Gui Clipping Routine

The clipping algorithms in this module are identical to the ones used in the Average module; the obvious difference is that the averaging step is not performed here. Below we review the assumptions and algorithms.

What this routine does:

This routine is used to clip away outliers from a distribution of data points. The data are grouped in wavelength bins whose amplitude can be specified by the user; in each bin the outliers are identified and discarded according to a set of different algorithms described below.

Binning the Data:

The binsize is defined either by the routine or by the user. It may be a variable width, changing with wavelength as defined by a resolution, or a fixed width in lambda. The routine tries to preset the binsize to a good choice based on the data.

SWS: The ``Auto'' binsize (a resolution value) represents 5 times the theoretical point source resolution (or one fifth of the resolution element) of the AOT used. The actual resolution can be different (e.g. for extended sources), and it is recommended that the user try out different bin sizes, using the default value as a starting point.

LWS: The ``Auto" binsize (a fixed width value) represents a width smaller than the minimum wavelength sampling possible for the  AOT used, but larger than the jitter in wavelength found in any scan. It is chosen so as to preserve the sample spacing of individual scans (see below).

CAM-CVF / PHT-S: The ``Auto'' binsize (a fixed width value) is computed as the median of the separation in wavelength between adjacent data points.

The bins are used to group the data for clipping, but the bin placement on the x-axis is not necessarily uniform.  Rather, the first (shortest wavelength) data point defines the start of the first bin. All data within a binwidth of that data point are included in the first bin. The next bin is defined to start at the wavelength of the next nearest data point in increasing wavelength, and etc.. The clipped points for each bin are plotted in red in the left panel of the Clip widget.

 2 Clipping Techniques available under ISAP:

Common Attributes of the two techniques :