What Requirements Drive NGAO Cost?

1. What Requirements Drive NGAO Cost? Richard Dekany NGAO Team Meeting September 11-12, 2008

2. Presentation Sequence • Laser power cost/benefit • Specific requirements • 50% EE in 70mas for 30% sky coverage • 170 nm RMS WFE for 10% sky coverage • 140 nm RMS WFE for bright NGS (goal?) • High-contrast LGS observations • Precision astrometry and photometry • Add’l cost saving ideas • Proposed WFE budget assumption changes • Conclusions

3. WFE budget changes (based on SDR and post-SDR feedback) • Reduce Na column density to 2 x 109atoms/cm2 • Approximately the 25% percentile column density • Increase multi-WFS tomographic error propagator • Multi-LGS centroid error is ~ 0.85 x the centroid error for a single beacon • Former ratio was 1/sqrt(NLGS) = 0.5 for NLGS = 4 (0.41 for NLGS = 6) • Required power to reach ~0.1” rms centroid error (all noise sources included) • 1 beacon = 25W (spigot) • 6 beacons = 137W (spigot) ~ 5.5x the 1 beacon power • Found and fixed a bug in the sky background calculation • Was using an IR band sky background in the HOWFS • Correction somewhat offsets the above increases to required laser power

4. NGAO lasers • Currently most expensive component procurement • SDR WBS 5.2 • Total Cost $FY087,289K for 2 x 50W ‘SOR-Type’ Lasers • Reduced from ~$FY088,925K for 3 x 50W (to realize ~$1,637K savings for SDR) • Greatest technical and programmatic risk • Commercial availability of such a laser is uncertain • Estimated savings of buying less laser power may not be realizable due to NRE costs • Technical assumptions at SDR • 75 W launched • 66.1 W reaching Na layer • 150 ph/cm2/sec/W return model (questioned at SDR) • ~10,000 ph/cm2/sec total return from all beacons

5. NGAO WFE vs. Laser Photoreturn

6. Requirement Drivers • 50% EE in 70mas for 30%+ sky coverage • Strongly depends on MOAO for IR TT stars • Typically >60% H EE vs. < 30% H EE w/o MOAO • Can generally reduce patrol range when using MOAO, compared to SCAO TT star correction (Need to revisit FoR requirement) • Weakly depends on PnS • Weakly depends on Nactuators • Weakly depends on Flaser return, WFS noise • Moderately depends on NLGS, Rasterism

7. Requirement Drivers • < 170 nm HO WFE for 10% sky coverage (includes KBO, Gal Center science cases) • Doesn’t depend on MOAO for IR TT stars • Doesn’t depends on PnS • Weakly depends on Nactuators • N=40 nearly as good as N=48 for 25W SOR return • Moderately depends on Plaser , WFS noise • 25W SOR return (meas err 61 nm w/ Nact = 48) better than 20W LMCT (meas err 84nm w/ Nact = 38) • Strongly depends on NLGS, Rasterism • NLGS = 3 --> 93nm on 20” radius asterism vs. NLGS = 1 --> 143nm • NLGS = 3+1 --> 85nm on 20” radius • Conspiracy of error budget terms, however, makes holding 170nm difficult & 190nm more likely obtainable

8. Requirement Drivers • < 140 nm HO WFE for bright NGS (goal?) • Doesn’t depend on MOAO for IR TT stars • Doesn’t depends on PnS • Strongly depends on Nactuators for mV = 6 • N=64 (atm fit 48nm, total 111nm) vs. N=40 (atm fit 71nm, total 121nm) • Weakly depends on Nactuators for mV = 9 • N=64 (atm fit 48nm, total 136nm) vs. N=40 (atm fit 71nm, total 134nm) • Moderately depends on WFS noise (for NGS mV = 9) • Doesn’t depends on NLGS, Rasterism

9. Requirement Drivers • Exo-Jup LGS (High-contrast LGS science) • Doesn’t depend on MOAO for IR TT stars • Doesn’t depends on PnS • Strongly depends on Nactuators • Correction of semi-static errors critical • Moderately depends on Flaser return, WFS noise, compute latency • Strongly depends on NLGS, Rasterism • NLGS = 3 gives err tomo 93nm on 20” radius asterism (3+1 85nm) • Strongly depends on (currently undescribed) instrument-integrated static speckle calibration system

10. Requirement Drivers • Precision Astrometry and Photometry • Weakly depends on MOAO for IR TT stars • Weakly depends on PnS • Moderately depends on Nactuators • To keep Strehl up • Moderately depends on Flaser return, WFS noise, compute latency • To keep Strehl up • Strongly depends on NLGS, Rasterism • To keep Strehl up • Strongly depends on accurate Cn2(h,t) sensor • Note • Compared to Keck 1 LGS, even RMS WFE of 220nm would give a significant improvement in photometry and astrometry

11. Add’l cost saving ideas • For more modest # of actuators (N = 40 - 52) • Eliminate 2nd relay in the science path • Saves: MEMS DM cost, MOAO calibration, risk mitigation, go-to error terms, science transmission losses • Costs: Increased 1st relay size, loss of MOAO bandwidth benefit • Reduce the size of 1st relay • Use only N = 10 - 14 in 1st relay • Saves: 1st optical relay costs • Costs: Less 1st relay correction of LGS & dIFS science, some increase in saturation errors (need to evaluate in detail, but probably not large)

12. Investigation Summary (starting point, not the end word) • NLGS = 3 (or 3+1) sufficient for all but d-IFU instrument • 50 W of SOR-type laser return would largely meet goals, when balanced with other system parameters • e.g Nsubap & frame rate, system transmission, CCD noise • Rasterism = 20” (fixed) appears sufficient for 10% sky coverage • Rasterism = 40 to 50” (fixed) preferred for 30% sky coverage • Nactuators = 40 sufficient for all but high-contrast science • Flaser return = 25W of 150 ph/cm2/W/sec sufficient for all but high-contrast science • Assumes CCID56 success, excellent laser beam quality • New indications from LAOS simulations that tomography error propagator much higher than expected forNLGS > 1 implies 50W baseline prudent • PnS concept appears DoA in light of this - would require purchase of additional lasers for patrolling LGS • By Implication: • All but high-contrast works with Nactuators ~ 40 probably workable in the ‘Large Relay’ architecture w/o Science Path MOAO (but with IR TT MOAO) • Consider design of semi-static high-order ‘calibration DM’ into NGAO NIR imager to emphasize its role as the LGS high-contrast instrument