Background Definition

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:

• a constant offset introduced by the electronics (bias),
• an optional constant pedestal due to the pre--flashing of the CCD,
• the dark current,
• general scattered light,
• diffuse light in the interorder space coming from adjacent orders.
• sky background spectrum

The first three components are removed during the preprocessing as described in Appendix . 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 .

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

• Bivariate polynomial interpolation
• Smoothing spline interpolation
• Background estimate by filtering
• Sky background definition