Getting Started

Documentation

Glish

Learn More

Programming

Contact Us

Version 1.9 Build 1488

News	FAQ
Search	Home

coordsys.velocitytofrequency - Function

	Package	general
	Module	images
	Tool	coordsys

Convert velocity to frequency

Synopsis
velocitytofrequency(value, frequnit, doppler, velunit)

Arguments

value	in	Velocity to convert
		Allowed:	Numeric or vector of numeric
frequnit	in	Unit of output frequencies
		Allowed:	String
		Default:	Intrinsic units
doppler	in	Velocity doppler definition
		Allowed:	String
		Default:	'radio'
velunit	in	Unit of input velocities
		Allowed:	String
		Default:	'km/s'

Returns
Double or vector of double or fail

Description

This function (short-hand name vtf) converts velocities to frequencies.

The input velocities are specified via a vector of numeric values, a specified unit (velunit), and a velocity doppler definition (doppler).

The frequencies are returned in a vector for which you specify the units (frequnit). If you don't give the unit, it is assumed that the units are those given by function units for the spectral coordinate.

This function will return a fail if there is no spectral coordinate in the Coordinate System. See also the function frequencytovelocity.

Example

- im := image('hcn.cube')
- cs := im.coordsys()
-
- local pa, wa;
- cs.findcoordinate(pa, wa, 'spectral')     # Find spectral axis
- pixel := cs.referencepixel();             # Use reference pixel for non-spectral
-
- nFreq := im.shape()[pa[1]];               # Length of spectral axis
- freq := [];
- for (i in 1:nFreq) { 
    pixel[pa[1]] := i;                      # Assign value for spectral axis of pixel coordinate
    w := cs.toworld(value=pixel, format='n')# Convert pixel to world
    freq[i] := w[wa[1]];                    # Fish out frequency
  }
- vel := cs.frequencytovelocity(value=freq, doppler='optical', velunit='km/s')
- freq2 := cs.velocitytofrequency(value=vel, doppler='optical', velunit='km/s')

In this example, we find the optical velocity in km/s of every pixel along the spectral axis of our image. First we obtain the Coordinate System from the image. Then we find which axis of the Coordinate System (image) pertain to the spectral coordinate. Then we loop over each pixel of the spectral axis, and convert a pixel coordinate (one for each axis of the image) to world. We obtain the value for the spectral axis from that world vector, and add it to the vector of frequencies. Then we convert that vector of frequencies to velocity. Then we convert it back to frequency. They better agree.