Lossy Compression of Weak-Lensing Data

Abstract

Future orbiting observatories will survey large areas of sky in order to constrain the physics of dark matter and dark energy using weak gravitational lensing and other methods. Lossy compression of the resultant data will improve the cost and feasibility of transmitting the images through the space communication network. We evaluate the consequences of the lossy compression algorithm of Bernstein et al. for the high-precision measurement of weak-lensing galaxy ellipticities. This square-root algorithm compresses each pixel independently, and the information discarded is, by construction, less than the Poisson error from photon shot noise. For simulated space-based images (without cosmic rays) digitized to the typical 16 bits pixel^-1, application of the lossy compression followed by imagewise lossless compression yields images with only 2.4 bits pixel^-1, a factor of 6.7 compression. We demonstrate that this compression introduces no bias in the sky background. The compression introduces a small amount of additional digitization noise to the images, and we demonstrate a corresponding small increase in ellipticity measurement noise. The ellipticity measurement method is biased by the addition of noise, so the additional digitization noise is expected to induce a multiplicative bias on the galaxies’ measured ellipticities. After correcting for this known noise-induced bias, we find a residual multiplicative ellipticity bias of m ≈ -4 × 10^-4. This bias is small when compared with the many other issues that precision weak-lensing surveys must confront; furthermore, we expect it to be reduced further with better calibration of ellipticity measurement methods.