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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 2nd 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 shapelets and HOLICs approaches, that the correlation between measurements of the same object is somewhat diminished, but produce similar scatter due to measurement noise.'
author:
- 'David M. Goldberg and Adrienne Leonard'
title: Measuring Flexion
---

Introduction
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Motivation
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Flexion has recently been introduced as a means of measuring small scale variations in weak gravitational lens fields (Goldberg & Bacon, 2005; Bacon, Goldberg, Rowe & Taylor, 2006, hereafter BGRT). Rather than simply measuring the ellipticities of arclets, this technique aims to measure the “arciness” and “skewness” (collectively referred to as “flexion”) of a lensed image. Flexion is a complementary approach to shear analysis in that it uses the odd moments (3rd multipole 