The command CALIBRATE/LONG approximates the dispersion relation for each searched row of the arc spectrum. The algorithm can be activated in different modes, controlled by the parameter WLCMTD:
The command CALIBRATE/LONG provides the following results:
The command CALIBRATE/TWICE performs a two-pass determination of the dispersion relation. In a first pass, the lines are identified by a standard CALIBRATE/LONG. Only the lines which have consistently identified at all rows are selected for the second pass, which then performs a new calibration on a stable set of arc lines. If after selection a good spectral coverage of the arc spectrum is secured, this method provides very stable estimates of the dispersion relation.
The command PLOT/CALIBRATE visualizes the lines found by the calibration process. The dispersion curve and the lines that were used to determine it are presented by PLOT/DELTA. Residuals to the dispersion curve are plotted by PLOT/RESIDUAL. For two-dimensional spectra, the command PLOT/DISTORTION can be used to check the stability of the dispersion relation along the slit.
The iterative identification loop consists of estimating the wavelength of all lines in the arc spectrum and associate them to laboratory wavelengths to refine the estimates of the dispersion relation. The line identification criterion will associate a computed wavelength to the nearest catalog wavelength if the residual:
is small compared to the distance of the next neighbours in both the arc spectrum and the catalog:
where is the distance to the next neighbour in the line catalog, the distance to the next neighbour in the arc spectrum and alpha the tolerance parameter. Optimal values of are in the range . The tolerance value is controlled by the parameter ALPHA.
Lines are identified in a first pass without consideration of the rms of the residual values by an iterative loop controlled by the parameter WLCNITER. The residuals for each line are then checked in order to reject outliers which residual is above the value specified by the final tolerance parameter TOL. The degree of the polynomials is controlled by the parameter DCX and the iterative loop is stopped if residuals are found to be larger than MAXDEV.