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We are searching for evidence that the dark matter in the halo of the Milky Way is made up of objects like brown dwarfs or planets: these objects have come to be known as MACHOs, for MAssive Compact Halo Objects. The signature of these objects is the occasional amplification of the light from extragalactic stars by the gravitational lens effect. The amplification can be large, but events are extremely rare: it is necessary to monitor photometrically several million stars for a period of years in order to obtain a useful detection rate. For this purpose we have built a two channel system that employs eight 2048*2048 CCDs, mounted on the 50 inch telescope at Mt. Stromlo, Australia. The high data rate (up to 8 GBytes per night) is accomodated by custom electronics and on-line data reduction.
We have taken ~26,000 images with this system, and reduction of part of these data have yielded databases containing light curves in two colors for more than twenty million stars. A search for microlensing has turned up almost fifty candidates (of which four are toward the Large Magellanic Cloud and the remainder are toward the Galactic bulge). The event rate toward the galactic bulge is higher than expected; the talk will discuss the meaning of this result, and will conclude with a discussion of future directions for this research.
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