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We present a strong lensing analysis of the galaxy cluster Abell 370  ( z = 0.375)  based on the recent multicolour images by Advanced Camera for Surveys obtained as part of the Early Release Observation (ERO) that followed the Hubble Service Mission #4. Back in 1987, the giant gravitational arc  ( z = 0.725)  in Abell 370 was one of the first pieces of evidence that massive clusters are dense enough to act as strong gravitational lenses. The new observations reveal in detail its disclike morphology, and we show that it can be interpreted as a complex five‐image configuration, with a total magnification factor of  32 ± 4 . Moreover, the high‐resolution multicolour information allowed us to identify 10 multiply imaged background galaxies. We derive a mean Einstein radius of  θ E = 39 ± 2  arcsec for a source redshift at   z = 2 , corresponding to a mass of   M (&lt;θ E ) = 2.82 ± 0.15 × 10 14  M ⊙   and   M (&lt;250 kpc) = 3.8 ± 0.2 × 10 14  M ⊙  , in good agreement with Subaru weak‐lensing measurements. The typical mass model error is smaller than 5 per cent, a factor of 3 of improvement compared to the previous lensing analysis. Abell 370 mass distribution is confirmed to be bimodal with very small offset between the dark matter, the X‐ray gas and the stellar mass. Combining this information with the velocity distribution reveals that Abell 370 is likely the merging of two equally massive clusters along the line of sight, explaining the very high‐mass density necessary to efficiently produce strong lensing. These new observations stress the importance of multicolour imaging for the identification of multiple images which is key to determining an accurate mass model. The very large Einstein radius makes Abell 370 one of the best clusters to search for high‐redshift galaxies through strong magnification in the central region.

Abell 370 revisited: refurbished Hubble imaging of the first strong lensing cluster