micron  u1        limb darkening from Allen (1973) p171
.30     1.04
.32     0.93
.35     0.84
.37     0.79
.38     0.84
.40     0.83
.45     0.73
.50     0.65
.55     0.59
.60     0.55
.80     0.41
1.0     0.34