The type of fit to be used for sky-background modelling.
The available functions are MEAN (the default), POLY and SPLINE.
Note that the required maximum number of coefficients for POLY and SPLINE are different. For POLY, the maximum number of coefficients corresponds to the number of degrees plus one, so for a cubic use coeffs=4. For a SPLINE the maximum number of coefficients corresponds to (knots+7)*2, so for 3 knots use coeffs=20. If you are using SPLINE fits then you should ensure that the value of TUNE_MAXPOLY is set accordingly, as well as using an appropriate value for TUNE_SKYPOLY and/or TUNE_SKYXPLY.
INITIAL SUGGESTED VALUE: MEAN.
Martin Clayton