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10.10 Line Measurements

The measurement of radial velocities is straightforward. To do so, type:


The H, W, F, Q vectors are the usual header, wavelength, flux, and quality flag vectors for your spectrum. The header and quality flags are optional.

RV allows you to interactively measure the radial velocities of features selected by using the graphics cursor. You will be prompted for the line identification and rest wavelength for each line you select. If you include the ``h'' and ``q'' vectors in the parameter list, the resulting plot will be labeled with the image information. For IUESIPS data ``q'' would be your $\epsilon$ vector; for NEWSIPS ``q'' would be your $\nu$ vector.

Table 10.3: FEATURE Outputs
Notation Definition
WLAB The original value for W0.
W1 Short wavelength limit to integration.
W2 Extremum of spectral line.
W3 Long wavelength limit to integration.

Rule RVn = (Wn-W0)$\ast$2.99792E5/W0
RV1 Radial velocity for short wavelength limit to integration.
RV2 Radial velocity for extremum of spectral line.
RV3 Radial velocity for the long wavelength limit to integration.

F1 Flux at the short wavelength limit to the integration.
F2 Flux at the spectral line extremum.
F3 Flux at the long wavelength limit to the integration.

Rule Continuum = (W-W1)$\ast$(F3-F1)/(W3-W1)+F1
FCONT Continuum flux. (Integral(Continuum)/(W3-W1))
RESI2 Residual flux at extremum. (F2/FCONT)

EW-A or EW-MA Equivalent width in Å or mÅ.

FTOT Total flux in the feature. (Integral(FLUX))
WTOT Flux weighted wavelength for feature.
WIDTOT Flux-weighted sigma, which for a gaussian profile can be related to the full width at half maximum.

Rule FNET, WNET and WIDNET are computed replacing F with F-Continuum.
FNET Net flux in feature.
WNET Flux weighted wavelength, with continuum not included.
WIDNET Flux weighted sigma for net flux only.

All integrals are with respect to wavelength from W1 to W2 using the trapezoidal rule.

You can use FEATURE to generate statistics for a spectral feature with a straight line continuum specified interactively with the graphics cursor. The calling sequence is


W,F Wavelengths and fluxes for your spectrum.
W0 Laboratory wavelength of the feature of interest for calculating radial velocities and for centering the plot.
title You may include a title for the results via this parameter.
noprint Normally, FEATURE asks you if you wish to send the results to the printer. You may suppress this question by setting this keyword.

The wavelengths computed are flux-weighted means; thus they are more accurate than those provided by POINT. This procedure generates a plot and all the calculated values so that a hardcopy of the screen records all the results, which may be sent to the printer. See Table 10.3 for an explanation of the statistics printed to the side of the plot. For more details on the manner in which the statistics are calculated use DOC_LIBRARY,'FEATURE'.

next up previous contents
Next: 10.11 Measuring Continuum Fluxes Up: 10 MORE WAYS TO Previous: 10.9 Identifying Spectral Lines