Compute moments from an image
The spectral moment distributions at each pixel are
determined. See the cookbook and User Reference Manual for
mathematical details.
The main control of the calculation is given by parameter
moments:
moments=-1 - mean value of the spectrum
moments=0 - integrated value of the spectrum
moments=1 - intensity weighted coordinate;traditionally used to get
'velocity fields'
moments=2 - intensity weighted dispersion of the coordinate; traditionally
used to get "velocity dispersion"
moments=3 - median of I
moments=4 - median coordinate
moments=5 - standard deviation about the mean of the spectrum
moments=6 - root mean square of the spectrum
moments=7 - absolute mean deviation of the spectrum
moments=8 - maximum value of the spectrum
moments=9 - coordinate of the maximum value of the spectrum
moments=10 - minimum value of the spectrum
moments=11 - coordinate of the minimum value of the spectrum
Keyword arguments:
imagename Name of input image
default: none; example: imagename="ngc5921_task.image"
moments List of moments you would like to compute
default: 0 (integrated spectrum);example: moments=[0,1]
see list above
axis The moment axis
default: (spectral axis); example: axis=spec
options: ra, dec, lattitude, longitude, spectral, stokes
mask Mask to use. See help par.mask. Default is none.
stretch Stretch the input mask if necessary and possible. See below.
region Region specification. See help par.region. Default is to not use a region.
box A box region on the directional plane
Only pixel values acceptable.
Default: none (whole 2-D plane);
Example: box="10,10,50,50"
box = "10,10,30,30,35,35,50,50" (two boxes)
chans channel numbers
Range of channel numbers to include in statistics
All spectral windows are included
Default:""= all; Example: chans="3~20"
stokes Stokes parameters to analyze.
Default: none (all); Example: stokes="IQUV";
Example:stokes="I,Q"
Options: "I","Q","U","V","RR","RL","LR","LL","XX","YX","XY","YY", ...
includepix Range of pixel values to include
default: [-1] (all pixels); example=[0.02,100.0]
excludepix Range of pixel values to exclude
default: [-1] (don"t exclude pixels); example=[100.,200.]
outfile Output image file name (or root for multiple moments)
default: "" (input+auto-determined suffix);example: outfile="source_moment"
If stretch is true and if the number of mask dimensions is less than
or equal to the number of image dimensions and some axes in the
mask are degenerate while the corresponding axes in the image are not,
the mask will be stetched in the degenerate axis dimensions. For example,
if the input image has shape [100, 200, 10] and the input
mask has shape [100, 200, 1] and stretch is true, the mask will be
stretched along the third dimension to shape [100, 200, 10]. However if
the mask is shape [100, 200, 2], stretching is not possible and an
error will result.
Example for finding the 1-momment, intensity-weighted
coordinate, often used for finding velocity fields.
immoments( axis="spec", imagename="myimage", moment=1, outfile="velocityfields" )
Example finding the spectral mean, -1 moment, on a specified region
of the image as defined by the box and stokes parameters
taskname="immoments"
default()
imagename = "myimage"
moment = -1
axis = "spec"
stoke = "I"
box = [55,12,97,32]
go
Example using a mask created with a second file to select the
data used to calculate the 0-moments, integrated values. In
this case the mask is from the calibrated.im file and all values
that have a value greater than 0.5 will be positive in the mask..
immoments( "clean.image", axis="spec", mask="calibrated.im>0.5", outfile="mom_withmask.im" )
If an image has multiple (per-channel beams) and the moment axis is equal to the
spectral axis, each channel will be convolved with a beam that is equal to the beam
having the largest area in the beamset prior to moment determination.
1.8.0