draw markers at the central positions of the clumps
described in a catalogue. For instance, the command "
listshow fred plot=mark"
will draw markers
identifying the positions of the clumps described in file fred.FIT, overlaying the markers on
top of the currently displayed image. Specifying "
plot=STCS"
instead of "
plot=mark"
will cause the spatial outline of the clump to be drawn if it is present in the catalogue (see
parameter SHAPE).
"
Normal"
, "
Uniform"
or "
Poisson"
. The
distribution for each clump parameter is specified by its own ADAM parameter containing
two values; the mean and the width of the distribution. If PARDIST is "
Normal"
, the
width is the standard deviation. If PARDIST is "
Uniform"
, the width is half the range
between the maximum and minimum parameter values. In either of these two cases, if a
width of zero is supplied, the relevant parameter is given a constant value equal to the
specified mean. If PARDIST is "
Poisson"
, the width is ignored. [current value] "
None"
, the spatial shape of each clump is
not recorded in the output catalogue. Otherwise, the catalogue will have an extra column
named "
Shape"
holding an STC-S description of the spatial coverage of each clump. "
STC-S"
is a textual format developed by the IVOA for describing regions within a WCS - see
http://www.ivoa.net/Documents/latest/STC-S.html for details. These STC-S desriptions can be
displayed by the KAPPA:LISTSHOW command, or using GAIA. Since STC-S cannot describe regions
within a pixel array, it is necessary to provide an NDF to define the WCS (using parameter LIKE) if
using this option. An error will be reported if the WCS in the NDF does not contain a pair of celestial
sky axes.
Polygon: Each polygon will have, at most, 15 vertices. If the data is 2-dimensional, the polygon is a fit
to the clump’
s outer boundary (the region containing all godo data values). If the data is
3-dimensional, the spatial footprint of each clump is determined by rejecting the least significant 10%
of spatial pixels, where "
significance"
is measured by the number of spectral channels that contribute
to the spatial pixel. The polygon is then a fit to the outer boundary of the remaining spatial
pixels.
Ellipse: All data values in the clump are projected onto the spatial plane and "
size"
of the collapsed
clump at four different position angles - all separated by 45 degrees - is found (see the OUTCAT
parameter for a description of clump "
size"
). The ellipse that generates the closest sizes at the four
position angles is then found and used as the clump shape.
Ellipse2: The above method for determining ellipses works well for clumps that are in fact elliptical,
but can generate extremely long thin ellipses for clumps are far from being ellitical. The "
Ellipse2"
option uses a different method for determining the best ellipse based on finding many marginal
profiles at one degree intervals of azimuth, and using the longest marginal profile as the major axis.
The ellipse is centred at the clump centroid.
Ellipse3: The same as "
Ellipse2"
except that the ellipse is centred at the clump peak, rather than the
clump centroid, and the pixel data values are used as weights when forming the mean radial distance
at each azimuth angle.
In general, ellipses will outline the brighter, inner regions of each clump, and polygons will include the fainter outer regions. [None]
"
velocity pixels per spatial pixel"
. See parameter PARDIST for additional
information. [current value] "
velocity pixels per
spatial pixel"
. See parameter PARDIST for additional information. [current value] If 3D data is created, pixel axes 1 and 2 are the spatial axes, and pixel axis 3 is the velocity axis.
The positions of the clumps are chosen from a uniform distribution on each axis.