Throughput and Emissivity for Alternatives to the Baseline AO Layout

1. Throughput and Emissivity for Alternatives to the Baseline AO Layout Don Gavel NGAO Telecon January 28, 2009

2. 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)

3. 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 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

4. 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

5. Performance Results

6. Performance at Tcold = -20 C

7. 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

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 1st 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 l = 2.3-2.4 microns wavelength range and very little difference shortward. • Recommendation is to stay with the baseline design