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ECH_FIT2D

Function

Model 2-D distortion using an arc frame.

Summary

This routine fits a 2-D polynomial surface to the distortion present in an echelle order. The order trace is used as an origin and axis, the X-distance running along it, and Y-offset below/above, are the coordinates of the fitted points. The fitted quantity is the delta-wavelength of an identified arc line. The identified line centres are located at all offsets above and below the trace. These centres are all assigned the identified wavelength for the line, and then their offset (in wavelength) relative to the same lines position on the origin axis calculated. This provides the following dataset for input in the surface fit:

  X - coord of a line centre
  Y - Offset from trace of the line centre
  Difference in wavelength from 1-D fit prediction for coord X.

Thus if no 2-D correction were necessary, all the differences would be zero. Formulating the fit in this way allows maximum significance of the quantity of interest.

Once a fit has been obtained, deviations from the fit of all sample points (line-centres) are calculated. Theses are then either interactively or automatically clipped until an acceptable fit is obtained. A typical interactive plot appears below.

In addition it is possible to request a plot showing an exaggerated view of the modelled arc lines. This make is easy to see if there is any systematic 2-D distortion present. A typical interactive plot appears below.

Available Topics

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Martin Clayton
mjc@star.ucl.ac.uk
Mon Dec 9 17:09:35 1996