15.37.20  OFFSET      

     OFFSET=n  If non zero the data is fitted as a function of (X-n).  For
non orthogonal functions such as plynomials, you should always specify the
offset  as  the  center  of  the  region  that you wish to fit.  This will
produce a better fit with lower errors on the coefficients.  The offset is
included  in  the  a1  coefficent for GAUSSIAN fits, so you do not need to
specify it.