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Errors

Errors arising in SGRID fall into three categories,
  1. Errors generated by spectral models due to inappropriate parameter values.
  2. Errors generated during minimisation when there are free parameters remaining after the grid axes are defined, eg. matrix of second derivatives of chi-squared or Cash statistic (with respect to the parameters) may become singular or ill-conditioned.
  3. Failure to optimise at a grid point due to insufficient iterations.
In the first two cases SGRID will generate a bad quality (MISSING) grid point for the combination of grid parameters giving the error. If this is a problem then that area of parameter space giving the the errors should not be gridded.

The third case is handled differently. On the assumption that a good fit would have been achieved had sufficient iterations been allowed, SGRID uses the value of the statistic but flags it by setting the IGNORE quality bit. The quality mask is set so that these points will be processed by other ASTERIX software. If it is required that these points be treated as bad like the first two cases above, then this can be achieved by using the MASK application to set the quality mask to 11111111.


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