A near-infrared tip-tilt sensor for the Keck I laser guide star adaptive optics system

Abstract

The sky coverage and performance of laser guide star (LGS) adaptive optics (AO) systems is limited by the natural guide star (NGS) used for low order correction. This limitation can be dramatically reduced by measuring the tip and tilt of the NGS in the near-infrared where the NGS is partially corrected by the LGS AO system and where stars are generally several magnitudes brighter than at visible wavelengths. We present the design of a near-infrared tip-tilt sensor that has recently been integrated with the Keck I telescope's LGS AO system along with some initial on-sky results. The implementation involved modifications to the AO bench, real-time control system, and higher level controls and operations software that will also be discussed. The tip-tilt sensor is a H2RG-based near-infrared camera with 0.05 arc second pixels. Low noise at high sample rates is achieved by only reading a small region of interest, from 2x2 to 16x16 pixels, centered on an NGS anywhere in the 100 arc second diameter field. The sensor operates at either Ks or H-band using light reflected by a choice of dichroic beamsplitters located in front of the OSIRIS integral field spectrograph.

Effect of segmented telescope phasing errors on adaptive optics performance

“…Each telescope is equipped with essentially identical AO systems. 1,2,15,16 There are notable differences between the sodium lasers and associated launch optics, 10,11 and the fact that Keck I has an infrared tip-tilt sensor 12,13 in addition to a visible light one. Three near infrared science instruments operate behind the AO systems.…”

LIFT on Keck: evaluation of performance and experiments towards an on-sky validation

“…Recently, a H2RG-based camera was implemented with the Keck I laser guide star adaptive optics system to work as a nearinfrared (H and/or Ks band) tip-tilt sensor. 8 Low noise is achieved by multiple reads of small regions of interest. The on-sky results have been excellent and have motivated a proposal to implement a near-infrared low-order wavefront sensor on Keck II.…”

New developments in instrumentation at the W. M. Keck Observatory

“…The first night of on-sky testing demonstrated closed loop operation using the near-IR tip-tilt sensor with NGS too faint to use in the visible wavelengths, and an improved Hband Strehl ratio. Further on-sky testing is scheduled for the fall of 2014 and further details of our near-IR tip-tilt sensor design and testing may be found in the paper by Wizinowich et al [5] in the proceedings of this conference. Our planned NGAO system will incorporate three near-IR tip-tilt sensors and the successful development of this first near-IR tip-tilt sensor provides an important reduction in the level of risk for the NGAO tip-tilt sensors…”