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The Abell catalogue is an all sky catalogue of galaxy clusters. It contains 4,073 rich clusters with a nominal redshift of z less than or equal to 0.2. Each galaxy cluster must meet certain criteria of richness, compactness, distance and galactic latitude to be included.

For example, to meet the richness criteria a cluster must contain at least 30 galaxies within a set magnitude range. There are six groups defined:
Group 0: 30-49 galaxies
Group 1: 50-79 galaxies
Group 2: 80-129 galaxies
Group 3: 130-199 galaxies
Group 4: 200-299 galaxies
Group 5: more than 299 galaxies

The original Northern Survey was published in 1958 by George O. Abell and contained 2,712 clusters of rich galaxies. A later Southern Survey added a further 1,361 clusters. The catalogue is of interest to amateur astronomers with large aperture scopes since it contains challenging galaxies that require dark locations.

Hubble Frontier Fields view of Abell 2744 (NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI))

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