General Description

A preliminary step to the wavelength calibration consists of extracting the orders of the WLC image which can then be used to determine the dispersion relation in two steps:

• The calibration lines are detected on the extracted orders by means of a simple thresholding algorithm. The center of the line is estimated by its center of gravity ( GRAVITY method) or by a gaussian fit to the line profile ( GAUSSIAN method). This is done with the command SEARCH/ECHELLE.

• A few lines are identified interactively on the 2D image display and a set of global dispersion coefficients are derived by comparing the identified lines with the line catalogue available in the system. This global model for the dispersion is a function of the wavelength and the spectral order number. Finally, dispersion coefficients for each order are computed using the global coefficients as a first approximation. A polynomial of degree 2 or 3 is sufficient to obtain, for each order, a good approximation of the wavelength scale.

  The command IDENTIFY/ECHELLE involves the echelle relation and requires the identification of two lines in overlapped regions of adjacent orders (method PAIR). The calibration can as well be performed for spectra which orders are not overlapped, this time requiring a minimum of four identifications (method ANGLE). Both methods are based on the echelle relation and therefore are not applicable if the disperser is not an echelle grating, as it is the case for EFOSC which involves a grism disperser. The method TWO-D allows to start directly the calibration with a two-dimensional fitting polynomial and requires more initial identifications. In case of several observations with the same, or near the same instrumental configuration, it is possible to use the global dispersion model from a previous calibration. The method GUESS implements this mode of operation. Two additional methods RESTART and ORDER are available. The selection of the method is performed by assigning a value to the echelle keyword WLCMTD.

  Solutions are computed either for each independent order ( WLCOPT=1D) or using a global bivariate polynomial ( WLCOPT=2D).