NGC6121 (M4)
 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)=    21.375 +-  0.146
 a(1)=     3.969 +-  0.250
 a(2)=     0.715 +-  0.193
 a(3)=     0.243 +-  0.135
Power-of-x coeffients a(0)+a(1)x+a(2)x^2+..., where x=1+(r/r_c)^2, r_c=1.90
 a(0)=     7.1875
 a(1)=     2.1253
 a(2)=    -0.6176
 a(3)=     0.2605
log r(arcsec)   mu_V    mu_V(fitted)  residual   weight  data set