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## Detecting and Identifying Arc Lines

For optimum results, it is important to use a comparison spectrum with as high a signal-to-noise ratio of the lines as possible. It is therefore advisible to flat-field  also the arc spectrum in order to correct for small-scale fluctuations. Another good idea is to filter the frame along the slit using the command FILTER/MEDIAN with a rectangular filter window of one pixel in the dispersion direction and several pixels in the perpendicular direction. For the row-by-row method one could also consider smoothing the spectra along the slit axis which would provide for a stronger coupling between neighbouring rows and thereby yield a solution which is intermediate between the extremes presented by the two pure methods.

• SEARCH/LONG: Finds the positions of reference lines in world coordinates. Positions are by default estimated by the center of fitted Gaussians. Other centering methods are available (Gravity, Maximum) but could result in systematic position errors (See Hensberge & Verschueren, Messenger, 58, 51). The results are stored in a table called line.tbl. The parameter YWIND corresponds the half-size of the row averaging window applied to adjacent rows of the spectrum for an improvement of the signal to noise ratio. The parameter YSTEP controls the step in rows between successive arc line detections. The value YSTEP=1 corresponds to the default row-by-row method and larger values can be used to get a quicker calibration. The algorithm detects lines whose strength exceeds a certain threshold (parameter THRES) above the local background. The local background results from a median estimate performed on a sliding window which size is controlled by the parameter WIDTH. The command PLOT/SEARCH allows to check the results at this stage. Note that for a two-dimensional spectrum, both options 1D and 2D can be used (See HELP PLOT/SEARCH).

• The command IDENTIFY/LONG allows an initial interactive identification, by wavelength, of some of the detected lines. Spectral line atlas are provided in the instrument operating manuals. The command PLOT/IDENT visualizes the interactive identifications.

Next: Getting the Dispersion Solution Up: Geometric Correction Previous: Geometric Correction
Petra Nass
1999-06-15