A novel technique to measure residual systematic segment piston errors of large aperture optical telescopes

An observational and modeling technique to look for possible systematic bias in the phasing of the segments of a large aperture optical telescope is presented. The technique uses on-sky images from adaptive optics (AO) science instruments in conjunction with detailed AO modeling to make predictions on possible periodic residual segment piston errors. The technique is demonstrated on the sky using the Keck II telescope to cancel two bright speckles seen in the NIRC2 science images by applying appropriate piston corrections to the telescope segments. Follow-up observations are planned for this summer to validate another model prediction, using this technique on low order aberrations to improve the image quality of AO science observations. The significance of the technique for AO observations with the existing and future extremely large segmented telescopes is discussed.