Microlensing surveys derive the microlensing optical depth towards various directions such as the Gal
actic Centre from the distribution of observed Einstein radius
crossing time. We show that the formula which is being used is invalid
for ``exotic'' lensing events. The corrected formula is derived.
The ``parallax'' effect (earth motion) requires no correction.
Corrections for the finite size of the source and wide binary lenses
are small (typically less than 1 \%), except for blended events.
Corrections for intermediate type binaries such as MACHO-LMC-9 can
be substantial. |
