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measures.frequency - Function

	Package	general
	Module	measures
	Tool	measures

define a frequency measure

Synopsis
frequency(rf, v0, off)

Arguments

rf	in	reference code
		Allowed:	coded string
v0	in	frequency/wavelength/...
		Allowed:	quantity
		Default:	0Hz
off	in	optional offset
		Allowed:	frequency measure
		Default:	F

Returns
frequency measure

Description
frequency defines a frequency measure from the CLI. It has to specify a reference code, frequency quantity value (see introduction for the action on a scalar quantity with either a vector or scalar value, and when a vector of quantities is given), and optionally it can specify an offset, which in itself has to be a frequency. Allowable reference codes are: REST LSRK LSRD BARY GEO TOPO GALACTO.
Note that additional ones may become available. Check in Glish with:

- dm.listcodes(dm.frequency())
[normal=REST LSRK LSRD BARY GEO TOPO GALACTO, extra=]

The frequency quantity values should be in one of the recognised units (examples all give same frequency):

• value with time units: a period (0.5s)
• value as frequency: 2Hz
• value in angular frequency: 720deg/s
• value as length: 149896km
• value as wave number: 4.19169e-8m-1
• value as enery (h.nu): 8.27134e-9ueV
• value as momentum: 4.42044e-42kg.m

Example

- dm.frequency('lsrk', '5GHz')
[type=frequency, refer=lsrk, m0=[value=5e+09, unit=Hz]] 
- dm.frequency('lsrk', '21cm')
[type=frequency, refer=lsrk, m0=[value=1.42758e+09, unit=Hz]]