NGC6681 (M70) c
 6 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)=    18.375 +-  0.130
 a(1)=     4.540 +-  0.245
 a(2)=     1.206 +-  0.171
 a(3)=     0.217 +-  0.162
Power-of-x coeffients a(0)+a(1)x+a(2)x^2+..., where x=r
 a(0)=     6.1319
 a(1)=     1.2670
 a(2)=     0.2914
 a(3)=     0.3125
log r(arcsec)   mu_V    mu_V(fitted)  residual   weight  data set