1. 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

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, 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!

4. 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’

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 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..

6. Observing scenarios
Wavelength range: 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 cycles to average PSFs = 20s real time exposure

7. 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)

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

9. 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

10. 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