What Requirements Drive NGAO Cost?What Requirements Drive NGAO Cost?

Richard DekanyRichard Dekany

NGAO Team MeetingNGAO Team Meeting

September 11-12, 2008September 11-12, 2008

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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

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WFE budget changes (based on SDR and post-SDR feedback)

• Reduce Na column density to 2 x 109 atoms/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

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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

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NGAO WFE vs. Laser Photoreturn

NGAO Performance vs. Photoreturn

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Relative photoreturn(1 = baseline; 150 ph/cm2/s/W, 100W, 4e9 cm-2 Na)

H-Strehl

N = 64 KBON = 32 KBON = 64 Gal Gal LensN = 32 Gal Gal Lens

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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

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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

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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

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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

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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

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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)

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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 for NLGS > 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