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The MACHO project is in its third year of a time resolved photometric survey of the LMC, SMC and galactic bulge to search for gravitational microlensing by intervening compact objects. We have analysed most of our first year's data; this analysis yielded 3 events that appear to be microlensing toward the LMC and nearly 50 events toward the galactic bulge. To infer $\rho_{macho}$ from these events we must take into account our inefficiency at detecting events due to temporal sampling and photometric confusion. Because we observe in very crowded fields many of the objects which we monitor consist of multiple unresolved stars. Microlensing in these confused objects is more difficult to detect than on single stars and the microlensing fit parameters can deviate substantially from a single star event.
We are addressing these problems by adding artificial stars into many CCD images to determine our response to flux changes in single point sources. Using this information, we can create realistic artificial photometry with which to determine our efficiencies and systematic errors in recovered event parameters. We calculate an upper limit to our efficiency by using the single star approximation for all objects that we monitor.
