arclocat in fwhm thresh
A R C L O C A T The input data must be a base NDF. They can be a single spectrum or a set of spectra. Examples for the latter are a long slit spectrum, a set of extracted fibre spectra, or a collapsed echellogram (a set of extracted orders from an echelle spectrograph). It is necessary that the spectroscopic axis be the first axis in the data set. It does not matter how many further axes there are, the data will be treated as a set of rows with each row a spectrum. The coverage in spectroscopic values of all spectra (rows) should either be similar (long slit or fibres) or roughly adjacent (echellogram). There must not yet be any spectroscopic value information: There must be no array of spectroscopic values or widths in the Specdre Extension. The pixel centre array for the spectroscopic axis (i.e. the first axis) must be NDF pixel coordinates (usually 0.5, 1.5, ...). The data must be arranged such that spectroscopic values increase left to right. In the case of rows with adjacent coverage spectroscopic values must also increase with row number. In a collapsed echellogram this usually means that for wavelength calibration the order number must decrease with increasing row number. In automatic mode this routine works on each row (spectrum) in turn. It scans through the spectrum and looks for pixels that exceed the local background level by at least the given threshold value. When such a pixel is spotted, a single-component line fit is tried no the local profile. The local profile is centred on the pixel suspected to be part of an emission feature. It includes 1.5 times the guessed FWHM on either side and a further 5 baseline pixels on either side. A local linear baseline is subtracted prior to the line fit. In order for the feature to be entered into the list of located features, the fit must succeed, the fitted peak must exceed the threshold, and the fitted peak must exceed the absolute difference of background levels between the left and right. When run with graphics dialogue this routine works on any choice of rows. It uses a pre-existing list of located features to which can be added or from which features can be deleted. Graphics dialogue can also be used to just check the locations. The graph displays the spectrum currently worked on in bin-style. The current list of located features is indicated by dashed vertical lines. The options in the graphical dialogue are: R - Choose different row to work on X - X-zoom 2x on cursor Y - Y-zoom 2x on cursor W - Unzoom to show whole row N - Pan left/right by 75% of current x range A - Add the feature under cursor to list (subject to line fit) S - Add the cursor position as feature to list D - Delete the feature nearest cursor from list Q - Quit, preserving the updated list of located features ? - Help The difference between the A and S options is that A tries a line fit to the local profile around the cursor, while S accepts the cursor x position as exact centre and the cursor y position as exact peak of a new feature; (the variance of the centre is set to 0.25, the variance of the peak to the bad value). The result of this routine is a list of Gauss or triangle features. Their locations in NDF pixel coordinates and their peak values are stored in the results structure of the Specdre Extension of the input data. If run in automatic mode, this routine will replace any previously existing results structure. If run with graphics dialogue, this routine will try to work with a pre-existing list of located features. But if the pre-existing results structure does not conform to the required format, then a new results structure is created. The list of located features (for each row) is always sorted such that the locations are strictly monotonically increasing. The results structure provides for a certain number of components. These have component type 'Gauss feature' or 'triangle feature'. Each component has two parameters 'centre' and 'peak'. The number of components is determined when the results structure is created, it is derived from the approximate width of features and the number of pixels in each spectrum.
arclocat in 4. 20. mode=triangle dialog=f This will scan through (all rows of) the NDF called "in". It looks out for features of 4 pixels full width at half maximum and with a peak value of at least 20 above the local background. The features are fitted as triangles. The search is automatic. Thus a new results structure in the input NDF's Specdre Extension is created with the locations (centres) and peaks of located features. arclocat in 4. mode=Gauss dialog=g rownum=5 This will use the graphic dialogue. Starting with the fifth row the user can use the mouse cursor to choose features that are to be deleted from or added to the list of located features. This can be used to improve on an automatic run, or when no features have been located so far. If you try to add a feature to the list, a Gauss fit is tried in the vicinity of the cursor-selected position.
This routine works in situ and modifies the input file.
FIGARO A general data reduction system