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A special mode of operation is triggered by setting the parameter TUNE_FFNXPLY. If the parameter is set to zero then no modelling in the X-direction will be performed. If the parameter is set to 1, then no polynomials are used (would be constant), but the balance factors are calculated using the local mean value based on a 5 pixel sample. This will normally be used when the flat field at the dekker limits cannot be modelled because its intensity changes too rapidly on a scale of 1 pixel due to undersampling of the profile.

In the situation where an independently produced balance factor fram is used, this may be fed to ECHOMOP as the flat-field frame. The parameter TUNE_PREBAL should then be set to YES. In this case the modelling will not take place and the balance factors will simply be copied from the supplied frame. This should be used for special cases where the polynomial model cannot generate appropriate values.