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NRAO Home > CASA > CASA Task Reference Manual |
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0.1.90 impv
Requires:
Synopsis
Construct a position-velocity image by choosing two points in the direction
plane.
Arguments
Inputs |
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imagename |
| Name of the input image
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| allowed: | string |
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| Default: |
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outfile |
| Output image name. If empty, no image is written.
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| allowed: | string |
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| Default: |
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mode |
| If ”coords”, use start and end values. If ”length”, use
center, length, and pa values.
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| allowed: | string |
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| Default: | coords |
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start |
| The starting pixel in the direction plane (array of two
values).
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| allowed: | any |
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| Default: | variant
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end |
| The ending pixel in the direction plane (array of two
values).
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| allowed: | any |
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| Default: | variant
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center |
| The center point in the direction plane (array of two
values). If specified, length and pa must also be specified
and neither of start nor end may be specified.
| |
| allowed: | any |
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| Default: | variant
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length |
| The length of the segment in the direction plane. If
specified, center and pa must also be specified and
neither of start nor end may be specified.
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| allowed: | any |
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| Default: | variant
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pa |
| The position angle of the segment in the direction plane,
measured from north through east. If specified, center
and length must also be specified and neither of start
nor end may be specified.
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| allowed: | any |
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| Default: | variant
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width |
| Width of slice for averaging pixels perpendicular to the
slice. Must be an odd positive integer or valid quantity.
See help for details.
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| allowed: | any |
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| Default: | variant 1 |
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unit |
| Unit for the offset axis in the resulting image. Must be
a unit of angular measure.
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| allowed: | string |
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| Default: | arcsec |
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overwrite |
| Overwrite the output if it exists?
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| allowed: | bool |
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| Default: | False |
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region |
| Region selection. Default is entire image. No selection is
permitted in the direction plane.
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| allowed: | any |
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| Default: | variant ”” |
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chans |
| Channels to use. Channels must be contiguous. Default
is to use all channels.
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| allowed: | string |
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| Default: |
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stokes |
| Stokes planes to use. Planes must be contiguous. Default
is to use all stokes.
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| allowed: | string |
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| Default: |
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mask |
| Mask to use. Default is none.
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| allowed: | string |
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| Default: | variant
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stretch |
| Stretch the mask if necessary and possible? Default False
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| allowed: | bool |
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| Default: | False |
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image
Example
PARAMETER SUMMARY
imagename Name of the input (CASA, FITS, MIRIAD) image
outfile Name of output CASA image. Must be specified.
mode Indicates which sets of parameters to use for defining the slice. mode="coords" means use
start and end parameters. mode="length" means use center, length, and pa parameters to define
the slice.
start The starting pixel in the direction plane (array of two values), such as [20, 5] or ["14h20m20.5s","-30d45m25.4s"].
Used iff mode="coords".
end The ending pixel in the direction plane (array of two values), such as [200, 300].
Used iff mode="coords".
center The center of the slice in the direction plane (array of two values), such as [20, 5] or ["14h20m20.5s","-30d45m25.4s"].
Used iff mode="length".
length The length of the slice in the direction plane. May be specified as a single numerical value, in which
case it is interpreted as the number of pixels, or as a valid quantity which must be conformant with
the direction axes units (eg "40arcsec", {"value": 40, "unit": "arcsec"}).
Used iff mode="length".
pa Position angle of the slice, measured from the direction of positive latitude of the positive longitude
(eg north through east in an equatorial coordinate system). Must be expressed as a valid angular
quantity (eg "40deg", {"value": 40, "unit": "deg"}).
Used iff mode="length".
width Width of slice for averaging pixels perpendicular to the slice which must be an odd positive integer or
valid quantity. The averaging using this value occurs after the image has been rotated so the slice lies horizontally.
An integer value is interpreted as the number of pixels to average.
A value of 1 means no averaging. A value of 3 means average one pixel on each
side of the slice and the pixel on the slice. A value of 5 means average 2 pixels
on each side of the slice and the pixel on the slice, etc. If a quantity (eg. "4arcsec", qa.quantity("4arcsec"))
is specified, the equivalent number of pixels is calculated, and if necessary, rounded up
to the next odd integer.
unit Allows the user to set the unit for the angular offset axis. Must be a unit of angular
measure.
overwrite If output file is specified, this parameter controls if an already existing file by the
same name can be overwritten. If true, the user is not prompted, the file
if it exists is automatically overwritten.
region Region specification. Default is to not use a region. If specified,
the entire direction plane must be specified. If specified do not specify chans or stokes.
chans Optional contiguous frequency channel number specification. Default is all channels.
If specified, do not specify region.
stokes Contiguous stokes planes specification. If specified, do not specify region.
mask Mask to use. Default is none.
stretch Stretch the input mask if necessary and possible. Only used if a mask is specified.
Create a position-velocity image. The way the slice is specified is controlled by the mode parameter. When
mode="coords", start end end are used to specified the points between which a slice is taken in the direction
coordinate. If mode="length" center, pa (position angle), and length are used to specify the slice. The spectral
extent of the resulting image will be that provided by the region specification or the entire spectral range of
the input image if no region is specified. One may not specify a region in direction space; that is accomplished by
specifying the slice as described previously. The parameters start and end may be specified as two
element arrays of numerical values, in which case these values will be interpreted as pixel locations in the input
image. Alternatively, they may be expressed as arrays of two strings each representing the direction. These strings
can either represent quantities (eg ["40.5deg", "0.5rad") or be sexigesimal format (eg ["14h20m20.5s","-30d45m25.4s"],
["14:20:20.5s","-30.45.25.4"]). In addition, they may be expressed as a single string containing the longitude-like and
latitude-like values and optionally a reference frame value, eg "J2000 14:20:20.5s -30.45.25.4".The center parameter can
be specified in the same way. The length parameter may be specified as a single numerical value, in which case it is
interpreted as the length in pixels, or a valid quantity, in which case it must have units conformant with the
direction axes units. The pa (position angle) parameter must be specified as a valid quantity with angular units.
The position angle is interpreted in the usual astronomical sense; eg measured from north through east in an equatorial
coordinate system. The slice in this case starts at the specified position angle and ends on the opposite side of
the specified center. Thus pa="45deg" means start at a point at a pa of 45 degrees relative to the specified center and
end at a point at a pa of 215 degrees relative to the center. Either start/end or center/pa/length must be specified;
if a parameter from one of these sets is specified, a parameter from the other set may not be specified. In either
case, the end points of the segment must fail within the input image, and they both must be at least 2 pixels from the
edge of the input image to facilite rotation (see below).
One may specify a width, which represents the number of pixels centered along and perpendicular
to the direction slice that are used for averaging along the slice. The width may be specified as an integer, in which
case it must be positive and odd. Alternatively, it may be specified as a valid quantity string (eg, "4arcsec") or
quantity record (eg qa.quantity("4arcsec"). In this case, units must be conformant to the direction axes units (usually
angular units) and the specified quantity will be rounded up, if necessary, to the next highest equivalent odd integer number
of pixels. The default value of 1 represents no averaging.
A value of 3 means average one pixel on each side of the slice and the pixel on the slice.
Note that this width is applied to pixels in the image after it has been rotated (see below for a description
of the algorithm used).
One may specify the unit for the angular offset axis.
Internally, the image is first rotated, padding if necessary to include relevant pixels that would otherwise
be excluded by the rotation operation, so that the slice is horizontal, with the starting pixel left of the
ending pixel. Then, the pixels within the specified width of the slice are averaged and the resulting image is
written and/or returned. The output image has a linear coordinate in place of the direction coordinate of the
input image, and the corresponding axis represents angular offset with the center pixel having a value of 0.
The equivalent coordinate system, with a (usually) rotated direction coordinate (eg, RA and Dec) is written
to the output image as a table record. It can be retrieved using the table tool as shown in the example below.
# create a pv image with the position axis running from ra, dec pixel positions of [45, 50] to [100, 120]
# in the input image
impv(imagename="my_spectral_cube.im", outfile="mypv.im", start=[45,50], end=[100,120])
# analyze the pv image, such as get statistics
pvstats = imstat("mypv.im")
#
# get the alternate coordinate system information
tb.open("mypv.im")
alternate_csys_record = tb.getkeyword("misc")["secondary_coordinates"]
tb.done()
More information about CASA may be found at the
CASA web page
Copyright © 2016 Associated Universities Inc., Washington, D.C.
This code is available under the terms of the GNU General Public Lincense
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