Sharpening of natural guide stars for low-order wavefront sensing using patrolling laser guide stars

Abstract

A laser guide star (LGS) adaptive optics (AO) system generally requires additional tip/tilt information derived using a natural guide star (NGS), while multi-LGS systems will benefit from measurement of additional low-order wavefront modes using one or more NGS's. If we use AO sharpened NGS's, we can improve both the measurement accuracy and accessible sky fraction while also minimizing the observational overhead of faint NGS acquisition. Multi-object adaptive optics (MOAO) sharpening of NGS is possible where a good estimate of the NGS wavefront can be made, for example where tomographic wavefront information is available. We describe a new approach for high Strehl ratio sharpening, based on additional patrolling laser beacons, to eliminate generalized anisoplanatism and minimize tomography error in the NGS direction.

PULSE: The Palomar Ultraviolet Laser for the Study of Exoplanets

“…However, a key benefit arises with PULSE: the NGS sensor subimages are sharpened by the action of the LGS control loop, boosting performance to the PULSE curve shown. This is comparable to the NGS faintness advantage of tip-tilt star sharpening well-known with sodium LGS systems 29 , except that here we do not expect diffraction-limited subimages. Rather models suggest we may see ~ 0.4" FWHM images within the Shack-Hartmann sensor, a modest but important gain over 1.1" seeing, allowing over 60% Strehl ratio for m V = 15.…”

Concept for the Keck Next Generation Adaptive Optics system

“…To selectively sharpen these 3 natural guide stars; we employ 3 dedicated "point-and-shoot" laser beacons that are positioned approximately 10" outside of each of TT star radii; thus making their deployable radius 175". [6] Figure 5. Schematic of the LGS WFS assembly enclosure.…”

“…The AO sharpening of tip/tilt stars allows much dimmer stars to be used, which dramatically improves the fractional sky coverage and performance in LGS AO observing mode. [6] In addition to LGS AO capability, the system will be operational in natural guide star (NGS) mode. A selectable sampling on the NGS wavefront sensor allows for a range of natural guide star brightness.…”

High-Speed Imaging and Wavefront Sensing With an Infrared Avalanche Photodiode Array

“…Recent innovations in infrared avalanche photodiode (APD) detectors, wherein the avalanche gain of photo-generated electrons occurs within the HgCdTe substrate, have reduced the effective read noise of sizable pixel arrays to below the critical 1 e − threshold (Feautrier et al 2014;Finger et al 2014). When paired with correspondingly low dark currents, there is the potential to drastically improve the many current and future applications of infrared arrays in astronomy, e.g., infrared photon counting (Beletic et al 2013;Rauscher et al 2015), improving the sky coverage of laserguided star adaptive optics (AO) systems using sharpened infrared tip-tilt stars (McCarthy et al 1998;Dekany et al 2008;Wang et al 2008;Wizinowich et al 2014), increasing the sensitivity of pyramid wavefront sensors (Peter et al 2010) and interferometers, e.g., S. Guieu et al (2015, in preparation), decreasing noise in post-coronagraphic and speckle nulling wavefront sensors in high-contrast systems (Martinache et al 2012;Cady et al 2013), and improving temporal bandwidth and sensitivity for IR photometric observations (Rafelski et al 2006;Mereghetti 2008). To prove this maturing technology in a challenging observing environment, we demonstrate the use of a Selex ES Advanced Photodiode for High-speed Infrared Array (SAPHIRA) with the Robo-AO visible-light laser AO system mounted to the robotic Palomar Observatory 1.5 m telescope (Cenko et al 2006).…”