NGAO Trade Study 3.1.2.1.7 GLAO for non-NGAO instruments

1. NGAO Trade Study 3.1.2.1.7GLAO for non-NGAO instruments Ralf Flicker, WMKO 7 March 2007 NGAO meeting #5, 03/07/07

2. 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)

3. Eligible Keck instruments • Depends on: • Whether NGAO/AM2 goes on K1 or K2 • Which existing instruments may become NGAO instruments • Instrument field of view, image quality, plate scales etc (next slide) • 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!

4. More on instruments • LRIS • Red: 0.21 arcsec/pix • Blue: 0.15 arcsec/pix • ESI: 0.15 arcsec/pix • DEIMOS: 0.12 arcsec/pix • Greg Wirth: • Typically you want the spectral FWHM to be about 2 px in order to by Nyquist sampled; hence, LRIS-red would be the most limited case (unable to take advantage of seeing better than 0.4 arcsec FWHM) while DEIMOS could benefit from seeing as good as 0.25 arcsec FWHM. (this neglects the effect of slit seeing) • NIRSPEC: 0.144/0.19 arcsec/pix • Jim Lyke: • From discussions with Ian McLean (NIRSPEC's PI), NIRSPEC was designed for median seeing of 0.4 ". The smallest slit width for NIRSPEC's high-resolution mode is 1 pixel or 0.144 arcsec. The smallest slit for low-resolution mode is 2 pixels or 0.38 arcsec. • MOSFIRE: 0.18 arcsec/pix (imaging) • Image quality: design delivers < 0.2″ images over 0.9–2.5 μm with no re-focus. • R= 4770 w/0.48" slit, 2 pix/resolution element, R = 3270 w/0.7″ slit, 3 pix/resolution

5. 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’

6. 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 shakenot included… • Also no misregistration, ncp aberrations, etc..

7. 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

8. 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)

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

10. 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 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

11. 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 • Speculation: • for NGAO: use a dichroic for NGSand pick-off arms for LGS, anddon’t use central LGS ? • Apart from cost, and technical issues(p.3) - GLAO seems generally a goodidea (if you have a AM2 and plentylasers already) • Still need to understand Keckinstruments image qualities andother restrictions better to knowwhich instruments might not besuitable for GLAO From the Gemini GLAO Feasibility Study Report