dopcor input output redshift
input
List of input spectra to be doppler corrected.
output
List of doppler corrected spectra. If no output list is specified then
the input spectra are modified. Also the output name may be
the same as the input name to replace the input spectra by the
calibrated spectra.
redshift
Redshift or radial velocity (km/s) to be removed? The spectra are corrected so
that the specified redshift is removed; i.e. spectra with a positive
velocity are shifted to shorter wavelengths and vice-versa. This parameter
may be either a number or an image header keyword with the desired redshift
or velocity value. An image header keyword may also have an initial minus
sign, '-', to specify the negative of a velocity or the redshift complement
(1/(1+z)-1) of a redshift. The choice between a redshift and a velocity is
made with the isvelocity parameter.
isvelocity = no
Is the value specified by the redshift parameter a velocity? If
no then the value is interpreted as a redshift and if it is yes then
it is interpreted as a physical velocity in kilometers per second. Note that
this is a relativistic velocity and not c*z! For nearby cosmological
velocities users should specify a redshift (z = v_cosmological / c).
add = no
Add doppler correction to existing correction in "multispec" spectra?
dispersion = yes
Apply a correction to the dispersion function?
flux = no
Apply a flux correction?
factor = 3
Flux correction factor as a power of 1+z when applying a flux correction.
apertures =
List of apertures to be corrected. If none are specified then all apertures
are corrected. An aperture list consists of comma separated aperture
number or aperture number ranges. A range is hypen separated and may
include an interval step following the character 'x'. See ranges
for further information. For N-dimensional spatial spectra such as
long slit and Fabry-Perot spectra this parameter is ignored.
verbose = no
Print corrections performed? The information includes the output image
name, the apertures, the redshift, and the flux correction factor.
The input spectra (as specified by the input image list and apertures) are corrected by removing a specified doppler shift and written to the specified output images. The correction is such that if the actual shift of the observed object is specified then the corrected spectra will be the rest spectra. The opposite sign for a velocity or the redshift complement (1/(1+z)-1) may be used to add a doppler shift to a spectrum.
The redshift or space velocity in km/s is specified either as a number or as an image header keyword containing the velocity or redshift. If a number is given it applies to all the input spectra while an image header keyword may differ for each image. The latter method of specifying a velocity is useful if velocity corrections are recorded in the image header. See rvcorrect for example.
The choice between a redshift and a space velocity for the redshift parameter is made using the isvelocity parameter. If isvelocity=yes then the header dispersion solution is modified according to the relativistic Doppler correction:
lambda_new = lamda_old * sqrt((1 + v/c)/(1 - v/c))
where v is the value of "redshift". If isvelocity=no, redshift is interpreted as a cosmological redshift and the header dispersion solution is modified to give:
lambda_new = lamda_old * z
where z is the value of "redshift"
If the add parameter is used and the image uses a "multispec" format where the previous doppler factor is stored separately then the new doppler factor is:
znew = (1 + z) * (1 + zold) - 1 = z + zold + z * zold
where z is the specified doppler factor, zold is the previous one, and znew is the final doppler factor. If the add parameter is no then the previous correction is replaced by the new correction. Note that for images using a linear or equispec coordinate system the corrections are always additive since a record is not kept of the previous correction. Also any flux correction is made based on the specified doppler correction rather than znew.
There are two corrections which may be made and the user selects one
or both of these. A correction to the dispersion function is selected
with the dispersion parameter. This correction is a term to be
applied to the dispersion coordinates defined for the image. The spectrum
is not resampled, only the dispersion coordinate function is affected.
A correction to the flux, pixel values, is selected with the flux
parameter. This correction is only significant for cosmological redshifts.
As such the correction is dependent on a cosmological model as well as
whether a total flux or surface brightness is measured. To provide the
range of possible corrections the flux correction factor is defined by
the factor parameter as the power of 1+z (where z is the
redshift) to be multiplied into the observed pixel values.
1. To dispersion and flux correct a quasar spectrum with redshift of
3.2 to a rest frame:
2. To correct a set of spectra (in place) to heliocentric rest the task
rvcorrect is used to set the VHELIO keyword using an observed
velocity of 0. Then:
3. To artificially add a redshift of 3.2 to a spectrum the complementary
redshift is computed:
REVISIONS
The add parameter was added.
EXAMPLES
cl> dopcor qso001.ms qso001rest.ms 3.2 flux+
cl> dopcor *.imh "" vhelio isvel+
cl> = 1/(1+3.2)-1
-0.76190476190476
cl> dopcor artspec "" -0.762 flux+
REVISIONS
DOPCOR V2.10.3
This task was extended to work on two and three dimensional spatial spectra
such as long slit and Fabry-Perot spectra.
DOPCOR V2.10.2
A sign error in converting velocity to redshift was fixed. A validity
check on the velocities and redshifts was added. The documentation
was corrected and improved.
DOPCOR V2.10
This task is new.
SEE ALSO
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