The recent measurements of weak lensing by large-scale structure present significant new opportunities for studies of the matter distribution in the universe .
Here , we present a new cosmic shear survey carried out with the Echelle Spectrograph and Imager on the Keck II telescope .
This covers a total of 0.6 square degrees in 173 fields probing independent lines of sight , hence minimising the impact of sample variance .
We also extend our measurements of cosmic shear with the William Herschel Telescope ( Bacon , Refregier & Ellis 2000 ) to a survey area of 1 square degree .
The joint measurements with two independent telescopes allow us to assess the impact of instrument-specific systematics , one of the major difficulties in cosmic shear measurements .
For both surveys , we carefully account for effects such as smearing by the point spread function and shearing due to telescope optics .
We find negligible residuals in both cases and recover mutually consistent cosmic shear signals , significant at the 5.1 \sigma level .
We present a simple method to compute the statistical error in the shear correlation function , including non-gaussian sample variance and the covariance between different angular bins .
We measure shear correlation functions for all fields and use these to ascertain the amplitude of the matter power spectrum , finding \sigma _ { 8 } \left ( \frac { \Omega _ { m } } { 0.3 } \right ) ^ { 0.68 } = 0.97 \pm 0.13 with 0.14 < \Omega _ { m } < 0.65 in a \Lambda CDM model with \Gamma = 0.21 .
These 68 \% CL uncertainties include sample variance , statistical noise , redshift uncertainty , and the error in the shear measurement method .
The results from our two independent surveys are both consistent with measurements of cosmic shear from other groups .
We discuss how our results compare with current normalisation from cluster abundance .