Description

Create a position-velocity image. The way the slice is specified is controlled by the mode parameter. When mode="coords", start and end are used to specify the points between which 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 (e.g. ["40.5deg", "0.5rad") or be sexigesimal format (e.g. ["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, e.g. "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; e.g. 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 225 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 fall 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 (e.g., "4arcsec") or quantity record (e.g. 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 (e.g., RA and Dec) is written to the output image as a table record. It can be retrieved using the table tool (see the examples tab, which is linked from the top of this page).