Multiplicative calibration factor; it is used to convert from the single
dish map units (e.g., main-beam brightness temperature) to janskys.
A default value of 0 can be used if the original data file is in unit of
Tmb, the main beam brightness temperature, because in this case the con-
version factor can be derived from the beam size. UV_SHORT will complain
if it cannot derive the appropriate value.