Measuring Flexion

Abstract

We describe practical approaches to measuring flexion in observed galaxies. In particular, we look at the issues involved in using the shapelets and HOLICs techniques as means of extracting second-order lensing information. We also develop an extension of HOLICs to estimate flexion in the presence of noise and with a nearly isotropic PSF. We test both approaches in simple simulated lenses, as well as a sample of possible background sources, from ACS observations of A1689. We find that because noise is weighted differently in the shapelets and HOLICs approaches, that the correlation between measurements of the same object is somewhat diminished, but the two approaches produce similar scatter due to measurement noise.