Galaxy cluster Abell 370

This is a J band image (1.25 micron) of the galaxy cluster Abell 370 (A370; redshift z = 0.375), obtained in 'jitter' mode, showing the famous gravitational arc just below the centre. The observations consisted of 24 exposures of 2 min each, made on randomly generated telescope positions within a sky region of 30 x 30 arcsec. The individual exposures have been sky subtracted using a running average determined from the same data and then re-centred and combined. The scale is 0.29 arcsec/pixel; the field is about 5 x 5 arcmin with North at the top and East to the left. The seeing was less optimal, about 1.2 arcsec. A J = 22 mag point source yields a signal-to-noise ratio of 4 within a 3 arcsec diameter aperture.