Detectors such as the IPCS often cause major geometric distortions in images created using them. ECHOMOP provides a mechanism for modelling such distortion, and using the model to provide corrections during the extraction process. It is also possible to generate a 'corrected' version of each order for visual examination, or processing by other software.
The distortion modelling uses a coordinate system based on X = calibrated wavelength at trace, Y = pixel offset from trace, and is thus performed independently for each order in turn.
The arc frame is used to locate the position of each identified arc line at a variety of offsets from the trace centre. The difference between the identified wavelength and that predicted by the wavelength polynomial is then calculated. These differences are fitted using Chebyshev polynomials.
Once a 2-D fit has been obtained, it is refined by either manual or automatic clipping of deviant points. When done manually the positions of all the points being fitted (i.e. arc line centres) may be plotted in a highly exaggerated form, in which systematic distortions of sub-pixel magnitude are readily apparent.
As the wavelength scale produced by the distortion fitting leads inevitably to some re-binning when the extraction takes place, it is normal to extract into a scrunched wavelength scale (e.g. constant bin size) and this is the default behavior of the 2-D extraction task/option.
Martin Clayton