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) 2

Tools & Assumptions Throughput and Emissivity Spreadsheet Sky data from A.Bouchez’s programs (KAON 501) Coatings data gathered from various sources –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 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 3

Options 4 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

Performance Results 5

Performance at Tcold = -20 C 6

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 7

8 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 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 1 st relay instead? 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 = microns wavelength range and very little difference shortward. Recommendation is to stay with the baseline design