An initial redshift source may be specified by
vel_init; if
t_absrej and
t_emrej set the lower and upper
acceptable limits for template pixels to be used in the continuum fit to
the template if
t_emchop is set to
yes.
A graph of the continuum-removed, apodized data is displayed if the
contsub_plot parameter is
yes.
If nsmooth is yes,
the displayed spectrum is smoothed by a 1-2-1 sliding filter
nsmooth times. The filtered cross-correlation with the best
R-value is displayed centered on the redshift
cvel (in km/sec) with a
width in km/sec of dvel.
If cvel is INDEF, the fit redshift is used;
if dvel is INDEF, the width is set to 20 times the peak
width. If the correlation is not displayed, absorption lines
(ablines=yes)
and/or emission lines
(emlines=yes)
may be labelled from line lists in the directory
linedir.
If curmode is yes,
the user can interact with the display using the terminal cursor to
zoom in on portions of the spectrum, rerun the cross-correlation, change
the display format, edit the spectrum, or several other functions.
Process Spectrum/Template Pair (XCORFIT subroutine)
First, the continuum, and, optionally, emission lines, are removed from the
rebinned object and template spectra.
Remove Continuum (ICSUBCON subroutine)
The IRAF curve fitting subroutines may be used interactively by setting
c_interactive to yes.
A function specified by
c_function of order
order is used to fit the
portion of the spectra specified by
sample, averaged in groups of
naverage pixels.
Points outside of limits
s_low_reject and
s_high_reject, specified in
standard deviations of the fit, are eliminated through
niterate iterations, with
grow pixels rejected on each
side of the rejected point.
For templates, the rejection limits are
t_low_reject and
t_high_reject.
Replace Lines with Continuum (ICSUBCON subroutine)
Emission and/or absorption lines may be removed from either or both of each
spectrum-template pair.
s_emchop controls whether
lines will be removed from the object spectrum.
If the template header parameter
SUBCONT is present, its value
overrides that of s_emchop.
s_absrej and
s_emrej set the lower and upper
acceptable limits for object spectrum pixels to be used in the continuum fit.
Apodize (XCORFIT subroutine)
Both spectra are then apodized, tapered at each end for the fraction
bell_window of the entire
spectrum. If
zeropad is yes,
the template and object spectra are padded with an equal length of null
(zero) spectrum to reduce noise in the Fourier transform. A graph of the
continuum-removed, apodized data is displayed if the
apodize_plot parameter is
yes.
Fourier Transform and Filter (XCORFIT subroutine)
The transformed object and template spectra are then filtered with a
cosine-bell filter where the low frequencies are filtered from
low_bin to
top_low and the high frequencies
are filtered from
top_nrun to
nrun. The template
header parameter
FI-FLAG controls whether
the template transform is filtered and whether the high-frequency filter
is turned off for both template and object transforms to leave in
emission lines. The Fourier power spectra of the object and template
spectra are displayed if fft_plot
is set to yes. The filtered Fourier power spectra of
the object and template spectra are displayed if
tfft_plot is set to yes.
Cross-Correlate (XCORFIT subroutine)
The filtered transforms are then cross-correlated and normalized.
If uxcor_plot is yes,
the unfiltered correlation is displayed. If the
xcor_plot parameter is
yes, this result is displayed,
and a specific peak may be selected. The maximum value within
pksrch
pixels of the cursor position is used. Otherwise, the highest correlation
peak between the velocities
minvel and
maxvel is used.
The redshift is calculated by fitting a parabola or similar function
specified by
pkmode to the portion of the peak
above
pkfrac
of the maximum value of that peak. The R-value and error are computed, and
control returns to XCFIT to set up the template for the next pass.
Report Results (XCFIT subroutine)
After all of the template spectra have been correlated against an object
spectrum, the template with the highest R-value is selected. The
are displayed as text to the devices specified by
logfiles in the format
specified by
report_mode.
Combine Emission and Cross-Correlation Velocities (VCOMBINE)
If there is only a cross-correlation velocity for this spectrum, that
becomes the final velocity and the final velocity error is based on the
correlation error.
If there is only an emission line velocity, it becomes the final velocity,
and the final velocity error is based on the emission error.
If both velocities are present and their difference squared divided by
the sum of the squares of their errors is less than 8, the error is the
two errors added in quadrature and the final velocity is the error-weighed
sum of the velocities. If they differ and the correlation R-value is
greater than 10 or it is greater than 4 and there are only one or two
emission lines found, that velocity is used. Otherwise, the emission line
velocity is used.
In all cases, an assumed dispersion error of 15 km/sec added in quadrature
to the final error value.
Display Results Graphically (XCPLOT subroutine)
If displot is yes,
the spectrum and, optionally, the selected correlation peak, are plotted to
device in the format specified by
dispmode. If
hardcopy is yes,
the same graph is sent to
plotter.
Save Results in Image Header (XCFIT subroutine)
If save_vel is yes,
cross-correlation redshift results are written into the spectrum image
header in a form appropriate to the spectrum format: two entries plus
one per template if multispec; otherwise one, value per keyword.
Last updated July 29, 1998 by Doug Mink
dmink@cfa.harvard.edu
[XCSAO]