Specifies the rejection threshold for the flat field.
Any pixels which deviate by more than TUNE_FFTHRESH sigma from the fitted polynomial model will be clipped and the model re-fitted.
The polynomial model of the flat field is made by fitting a polynomial at each X- and Y-increment. These polynomials are then used to calculate the balance factors at 'Object' pixel positions.
The default value is 10.0 sigma.
Martin Clayton