Gemini South multi-conjugate adaptive optics (GeMS) laser guide star facility on-sky performance results

Abstract

With two to three deformable mirrors, three Natural Guide Stars (NGS) and five sodium Laser Guide Stars (LGS), the Gemini Multi-Conjugate Adaptive Optics System (Gemini MCAO a.k.a. GeMS) will be the first facility-class MCAO capability to be offered for regular science observations starting in 2013A. The engineering and science commissioning phase of the project was kicked off in January 2011 when the Gemini South Laser Guide Star Facility (GS LGSF) propagated its 50W laser above the summit of Cerro Pachón, Chile. GeMS commissioning has proceeded throughout 2011 and the first half of 2012 at a pace of one 6-to 10-night run per month with a 5-month pause during the 2011 Chilean winter. This paper focuses on the LGSF-side of the project and provides an overview of the LGSF system and subsystems, their top-level specifications, design, integration with the telescope, and performance throughout commissioning and beyond. Subsystems of the GS LGSF include: (i) a diode-pumped solid-state 1.06+1.32 micron sum-frequency laser capable of producing over 50W of output power at the sodium wavelength (589nm); (ii) Beam Transfer Optics (BTO) that transport the 50W beam up the telescope, split the beam five-ways and configure the five 10W beams for projection by the Laser Launch Telescope (LLT) located behind the Gemini South 8m telescope secondary mirror; and (iii) a variety of safety systems to ensure safe laser operations for observatory personnel and equipment, neighbor observatories, as well as passing aircrafts and satellites.

Getting ready for GeMS 2.0: A workhorse AO facility

“…Our current laser, a 50W solid-state laser delivered by Lockheed Martin Coherent Technologies (LMCT) in March 2010, has seen its first light on-sky in January 2011. 15 Two infrared laser arms are independently amplified several times before being mixed into a sum frequency generation birefringent crystal to produce a laser line centered at the D2a sodium resonance. This very complex system is located inside a container box mounted on the side of the telescope platform providing a control environment in a gravity invariant area.…”

Beam shaping for laser-based adaptive optics in astronomy

“…During the 2011/2012 commissioning campaign, the measured M 2 factors on GeMS were below 1.5 and 1.3, in x and y respectively [4]. A beam quality M 2 close to 1 has not been achieved so far with GeMS laser.…”

“…The dotted line represents the half of the maximum to quantify the FWHM and it shows that the beam diameter is between 1.5 and 2 times larger along x than along y. The beam quality is always worse in the x-axis with GeMS because it corresponds to the unguided dimension of the amplification stages in the waveguide amplifier module [4]. In addition, a slow increase of the M 2 values over the length of the runs has also been noticed, suggesting that the system suffers from constant misalignment [4].…”

“…(1)), the reduced size of the short-exposure laser spot with a perfect Gaussian beam compared to the distorted beam would be equivalent to an increase of the received number of photons by 30 to 45%. The beam degradation is understood as the result of quasi-static aberrations in both the laser bench and the beam transfer optics (BTO) path [4]. First, the co-alignment of the optical system for sum frequency generation used in GeMS is highly subject to temperature hysteresis, as well as telescope vibrations.…”

“…For the first two years of operations of GeMS during the season of low sodium concentration (December-January), the system frame rate had to be frequently reduced compared to its maximum of 800 Hz in order to guarantee the average detection of at least 135 photons per subaperture per frame from each laser guide star [3,4]. This minimum flux is required for adequate closed-loop AO correction by GeMS.…”