0 Assigned ID: 1081 ----- Attempting Submission 1 (Fri Apr 10 23:49:04 GMT 2015) ----- Mass substructure is a key test of the standard cosmology, and galaxy clusters are an ideal laboratory for measuring substructure with gravitational lensing. By including flexion, a higher-order image distortion, along with strong and weak lensing in our mass reconstructions, we can measure substructure with masses as small as 10^12 M_sun. To increase the legacy value of the rich set of HST cluster data images, we propose to add a flexion mass measurement and improved mass reconstruction for 26 clusters. With this sample and our substructure mass detection threshold, we will constrain the subhalo mass function to 10% and constrain a deviation from halo self-similarity predicted in N-body simulations. This is an important test of our understanding of structure formation in the Universe. In addition to the mass modeling, we propose an analysis of intrinsic flexion - scatter in the shape measurements due to intrinsic galaxy morphology, as well as PSF systematics and source selection biases, using the parallel fields of the cluster observations as our data set. This will be the most in-depth study of flexion systematics to date, particularly considering that we will be using real galaxy images instead of simulated data, and critical to validating the mass maps produced using flexion. We will release the mass maps and associated lensing maps to the community, increasing the value of the data. COSMOLOGY 0 Regular false false false false false SUPPORTED