NGAO Trade Study 3.1.2.1.7 GLAO for non-NGAO instruments Ralf Flicker, WMKO 6 March 2007 NGAO meeting #5, 03/07/07

WBS definition WBS 3.1.2.1.7 (GLAO for non-NGAO instruments) “Consider the relative performance, cost, risk, and schedule of GLAO compensation using an ASM as a wide-field optical relay for non-NGAO instruments. Complete when expected performance benefit for each instrument documented.” GLAO = ground-layer adaptive optics Modest image quality improvement (~2x EE) over large fields of view Adaptive secondary mirror (AM2, ASM) High-order deformable surface common to all instruments Rationale: IF (NGAO && AM2) THEN can use AM2+NGAO LGS for non-NGAO instruments; possibly use wide technical field (5’ ?) of NGAO for doing wide-field GLAO with non-NGAO instruments Requires additional optics (~$1M)

Eligible Keck instruments Depends on: Whether NGAO/AM2 goes on K1 or K2 Which existing instruments may become NGAO instruments Instrument field of view, internal image quality Lots of other technical issues: more BTO to acquire LGS/NGS for NGAO (beam splitters or pick-off arms?) pick-off arms  science inst. vignetting? AM2–HOWFS rotation? (several options) Plate scale changes & distortions over large FoV? Instrument slit widths? NGAO LOWFS sensitivity issue, if optimized for partially corrected NGS Happy to take your input here!

Simulations: LGS/NGS FoV Ran a number of numerical AO simulations with the YAO package to investigate GLAO performance over a wide range of parameters Alternative NGS positions • 1 NGS (central) • 4 NGS (plus “+”) LGS LGS FoV • 2’  2’ • 5’  5’ Extended FoV • 3’  3’ • 7.5’  7.5’

GLAO system assumptions 5 LGS in a quincunx asterism (15 W out per laser) 32x32 sub-aperture Shack-Hartmann wavefront sensors Lower order than baseline NGAO: maybe realistic for AM2 (also tried 64x64 and did not see huge difference) J-band tip/tilt NGS (SH type) No tomography with LGS: simple WFS averaging LGS: 1 kHz sample rate, 1 additional frame delay (NGS @500 Hz) Switched on (almost) every feature of the code: Static + dynamic segment aberrations (30 Hz vibrations) LGS spot elongation Rayleigh back-scattering (fratricide effect) Uplink LGS t/t control LGS centroid gain optimization (François dithering method) Photon + read-out noise for both LGS and NGS 10% DM hysteresis Wind shake not included… Also no misregistration, ncp aberrations, etc..

Observing scenarios Wavelength range: 0.55-2.19 micron (V, R, I, z, J, H, K) Field of view: 2’2’ or 5’5’ Number of NGS: 1 (central) or 4 (plus “+” asterism) NGS brightness adjusted for each case, based on R.Clare sky coverage simulation median numbers ≈ 50% SC Cn2 turbulence profile: CN–M3 (47% GL) or 13–N (67% GL) Ran 20000 cycles to average PSFs = 20s real time exposure

Sample simulation results (#1) FWHM (mas) Strehl Ratio Encircled energy within 225 mas (same area as a 0.2” pixel - common bench mark used by Gemini and MUSE) Diameter of 50% encircled energy (mas) Simulation scenario #1 (1 NGS, CN–M3, 2’ FoV)

Comparing all scenarios CN–M3 13–N = 4 NGS No symbol = 1 NGS 2’ FoV 5’ FoV

Conclusions Potentially interesting/useful image quality improvements GLAO by NGAO (for non-NGAO instruments) could work over a large FoV (~6’ square) and a wide range of observing conditions, with performance in the range of a factor 1.2–4 improvement of FWHM and EE Large sky coverage Although sky coverage calculations have not been done, current results (with realistic NGS magnitudes) suggest a generous sky coverage for GLAO Turbulence profile the most sensitive parameter for GLAO success Fraction of turbulence in the ground-layer MOSFIRE (&LRIS) could take great advantage of wide-field GLAO if NGAO+AM2 goes on K1 DEIMOS & ESI on K2 candidates for wide-field image enhancement Instrument image qualities currently not known (to me!) No reason narrow-field instruments like NIRC2, OSIRIS and NIRSPEC would not benefit from GLAO correction also, but the more interesting application (IMO) is the wide-field science that is not planned for NGAO

Conclusions (cont.) Not clear what the impact of LGS/NGS pick-offs will be to non-NGAO instruments (one LGS at the center of the field!) Vignetting of science FoV Apart from cost, and technical issues (p.3) - GLAO seems generally a good idea (if you have a AM2 and plenty lasers already) Still need to understand Keck instruments image qualities and other restrictions better to know which instruments might not be suitable for GLAO From the Gemini GLAO Feasibility Study Report