Title: On-sky closed loop correction of atmospheric dispersion for high-contrast coronagraphy and astrometry,
Abstract: Adaptive optic (AO) systems delivering high levels of wavefront correction
are now common at observatories. One of the main limitations to image quality
after wavefront correction comes from atmospheric refraction. An Atmospheric
dispersion compensator (ADC) is employed to correct for atmospheric refraction.
The correction is applied based on a look-up table consisting of dispersion
values as a function of telescope elevation angle. The look-up table based
correction of atmospheric dispersion results in imperfect compensation leading
to the presence of residual dispersion in the point-spread function (PSF) and
is insufficient when sub-milliarcsecond precision is required. The presence of
residual dispersion can limit the achievable contrast while employing
high-performance coronagraphs or can compromise high-precision astrometric
measurements. In this paper, we present the first on-sky closed-loop correction
of atmospheric dispersion by directly using science path images. The concept
behind the measurement of dispersion utilizes the chromatic scaling of focal
plane speckles. An adaptive speckle grid generated with a deformable mirror
(DM) that has a sufficiently large number of actuators is used to accurately
measure the residual dispersion and subsequently correct it by driving the ADC.
We have demonstrated with the Subaru Coronagraphic Extreme AO (SCExAO) system
on-sky closed-loop correction of residual dispersion to < 1 mas across H-band.
This work will aid in the direct detection of habitable exoplanets with
upcoming extremely large telescopes (ELTs) and also provide a diagnostic tool
to test the performance of instruments which require sub-milliarcsecond
correction.