Specifies the mixing fraction for two wavelength ranges (fitted).
This is only required when there are arc exposures bracketing the science data and there is a significant difference between the two.
The fraction is used as follows:
Lambda = Lambda1 + (Lambda2 - Lambda1) * Fraction
No facility is provided for calculating the appropriate fraction, but it will usually be estimated as the the ratio of elapsed times between the object frame, and the before/after arc frames (assumes a constant 'drift' of the wavelength scales.
Martin Clayton