  We present a deep survey of Halpha emission-line stars in the M42 region
using wide-field objective prism films. A total of 1699 Halpha emission-line
stars were identified, of which 1025 were previously unknown, within an area of
5.5 x 5.5 degrees centred on the Trapezium Cluster. We present Halpha strength
estimates, positions, and JHKs photometry extracted from 2MASS, and comparisons
to previous surveys. The spatial distribution of the bulk of the stars follows
the molecular cloud as seen in CO and these stars are likely to belong to the
very young population of stars associated with the Orion Nebula Cluster.
Additionally, there is a scattered population of Halpha emission-line stars
distributed all over the region surveyed, which may consist partly of
foreground stars associated with the young NGC 1980 cluster, as well as some
foreground and background dMe or Be stars. The present catalogue adds a large
number of candidate low-mass young stars belonging to the Orion population,
selected independently of their infrared excess or X-ray emission.
