Time delay lens modelling challenge

In recent years, breakthroughs in methods and data have enabled gravitational time delays to emerge as a very powerful tool measure the Hubble constant $H_0$. However, published state-of-the-art analyses require of order 1 year expert investigator up million hours computing per system. Furthermore, precision improves, it is crucial identify mitigate systematic uncertainties. With this delay lens modelling challenge we aim assess level accuracy techniques that are currently fast enough handle 50 lenses, via blind analysis simulated datasets. The results Rung 2 show use only point source positions tend lower ($10 - 20\%$) while remaining accurate. 2, exploit full information imaging kinematic datasets can recover $H_0$ within target ($ |A| < 2\%$) ($< 6\%$ system), even presence poorly known spread function complex morphology. A post-unblinding 3 showed numerical ray-traced cosmological simulations be insufficient test methodology at percent level, making difficult interpret. new with improved needed make further progress investigation For completeness, present an appendix, them discuss various approaches mitigating against similar subtle generation effects future challenges.