We present deep optical observations of the gravitational lens system CLASS B0218+357, from which we derive an estimate of the Hubble constant. Extensive radio observations have reduced the degeneracies between Hubbles constant and the mass model in this lens to one involving only the position of the radio-quiet lensing galaxy relative to the lensed images. B0218+357 has an image separation of only 334 mas, so optical observations have previously been unable to resolve the lens galaxy from the bright lensed images. Using the new Advanced Camera for Surveys installed on the Hubble Space Telescope, we have measured the separation between the lens galaxy centre and the brightest image. The position found, and hence our estimate of Hubbles constant, depends on our approach to the spiral arms in B0218+357. If the most prominent arms are left unmasked, we find a value for Hubbles constant of 70+/-5 km/s /Mpc (95% confidence). If the spiral arms are masked out, we find a value of 61+/-7 km/s /Mpc (95% confidence).
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