  We have carried out a major survey for visual binaries towards the Orion
Nebula Cluster using HST images obtained with an H-alpha filter. Among 781
likely ONC members more than 60" from theta-1 Ori C, we find 78 multiple
systems (75 binaries and 3 triples), of which 55 are new discoveries, in the
range from 0.1" to 1.5". About 9 binaries are likely line-of-sight
associations. We find a binary fraction of 8.8%+-1.1% within the limited
separation range from 67.5 to 675 AU. The field binary fraction in the same
range is a factor 1.5 higher. Within the range 150 AU to 675 AU we find that T
Tauri associations have a factor 2.2 more binaries than the ONC. The binary
separation distribution function of the ONC shows unusual structure, with a
sudden steep decrease in the number of binaries as the separation increases
beyond 0.5", corresponding to 225 AU. We have measured the ratio of binaries
wider than 0.5" to binaries closer than 0.5" as a function of distance from the
Trapezium, and find that this ratio is significantly depressed in the inner
region of the ONC. The deficit of wide binaries in the central part of the
cluster is likely due to dissolution or orbital change during their passage
through the potential well of the inner cluster region. Many of the companions
are likely to be brown dwarfs.
