Research projects

My past academic research focused primarily on weak gravitational lensing and image analysis in noisy astrophysical data, for experiments such as the Dark Energy Survey and NASA's Nancy Grace Roman Space Telescope.

More recently I have published far less frequently, but I have been working on an idea that has interested me for over a decade.  Overfitting creates striking patterns in residuals that a Mean Squared Error loss function is not sensitive to.  I think we can generalize the iid multivariate Gaussian loss or likelihood function to be sensitive to the apparent entropy of distributions of residuals when fitting data, which would provide directly motivated regularization, at modest computational cost.  I hope to find the spare time to write as much on this topic as I can!

Astrophysics projects


With Rachel Mandelbaum (Carnegie Mellon) I helped lead the GREAT3 Challenge project (the third GRavitational lEnsing Accuracy Testing Challenge). For information about the Challenge, please see the GREAT3 results paper.


To create the GREAT3 Challenge data we established an open-source software project called GalSim: the modular galaxy image simulation toolkit. This was an ambitious project to provide the community with a free, well-vetted toolkit for simulating realistic galaxy images, based on physical models of telescope optics, the atmosphere, and using real galaxy images from the Hubble Space Telescope.  You can read about GalSim in the paper here or here.  I am very proud that GalSim is now one of the core image simulation tools adopted by the incredible Large Synoptic Survey Telescope project, ESA's Euclid telescope mission, and many other weak lensing experiments.

Along with close collaborators like Rachel, and Mike Jarvis (UPenn), GalSim has been blessed by a wonderful team of GalSim developers.

The Dark Energy Survey

At University College London I was part of the Capitalizing on Gravitational Shear (COGS) project (PI: Sarah Bridle), and together with Sarah Bridle, Joe Zuntz, Lisa Voigt, Michael Hirsch & Tomasz Kacprzak we worked towards the accurate measurement of cosmological lensing information in the Dark Energy Survey (DES). This huge telescope project is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision.


A big part of the work for DES is the open-source im3shape galaxy shape estimation software, described here. This is a maximum-likelihood galaxy shape inference tool, designed from the outset to be as modular and easily extensible as possible. The project was led by Joe Zuntz, and results were very encouraging in terms of both robustness and overall bias.

Optimal Linear Image Combination: IMCOM

In a very enjoyable project funded by the NASA Nancy Grace Roman Space Telescope mission, I worked on the implementation of an image combination algorithm devised by Christopher Hirata which we called IMCOM. The paper is available here.

The IMCOM algorithm allows for careful treatment of aliasing in undersampled imaging data, and can be used to test the feasibility of multi-exposure observing strategies for space-based survey missions. IMCOM has been used to explore focal plane undersampling for the optical space mission such Euclid, and for the Nancy Grace Roman Telescope has been used in laboratory analysis work for infrared focal plane arrays, and alongside GalSim for infrared weak lensing simulations.

The IMCOM software is now freely available on GitHub.


A modern analysis of the current largest area of deep, optical, Canada-France-Hawaii Telescope data using gravitational lensing. A forerunner of DES, the CFHTLenS project has constructed the largest map of dark matter yet made, and placed new constraints on the validity of Einstein's General Relativity on cosmological scales.

Public software

Some of the freely-available software I have been involved with helping build in support of the projects described above:

  • IMCOM (IMage COMbination) - code for exploring (and combining) undersampled image data (Fortran 95)
  • GalSim: The modular galaxy image simulation toolkit - weak gravitational lensing-focused software toolkit for generating realistic galaxy and telescope PSF images (Python wrapping C++)
  • great3-public - initially helper and utility scripts for shape measurement method authors preparing submissions to the GREAT3 Challenge, now the entire code used to build GREAT3 (Python)
  • im3shape - weak lensing shape measurement code, led by Joe Zuntz at the University of Manchester as part of COGS.
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