  The Event Horizon Telescope (EHT) images of the supermassive black hole at
the center of the galaxy M 87 provided the first image of the accretion
environment on horizon scales. General relativity predicts that the image of
the shadow should be nearly circular, given the inclination angle of the black
hole M 87*. A robust detection of ellipticity in the image reconstructions of M
87* could signal new gravitational physics on horizon scales. Here we analyze
whether the imaging parameters used in EHT analyses are sensitive to ring
ellipticity and measure the constraints on the ellipticity of M 87*. We find
that the top set is unable to recover ellipticity. Even for simple geometric
models, the true ellipticity is biased low, preferring circular rings.
Therefore, to place a constraint on the ellipticity of M 87*, we measure the
ellipticity of 550 simulated data sets of GRMHD simulations. We find that
images with intrinsic axis ratios of 2:1 are consistent with the ellipticity
seen from the EHT image reconstructions.
