Transcript ThroughputEmissivityStudy.ppt

Throughput and Emissivity for
Alternatives to the Baseline AO Layout
Don Gavel
NGAO Telecon
January 28, 2009
Baseline vs Alternative
• Baseline is to “cool” the AO system: place the AO fore-optics at -10
degrees C to reduce emissivity (FR-14 says -20 deg C), and a
cascaded woofer/tweeter relay architecture
– Requires an input window (double pane: 4 surfaces, two of them warm)
– Cascaded AO relay has upwards of 10+ surfaces prior to science
instrument window
• Cost saving alternative: can we get away without refrigeration if we:
– Replace K-mirror with multiple vertical rotators
– Single relay with one high order high stroke DM (~4 high quality
reflections) ahead of instrument window
• Could this meet <30% unattenuated (sky+tel) spec? (from ScRD
KAON 455)
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Tools & Assumptions
• Throughput and Emissivity Spreadsheet
• Sky data from A.Bouchez’s programs (KAON 501)
• Coatings data gathered from various sources
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HGNa: Lick “Holy-Grail” - enhanced at 589 nm
Gold for IR only reflectors
Assume “Perfect” Na and IR/Vis dichroics (absent specific designs)
Easy to add or modify coating data in T&E Spreadsheet
• Window needed on input to AO chamber if cooled, or on the MEMS
in non-environmentally protected chamber
– Infrasil or CaF will pass full band
– Coating on this window, AR from 589-2500 nm, is not a standard
catalog item. We need to talk to coating vendors about it.
– No-coating Fresnel loss is 4% per surface
– Assume for now: loss is 1% per surface (conservative?)
– Front window of double-pane is warm on both sides
• Assumes DM is mounted on the fast tip/tilt platform
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Options
• Baseline
• Option 1
– No cooling, No K mirror
– Cascaded relay
– Fold mirror to vertical instrument
• Option 2
– No cooling, No K mirror
– Single relay
– Fold mirror to vertical instrument
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Performance Results
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Performance at Tcold = -20 C
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Performance Comparison Conclusions
• Just counting AO surfaces (into, say, narrow field instrument) the
option 2 design has the lowest emissivity and highest throughput
• Counting 6 gold surfaces in pickoff mirrors + DMs into off-axis arm of
a dIFS, the baseline design has the lowest emissivity
• The baseline design meets the < 30% of Sky+Telescope
requirement at all wavelengths shortward of 2 microns
• No option meets spec at 2.1 microns and longer (all are > 40% of
Sky+Telescope) (baseline -20 C meets it at 2.2 um)
• The option 2 design has 10% more throughput (97% vs 87%) at the
589nm laser line
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Cost Comparison
•
Rotating instrument instead of using a K mirror
– Rotation bearings and wraps ~$100K-$200K- more than K mirror
– Extra flexure mitigation (for side-looking instrument)
•
Savings of no refrigerator:
– No compressor, coolant, plumbing, insulation ~$20K
– No front window or window coating ~$40K
•
Extra cost of DM
– Xinetics DM? $1M vs $300K woofer + $400K MEMS tweeter
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Gained 10% of the laser return. Equivalent to about 7Wx$73K/W =~$500K
savings – but this conclusion is very sensitive to assumptions about the window
coating
– Achieved by derotating after the LGS pickoff. Could we put the K-mirror after the 1st
relay instead?
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Cost Conclusion
– For vertical instrument barrels: probably a wash in performance and a wash in cost.
– Or, could pay more (flexure compensation) with side-barrel instrument and get ~10%
better fraction of Sky+Telescope emissivity at l = 2.3-2.4 microns wavelength range
and very little difference shortward.
•
Recommendation is to stay with the baseline design
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