Precision photometric calibration with alternating incoherent satellite speckles

Exoplanet characterization relies on extracting precision astrometry (orbits) and photometry (spectra, time variation) measurements from high contrast images. Obtaining such measurements is challenging due to residual starlight speckles and the fact that the central stellar PSF core is blocked by the coronagraph. In post coronagraphic images it becomes quite difficult to measure the relative position and intensity of companion with respect to the stellar as it is blocked by the coronagraph. This issue has been addressed by imposing a periodic phase grid on the Deformable Mirror of SCExAO system to generate satellite speckles which are incoherent with underlying speckle halo for calibration. We report on-sky measurements of the photometric stability of such incoherent speckles over a 15 minute period time. We demonstrate that for relatively fainter speckles, unknown background noise (speckles) creates a bias in the photometric measurement. We show that, by subtracting two frames with two different speckle patterns, we address this issue and obtain unbiased measurements. This technique will be beneficial for future large telescopes where they have the potential to obtain diffraction limited images.