The lens parallax method: determining the redshifts of the faint blue   galaxies through gravitational lensing
  We propose a new technique, which we call the lens parallax method, to determine simultaneously the redshift distribution of the faint blue galaxies and the mass distribution of foreground clusters of galaxies. The method is based on gravitational lensing and uses the fact that the mean angular size of the faint blue galaxies is a well-determined function of surface brightness. The method requires first a calibration of the angular-size vs. surface brightness relation in unlensed blank fields and a determination of the mean redshift of the brightest of the faint blue galaxies. By combining this information with observations of the distorted images of background galaxies in the fields of foreground clusters, we show that it is possible to obtain the mean redshift of the galaxies as a function of their surface brightness. With a sample of about ten moderate redshift rich clusters and using ten bins in surface brightness, a bin-to-bin signal-to-noise ratio of $\sim3.5$ can be achieved. The method simultaneously allows a determination of the convergence and shear of the lensing cluster as a function of position, and through this the mass distribution of the lens can be obtained. The lens parallax method can be used in conjunction with, and will improve the accuracy of, other previously proposed techniques for mapping clusters. 
Ramesh Narayan and Matthias Bartelmann