Gravitational Lensing and Telescope Aberrations

Abstract

This is a work in three parts, each of which addresses challenges to weak gravitational lensing flexion measurements. Part one is a derivation and analysis of the aberration shapes and patterns imparted onto images by misaligned telescopes. As telescope aberrations will generally interfere with lensing measurements, it is important to be able to quantify and ultimately minimize them. The conclusions of this first part are (1) misaligned telescopes produce the same aberration shapes, but different field patterns, as aligned telescopes, (2) these misalignment patterns are generic for onaxis telescopes and can be modeled with relatively few parameters, and (3) with well-placed wavefront sensors, telescopes can be kept near enough to alignment so that any remaining aberrations are benign. The second part of this work explores the effect of any un-removed telescope aberrations on images of flexed galaxies. Telescope astigmatism, coma, and trefoil distort stellar images in much the same way that gravitational shear and flexion distort galaxy images. In this chapter, I derive simple models for lensed galaxies and for aberrated PSFs, and convolve the two models to determine the analytic form for the gravitationally lensed and optically distorted galaxy shapes that one might expect to see in actual telescope images. Given this representation of the galaxy image, one can analytically disentangle the gravitational signal from the other distortions. The final chapter is an observational study of weak gravitational lensing flexion in Abell 1689. Using the analytic models for images of lensed and aberrated galaxies that I derived in the preceding section, I attempt to measure galaxy halo truncation from galaxy-galaxy lensing flexion in Abell 1689. While I am able to successfully measure flexed and distorted galaxy shapes and extract the de-aberrated flexion signal, the measurement of galaxy-galaxy gravitational flexion is ultimately thwarted by inherent shape noise in Abell 1689's background source galaxy population. I characterize this shape noise, concluding that it is much larger than previously reported. I further analyze the other hindrances to the lensing flexion measurement and conclude with a recipe for perhaps succeeding in the future.