NGAO Trade Study : LOWFS type and architecture Stephan Kellner, Ralf Flicker NGAO Team meeting #4, WMKO Kamuela HI, 1/22/2007 Status report

2 LOWFS architecture and type TS Original WBS : –Consider the cost/benefit and technical maturity of MEMS-based correction within the LOWFS, using MOAO control techniques. Include consideration of additional metrology systems required, if any. Compare with cost/benefit of MCAO system to provide tip/tilt star sharpening. Complete when LOWFS requirements and sky coverage estimates have been documented. Re-scoping: –Include WFS type (SH vs PYR) in performance/cost TS (moved from WBS ) –Will not calculate sky coverage initially in this TS. WFS architectures are compared on a 1-on-1 basis, under the assumption that whichever performs the better at a given NGS magnitude will provide the greater sky coverage.

3 Current status Work on system modeling started in Jan 07 –Stephan Kellner working remotely on computers at WMKO in Waimea –1/19/07: model almost complete We decided that a fairly sophisticated numerical simulation was the way to go (no simple analytical formulas to account for everything) Using the IDL based AO simulation package “CAOS” –Yes, yet another simulation code! (number 5, I think…) –Authors: M. Carbillet et al. (Arcetri) –Good: modular application builder, no coding required Comes with a pyramid sensor (PYR) module –Bad: inefficient, memory expensive = not so good for very large (e.g. NGAO) simulations, but ok for this particular TS

4 Analysis approach Build high-order conventional NGS AO system model –Not feasible to model high-order multi-LGS tomography in CAOS –use off-axis position of NGS to adjust level of partial high-order correction, emulating varying levels of tomography (anisoplanatism) error on the LOWFS Build two LOWFS sub-systems (SH & PYR) running in parallel –compare low-order correction performance side-by-side as a function of Level of partial high-order correction & internal MOAO correction (on/off) NGS magnitude Number of modes sensed/corrected (2 or 5) Wavelength band (visible - IR)

5 CAOS system model On-axis GS Off-axis GS LOWFS unit (open loop) Wave front adder: Offaxis residual wavefront error plus TT etc LO loop HO loop Still needs some work…

6 System & observing parameters no!

7 LOWFS type TS - current plan S. Kellner to proceed with modeling remotely (from Germany) –Originally 160 hr allocated to –SK well within this limit (assuming he is getting paid) Continuous support from R. Flicker –On vacation January 2007 (but will be in communication) Finish modeling this week (1/26/07) First trade study results early February 2007 Final results reported at NGAO meeting #5 (or sooner)