Space Telescope Science
If you need to check whether the prescription for your eye glasses or contact lenses is still accurate, you visit an ophthalmologist for an eye exam. The doctor will ask you to read an eye chart, which tests your visual acuity. Your score helps the doctor determine whether to change your prescription.
Astronomers don't have a giant eye chart to check the prescription for natural cosmic lenses, created by galaxy clusters. The gravity of these cosmic lenses warps space around them, magnifying and brightening the light from distant objects behind them. Without these lenses, background objects would be too dim to be detected by even NASA's Hubble Space Telescope. But how do astronomers know whether the prescription for these zoom lenses, which tells them how much an object will be magnified, is accurate? Astronomers using the Hubble telescope have discovered the next best thing to a giant cosmic eye chart: the light from distant exploding stars behind galaxy clusters.