  The biggest uncertainty in determining microlensing parameters comes from the
blending of source star images because the current experiments are being
carried out toward very dense star fields: the Galactic bulge and Magellanic
Clouds. The experiments try to correct the blending effects for individual
events by introducing an additional lensing parameter, the residual flux, but
this method suffers from very large uncertainties in the derived lensing
parameters due to the degeneracies among the parameters. In this paper, I
propose to use the Hubble Space Telescope (HST) to correct blending effects.
With the high resolving power of the HST combined with the color information
from ground-based observations, one can uniquely identify the lensed source
star in the blended seeing disk, and thus the uncertainty in the derived time
scale can be significantly reduced.
