NGST's view of lensed QSOs

The projected Next Generation Space Telescope (NGST) is likely to detect a large number of high-redshift QSOs lensed by spiral galaxies. We calculate the expected number density of detectable QSOs which are multiply imaged by spirals and we investigate the influence of various evolution effects on that number. We find that NGST is likely to find of order ten lensed QSOs per square degree at 26th magnitude in the V and L bands, and that various observable quantities such as the total number density of lensed QSOs in these two bands, the ratio between the number densities of lensed QSOs in the V and L bands, the fraction of QSOs with more than two images and so forth can be used to constrain the evolution of the QSOs, the spirals, the dust in spirals, and the masses of spiral disks.