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image.pv - Function

1.1.1 Construct a position-velocity image between two points in the direction plane.


Description

Create a position-velocity image by specifying either two points between which a slice is taken in the direction coordinate or a center, position angle, and length describing 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 start and end points or the center, length, and position angle of the slice. 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 and latitude values and optionally a reference frame value, eg ”J2000 14:20:20.5s -30.45.25.4”. The center parameter is 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; ie measured from north through east. 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 is 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). The end points of the specified 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 the unit for the angular offset axis.

A true value for the wantreturn parameter indicates that an image analysis tool attached to the created image should be returned. Nothing is returned if wantreturn is false, but then outfile should be specified (unless perhaps you are debugging).

Internally, the image is first rotated, padding first 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.

Arguments





Inputs

outfile

Output image name. If empty, no image is written. Default ””

allowed:

string

Default:

start

The starting point in the direction plane (array of two values). If specified, end must also be specified and none of center, pa, nor length may be specified.

allowed:

any

Default:

variant

end

The ending point in the direction plane (array of two values). If specified, start must also be specified and none of center, pa, nor length may be specified.

allowed:

any

Default:

variant

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

Default:

variant

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.

allowed:

any

Default:

variant

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.

allowed:

any

Default:

variant

width

Width in pixels for averaging pixels perpendicular to the slice. Must be an odd integer >= 1 (1 means only use the pixels along the slice), or a quantity which will be rounded up if necessary so it corresponds to the next highest odd number of pixels.

allowed:

any

Default:

variant 1

unit

Unit for the offset axis in the resulting image. Must be a unit of angular measure.

allowed:

string

Default:

arcsec

overwrite

Overwrite the output if it exists?

allowed:

bool

Default:

false

region

Region selection. See ”help par.region” for details. Default is to use the full image. No selection is permitted in the direction plane.

allowed:

any

Default:

variant

chans

Channels to use. See ”help par.chans” for details. Channels must be contiguous. Default is to use all channels.

allowed:

string

Default:

stokes

Stokes planes to use. See ”help par.stokes” for details. Planes must be contiguous. Default is to use all stokes planes.

allowed:

string

Default:

mask

Mask to use. See help par.mask. Default is none.

allowed:

string

Default:

variant

stretch

Stretch the mask if necessary and possible? See help par.stretch. Default False

allowed:

bool

Default:

false

wantreturn

Return an image analysis tool attached to the created image?

allowed:

bool

Default:

true

Returns
image

Example

 
        ia.open("my_spectral_cube.im")  
        # create a pv image with the position axis running from ra, dec pixel positions of [45, 50] to [100, 120]  
        # in the input image  
        mypv = ia.pv(outfile="pv.im", start=[45,50], end=[100,120], wantreturn=true)  
        ia.done()  
        # analyze the pv image, such as get statistics  
        pvstats = mypv.statistics()  
        # when done, close the tool to release system resources  
        mypv.done()  
 
        # get the alternate coordinate system information  
        tb.open("pv.im")  
        alternate_csys_record = tb.getkeyword("misc")["secondary_coordinates"]  
        tb.done()  
 

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