Line Detection in the Comparison Spectrum

The first step consists of detecting the emission lines of the comparison spectrum to produce the line table line.tbl. It can be noted that the command SAVE/LONG will store all keyword values as descriptors of this table, so that a session can be saved only if the table line.tbl is available. The commands:

Midas...> SET/LONG WLC= ccd0003 THRES=20. WIDTH=8 YWIDTH=3 YSTEP=1
Midas...> SEARCH/LONG

initialize the line searching parameters and SEARCH/LONG performs the following operations:

• Average 2*+1 rows of image in the Y direction at a step
• for each resulting row, detect emission lines above a local background computed by median estimate in a sliding window of size and with a peak intensity counts above the local background;
• determine line centers by fitting Gaussians to the line profiles.

The parameter can be used to improve the signal-to-noise ratio of each row by averaging adjacent rows. The parameter is by default equal to 1 which corresponds to the standard row-by-row method but can be set to a larger value to accelerate the wavelength calibration. The detections are stored in the following columns of the table line.tbl:

Results can be checked with the command PLOT/SEARCH. For one-dimensional spectra,

Midas...> PLOT/SEARCH

plots the central row of the spectrum as well as the position of the detected lines. This behavior is obtained for 2D spectra by the additional option 1D, as in:

Midas...> PLOT/SEARCH 1D

whereas typing the command without option provides a bi-dimensional graph of the X- and Y-positions of the detected lines.