1.1.3.0 coordsys.velocitytofrequency

1.1.3 Convert velocity to frequency
Description

This function 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.


Inputs
value	Velocity to convert
	allowed:	doubleArray
	Default:
frequnit	Unit of output frequencies. Default is intrinisic units.
	allowed:	string
	Default:
doppler	Velocity doppler definition
	allowed:	string
	Default:	radio
velunit	Unit of input velocities
	allowed:	string
	Default:	km/s

"""
#
print "\t----\t velocitytofrequency Ex 1 \t----"
ia.fromshape(’hcn.cube’,[64,64,32,4], overwrite=true)
csys = ia.coordsys()
rtn = csys.findcoordinate(’spectral’)   # Find spectral axis
pixel = csys.referencepixel();          # Use reference pixel for non-spectral
pa = rtn[’pixel’]
wa = rtn[’world’]
nFreq = ia.shape()[pa]                  # Length of spectral axis
freq = []
for i in range(nFreq):
  pixel[pa] = i;                        # Assign value for spectral axis of pixel coordinate
  w = csys.toworld(value=pixel, format=’n’)# Convert pixel to world
  freq.append(w[’numeric’][wa])         # Fish out frequency
print "freq=", freq
vel = csys.frequencytovelocity(value=freq, doppler=’optical’, velunit=’km/s’)
freq2 = csys.velocitytofrequency(value=vel, doppler=’optical’, velunit=’km/s’)
print "vel=",vel
print "freq2=",freq2
csys.done()
#
exit() # This is last example so exit casapy if you wish.
#
"""

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.

coordsys.velocitytofrequency - Function