Extending the MACHO search to about 10 exp 6 solar masses

The search for a microlensing (changing light-curve) signature of massive compact halo objects (Machos) by the Macho Collaboration is currently believed to be sensitive in the range 10^-7^-10²^ Mₛun_. Microlensing events from higher mass objects last longer than the 4 yr duration of the planned experiments and therefore, according to current beliefs, cannot be distinguished from long-term variables. In fact the signature of Machos in the range 10²^-10³^ Mₛun_ can be distinguished from background events by the annual modulation in light magnification induced by the Earth's motion. For Machos in the range 10³^-10⁶^ Mₛun_, Hubble Space Telescope (HST), or even ground-based measurements can resolve the split lensed images, thus confirming the lens interpretation of an event. If the HST's optics were repaired, it could resolve images for Machos >~ 300 Mₛun_. The lower mass limit can be reduced to 4 x 10^-9^ Mₛun_ by conducting 1 month of rapid repeat observations of a single field. The standard view is that a Macho light curve yields only one physically relevant parameter, the time scale of the event. The time scale is a combination of the four parameters one would like to know: the mass, the distance, and the two components of transverse velocity of the Macho. I show that for masses 4-100 Mₛun_, annual parallax oscillations in the light curve can be used to determine the transverse velocity. In the range 10^-3^-10⁶^ Mₛun_ such measurements can be made using a small special-purpose satellite telescope. For masses 10³^-10⁶^ Mₛun_, one may determine all four Macho parameters by combining a number of techniques.