Improving Coronagraphic Performance with Active Atmospheric Dispersion Control on MagAO-X

Refractive index changes in the Earth’s atmosphere cause differential atmospheric dispersion, an unwanted wavelength dependency in the observations of ground-based astronomical telescopes. These dispersion effects cause smearing of the telescope’s point-spread function (PSF), resulting in difficulty when performing high-contrast imaging at small inner-working angles due to reduced coronagraph light-blocking efficiency. An Atmospheric Dispersion Corrector (ADC) is used to compensate for these effects. Current approaches use analytical models to anticipate dispersion effects at different telescope zenith angles; however, uncertainties in these models lead to over- or under-correction of the true atmospheric dispersion by the ADC. As an alternative, we present a closed-loop approach to this problem using the Magellan Adaptive Optics eXtreme (MagAO-X) instrument to measure and correct for dispersion effects as they appear. Residual atmospheric dispersion can be measured from artificial speckles on our science image, which are created by sinusoidal phase patterns on the deformable mirror. We use a forward model phase retrieval approach to determine the proper correction parameters to send to the ADC. Increasing the precision of our dispersion compensation is especially important as we prepare for the next generation of ground-based observations with the Giant Magellan Telescope.