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Name

ccd2vel - convert map of wave/freq/velocities to doppler velocities

Synopsis

ccd2vel [parameter=value]

Description

ccd2vel converts a map to a doppler velocity map. The input map units can be in wavelength (restwave= is needed), frequency (restfreq= is needed) or one of three doppler velocity frames of reference (radio, optical, relativistic). The output map will be in one of those three doppler velocity frames of reference (radio, optical, relativistic). Output units are normally km/s.

This program ignores references to being in a Barycenric, Heliocentric or LSR frame. See comments below in the CASA and MIRIAD sections.

Parameters

The following parameters are recognized in any order if the keyword is also given
in=
Optional input image file. No default.
out=
Output image file. No Default.
from=rel|rad|opt
Frame of reference (relativistic, radio, optical). When using restwave= or restfreq= this keyword is ignored. [rel]
to=rel|rad|opt
Frame of reference (relativistic, radio, optical) [rel]
restwave=
Rest wavelength, in the units of the map. Only one of restwave of restfreq can be used.
restfreq=
Rest frequency, in the units of the map
clip=min,max
Clipping of output (none, or two values needed). Default: no clipping.
units=km/s
Desired output units (km/s, m/s) [km/s]

Examples

Here is how one converts a fits cube with an optical wavelengths 3rd axis to a relatistic velocity field:
   fitsccd Ha.cube.fits  - | ccdmom - - mom=3 | ccd2vel - Ha-3.vel opt rel 6562.80

and here an optical velocity field into a relativistic
   NGC2347.VoptFromHa.fits - | ccd2vel - NGC2347.VrelFromHa.ccd opt rel

Casa

From the tclean documentation on the veltype keyword:

Velocity type (radio, z, ratio, beta, gamma, optical) For start and/or width specified in velocity, specifies the velocity definition

Options: ’radio’,’optical’,’z’,’beta’,’gamma’

NOTE: the viewer always defaults to displaying the ’radio’ frame, but that can be changed in the position tracking pull down.

The different types (with F = f/f0, the frequency ratio), are:

                       Z = (-1 + 1/F)
                      RATIO = (F) *
                      RADIO = (1 - F)
                      OPTICAL == Z
                      BETA = ((1 - F2)/(1 + F2))
                      GAMMA = ((1 + F2)/2F) *
                      RELATIVISTIC == BETA (== v/c)
                      DEFAULT == RADIO
                      Note that the ones with an ’*’ have no real interpretation

                      (although the calculation will proceed) if given
as a velocity.

Miriad

From the VELSW documentation:

VELSW is a Miriad task used to change the type of a spectral axis. For example, a frequency axis could be changed to redshift or relativistic velocity.

The resulting axis may be non-linear. If so, it will be marked by a code, e.g. VOPT-F2W, meaning that it is actually linear in frequency (F) but is labelled as a spectral type that is linear in wavelength (W). Such non-linear axes are handled properly by the coordinate routines. However, when plotted, non-linear axes are only correct to first-order at the reference point.

Switching between velocity and other spectral types, via the Doppler formula, is only possible if a rest frequency is recorded in the header. It is also possible to change the Doppler reference frame to barycentric or LSRK (only).

Note that velocity axes labelled as {VELO,FELO}-{HEL,LSR} in Miriad have been Doppler-shifted to the specified frame (barycentric or LSRK) as would be expected. However, frequency axes labelled as FREQ-{HEL,LSR} are actually topocentric, the Doppler frame here refers only to the value of vobs recorded in the header. This convention also applies to FITS images written and read by Miriad - the presence of vobs with FREQ-{HEL,LSR} in the header of a Miriad image read from such a FITS file is indicative. However, there is a potential inconsistency when FITS files generated by other packages are read into Miriad. See option altspc below.


Keyword: axis
         The spectral type to change to:
           FREQ: frequency,
           ENER: photon energy (linear with frequency),
           WAVN: wavenumber (linear with frequency),
           VRAD: radio velocity (linear with frequency),
           WAVE: wavelength,
           VOPT: optical velocity (linear with wavelength),
           ZOPT: redshift (linear with wavelength),
           AWAV: air wavelength (applicable to optical regime only),
           VELO: apparent radial velocity (relativistic),
           BETA: relativistic beta factor (= VELO/c).

Note that there is no minimum-match on these four-letter spectral types. For backwards compatibility, ’frequency’, ’radio’, and ’optical’ (with min-match) are understood as synonyms for ’FREQ’, ’VRAD’, and ’VOPT’.

AIPS-convention types, {FREQ,VELO,FELO}-{HEL,LSR}, are also understood and will be translated. Here VELO means radio velocity (VRAD), and FELO is optical velocity (VOPT-F2W). However, these types are deprecated, and there is no way to switch to them explicitly.

An optional second value defines the Doppler frame to transform to (min-match applies):

   BARYCENT: Solar System barycentre (aka BARYCENT{RE,ER,RIC}, or HEL),
   LSRK: Kinematic Local Standard of Rest.
The default for both the spectral type and the Doppler frame is to leave them as is, performing only required translations.

See Also

ccdsky(1NEMO) , ccdmath(1NEMO) , fitsccd(1NEMO)

Author

Peter Teuben

Update History


27-Apr-17    V0.3 Created    PJT


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