SCExAO: lessons learned for a future TMT instrument

The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument has evolved tremendously for the past few years. Its modular design is an asset to test new technologies and functionalities. Although ExAO capabilities were only achieved recently, providing high-contrast images of a new disk and several known targets, lessons have already been learned for future ELT instruments. Some critical issues were identified during the development, forcing us to rethink the priorities of the wavefront correction. Telescope vibrations were carefully characterized, using accelerometers and wavefront sensors (WFS) telemetry. This analysis shaped a new vibration mitigation scheme, using the accelerometer data and more advanced predictive control. The low-wind effect (LWE) that splits the PSF was also measured. Although the LWE is not naturally present on the telescope, it can be artificially created by pupil misalignments in the Pyramid WFS (PyWFS). The sensitivity of the PyWFS to the LWE modes was measured and showed that a trade-off has to be made between sensitivity and accuracy. We are currently developing and testing focal plane WFS approaches to address this effect. Performing the ExAO correction independently behind a pre-existing classical AO system has the great advantage of simplifying the design and development of the instrument, but it has some practical limits. Communication is necessary between the various control loops. We are now converging on a design with better communication, better hardware, and multiple WFS in visible and infrared, to be more adaptable to the targets we observe. These lessons will be invaluable for designing an optimized high-contrast imager for TMT and other ELTs.