The estimation of the background is one of the critical points in the reduction of echelle spectra for two reasons. On one side, a correct estimate of the background level is necessary to compute the true flux of the object spectrum; on the other side, a wrong estimate of the background in either the flat field image (FLAT), the object (OBJ) or (optionally) the standard star (STD), will severely affect the accuracy with which instrumental effects - such as the blaze - can be corrected for. The background in an echelle image consists of:
The first three components are removed during the preprocessing as described in Appendix D. Correction for the general scattered light and diffuse light background is presented in the three following sections. Correction for the sky background is presented in Section 7.4.4.
The remaining background due to scattered light is estimated from points located in the interorder space. These locations are defined when performing the order definition (command DEFINE/ECHELLE or DEFINE/HOUGH) and can be displayed by command LOAD/ECHELLE. The order definition is used to create a table back.tbl containing the background positions. Unscanned areas of the CCD can be avoided in the background estimate by using the command SCAN/ECHELLE. If bright features (e.g. sky lines) are located at the same location as background points, these locations must be unselected from table back.tbl using SELECT/BACKGROUND. Selection of the method is possible by assignment of a value to the echelle keyword BKGMTD and the background estimate, subtraction and update of the descriptor BACKGROUND is performed by the command SUBTRACT/BACKGROUND