Transcript 080911_NGAO_PD_Team_Meeting_3.ppt

What Requirements Drive NGAO Cost?
Richard Dekany
NGAO Team Meeting
September 11-12, 2008
Presentation Sequence
• Laser power cost/benefit
• Specific requirements
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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
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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
0.75
0.70
0.65
H-Strehl
0.60
0.55
N
N
N
N
0.50
0.45
=
=
=
=
64
32
64
32
KBO
KBO
Gal Gal Lens
Gal Gal Lens
0.40
0.35
0.30
0.25
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)
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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)
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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)
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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
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Rasterism = 20” (fixed) appears sufficient for 10% sky coverage
– Rasterism = 40 to 50” (fixed) preferred for 30% sky coverage
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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
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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
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