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

2. 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. 3 WFE budget changes (based on SDR and post-SDR feedback) Reduce Na column density to 2 x 10 9 atoms/cm 2 –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(N LGS ) = 0.5 for N LGS = 4 (0.41 for N LGS = 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. 4 NGAO lasers Currently most expensive component procurement –SDR WBS 5.2 Total Cost $ FY08 7,289K for 2 x 50W ‘SOR-Type’ Lasers –Reduced from ~$ FY08 8,925K 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/cm 2 /sec/W return model (questioned at SDR) –~10,000 ph/cm 2 /sec total return from all beacons

5. 5 NGAO WFE vs. Laser Photoreturn

6. 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 N actuators –Weakly depends on F laser return, WFS noise –Moderately depends on N LGS, R asterism

7. 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 N actuators N=40 nearly as good as N=48 for 25W SOR return –Moderately depends on P laser, WFS noise 25W SOR return (meas err 61 nm w/ N act = 48) better than 20W LMCT (meas err 84nm w/ N act = 38) –Strongly depends on N LGS, R asterism N LGS = 3 --> 93nm on 20” radius asterism vs. N LGS = 1 --> 143nm N LGS = 3+1 --> 85nm on 20” radius Conspiracy of error budget terms, however, makes holding 170nm difficult & 190nm more likely obtainable

8. 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 N actuators for mV = 6 N=64 (atm fit 48nm, total 111nm) vs. N=40 (atm fit 71nm, total 121nm) –Weakly depends on N actuators 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 N LGS, R asterism

9. 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 N actuators Correction of semi-static errors critical –Moderately depends on F laser return, WFS noise, compute latency –Strongly depends on N LGS, R asterism N LGS = 3 gives err tomo 93nm on 20” radius asterism (3+1 85nm) –Strongly depends on (currently undescribed) instrument-integrated static speckle calibration system

10. 10 Requirement Drivers Precision Astrometry and Photometry –Weakly depends on MOAO for IR TT stars –Weakly depends on PnS –Moderately depends on N actuators To keep Strehl up –Moderately depends on F laser return, WFS noise, compute latency To keep Strehl up –Strongly depends on N LGS, R asterism To keep Strehl up –Strongly depends on accurate C n 2 (h,t) sensor Note –Compared to Keck 1 LGS, even RMS WFE of 220nm would give a significant improvement in photometry and astrometry

11. 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. 12 Investigation Summary (starting point, not the end word) N LGS = 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 N subap & frame rate, system transmission, CCD noise R asterism = 20” (fixed) appears sufficient for 10% sky coverage –R asterism = 40 to 50” (fixed) preferred for 30% sky coverage N actuators = 40 sufficient for all but high-contrast science F laser return = 25W of 150 ph/cm 2 /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 for N LGS > 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 N actuators ~ 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