subroutine CAL_VignettingFactor(theta, phi, Elo, Ehi, factor)

- theta: off-axis angle position in the FOCCOORD
reference system at which the effective area curve is requested.
- phi: azimut angle position in the FOCCOORD
reference system at which the effective area curve is requested.
- Elo, Ehi: energy band at which the effective area will be calculated.

Michael Smith 2011-09-20