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##### coordsys.frequencytovelocity - Function

1.1.3 Convert frequency to velocity

Description

This function converts frequencies to velocities.

The input frequencies are specified via a vector of numeric values and a specified unit (frequnit). If you don’t give a frequency unit, it is assumed that the units are those given by function coordsys units() for the spectral coordinate.

This function does not make any frame conversions (e.g. LSR to BARY) but you can specifiy the velocity doppler definition via the doppler argument (see image summary() for possible values).

The velocities are returned in a vector for which you specify the units (velunit - default is km/s).

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

Arguments

 Inputs value Frequency to convert allowed: doubleArray Default: frequnit Unit of input frequencies. Default is unit of the spectral coordinate. allowed: string Default: doppler Velocity doppler definition allowed: string Default: radio velunit Unit of output velocities allowed: string Default: km/s
Returns
doubleArray

Example

"""
#
print "\t----\t frequencytovelocity Ex 1 \t----"
im = ia.fromshape(shape=[10,10,10])
csys = ia.coordsys()
rtn = csys.findcoordinate(’spectral’)   # Find spectral axis
pa=rtn[’pixel’]
wa=rtn[’world’]
pixel = csys.referencepixel();          # Use reference pixel for non-spectral
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
#freq= [1414995000.0, 1414996000.0, 1414997000.0, 1414998000.0,
# 1414999000.0, 1415000000.0, 1415001000.0, 1415002000.0, 1415003000.0, 1415004000.0]
vel = csys.frequencytovelocity(value=freq, doppler=’optical’, velunit=’km/s’)
print "vel=", vel
#vel= [1146.3662963847394, 1146.153618169159, 1145.9409402542183, 1145.7282626398826,
# 1145.5155853261515, 1145.3029083129911, 1145.0902316004676, 1144.8775551885467,
# 1144.6648790772279, 1144.4522032665104]
#
"""

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.

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