NGC6356
 3 iterations.
Chebyshev coeffients a(0)T_0(u)+a(1)T_1(u)+..., where u is log (r/arcsec) mapped to [-1,1]
 a(0)=    19.466 +-  0.158
 a(1)=     2.829 +-  0.255
 a(2)=     0.366 +-  0.183
Power-of-x coeffients a(0)+a(1)x+a(2)x^2+..., where x=1+(r/r_c), r_c=1.22
 a(0)=     7.2698
 a(1)=     1.0833
 a(2)=     0.4629
log r(arcsec)   mu_V    mu_V(fitted)  residual   weight  data set