To directly image and characterize exoplanets, many developments of high-contrast imaging (HCI) systems are ongoing for current ground-based telescopes as well as future extremely large telescopes and space-based telescopes. Despite recent developments in HCI, the contrast of the HCI systems is limited by non-common path aberrations (NCPAs) and residual errors of the adaptive optics (AO) system. In order to overcome these limitations, HCI systems need focal plane wavefront sensing and control (FPWFS&C) techniques.
We present the implementation of two FPWFS&C techniques, electric field conjugation (EFC) and spatial linear dark field control (LDFC), on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument. First, we generate a half-dark hole in the focal plane image using EFC. Once the bright field and dark field (dark hole) have been established by EFC, as a second step, we deploy spatial LDFC to maintain the contrast of the half-dark hole generated by EFC. We could also use EFC to preserve the contrast of the dark hole, but it requires field modulation, which interferes with the science image acquisition. Because of this reason, we use spatial LDFC as an alternative way to maintain the contrast without modulation.
In actual demonstrations, we obtained a dark hole contrast of ∼2×10−7 with a classical Lyot coronagraph of 114 mas diameter, at a 1550 nm wavelength using EFC. This result is the first EFC implementation and the deepest contrast obtained on the SCExAO testbed. Using spatial LDFC, we also ideally removed focal plane speckles generated by static phase error and restored the initial contrast. Our results provide a promising path forward to generating the high-contrast dark hole using EFC and stabilizing the contrast of the dark hole without interrupting the science acquisition using spatial LDFC.
