Specifies the fraction of the X-dimension of the frame to be used for determining the profiles.
This is set low (0.2 = 20%) by default for speed.
Very poor data may benefit from an increase in this parameter, which provides a more accurate sampling of the object profile, and thus more accurate extraction weights. The central fraction from each order is sampled and the average profile over all orders is calculated.
A special mode is selected by setting TUNE_USE_NXF=1, which makes the sampling independent for each order.
The default value is 0.2. (i.e. 20%).
Martin Clayton