Making Robo-AO go faint: laser-only adaptive optics
Ground-based telescopes have to look through a lot of atmosphere when observing the night sky. To get high resolution images, complex adaptive optics systems can correct for atmospheric blurring by comparing the wavefront of incoming starlight from a nearby star (called a guide star) to the expected wavefront and adjusting. These fortuitous guide stars must be fairly bright, and are not always found near the desired target star. By reflecting a laser signal at very high altitudes, a laser guide star AO (LGS AO) system can do away with the need for a natural guide star (NGS) for all but one aspect of the atmospheric correction.
This remaining aspect is called tip-tilt error, and it still requires a NGS, albeit with less stringent requirements on its brightness, allowing a higher fraction of the sky to be covered by the AO system. Tip-tilt is the random apparent motion of the star on the sky due to atmospheric effects, and removing it without a sufficiently bright NGS severely limits the sky coverage of LGS AO systems.
Robo-AO, the first autonomous LGS AO system, observes hundreds of targets a night due to its low observation overheads, a magnitude increase in observing efficiency over classic LGS AO systems. I reduced 42,000 Robo-AO observations with no tip-tilt correction at all to test the typical improvement in effective seeing possible for laser-only AO. Turns out it’s a 39+/-19% improvement in FWHM! Furthermore, 50% encircled-energy performance without tip-tilt correction remains comparable to diffraction-limited, standard Robo-AO performance. Faint-target science programs primarily limited by 50% encircled-energy (e.g. those employing integral field spectrographs placed behind the AO system) may see significant benefits to sky coverage from employing laser-only AO.