EWAVE will first match emission peaks in an extracted echelle arc spectrum (where the row number of the Vista image corresponds to the order number of the spectrum in that row, and the column number will be the column from the original (raw) image). For each order, the program looks for peaks and attempts to identify them based on an initial estimate of the central wavelength of that order. It assumes that none of the lines are blended. After the identifications are made a two-D polynomial is fit using a non-linear least square method. The two coordinates of this polynomial are the column number and 1/m where m is the order number. The default orders of the polynomial are 2 for the column number and 1 for the 1/m coordinate. It is possible at the moment to fit a 3 x 1 polynomial instead of a 2 x 1 polynomial using PORD=3 but this generally does not improve the accuracy of the fit. The fit parameters are iterated up to five times.
The `XOFF=' keyword is used when the ccd is offset from the nominal pattern center (grating rotation = 493830). It is positive when the ccd is to the left of the pattern center. It is zero by default. The keyword `REJ=' is invoked to allow the routine to converge more rapidly. A line is rejected if the difference between the wavelength of the line and its wavelength determined from the polynomial fit is larger than `rej' mA. It is invoked after the second iteration and is 100 mA by default.