TUNE_SKYPOLY > 0 specifies the degree of polynomial to use when trying to fit to the sky intensity at each X- (lambda-) increment.
These polynomials are used to predict the sky intensity in object pixels.
The default value of 0 gives sky averaging (i.e., the sky model is constant at each order/column).
Set TUNE_SKYPOLY=-1 when there is no sky to be modeled (i.e., sky model is zero everywhere).
The default value is: 0.
Martin Clayton