1. Don Gavel: Keck NGAO meeting April 25, 2007 1 Some Comments on NGAO System Design and Specification Donald Gavel NGAO Team Meeting 6 April 25, 2007

2. Don Gavel: Keck NGAO meeting April 25, 2007 2 We need to be careful when writing specifications SRD = System Requirements DocumentSRD = System Requirements Document Make the specifications traceable, either:Make the specifications traceable, either: Science case driverScience case driver Engineering selection based on performanceEngineering selection based on performance or Costor Cost In particular, what is driving the wide field? Science or Technical consideration (e.g. capturing TT stars)? Critique of the present SRDCritique of the present SRD Implicitly assumes a single AO system will be built, when there is at least 2, maybe 4 under considerationImplicitly assumes a single AO system will be built, when there is at least 2, maybe 4 under consideration (on-axis, dMOAO, interferometer, contiguous wide-field)

3. Don Gavel: Keck NGAO meeting April 25, 2007 3 Big Trade Items High Strehl –or- High Sky Coverage?High Strehl –or- High Sky Coverage? Fixed #LGS (or laser power)Fixed #LGS (or laser power) Fixed #DMsFixed #DMs These are cost limitsThese are cost limits High Strehl –and- High Sky CoverageHigh Strehl –and- High Sky Coverage Requires both more LGS and more DMs, alwaysRequires both more LGS and more DMs, always This is “architecture” neutral – doesn’t depend whether it’s MCAO or MOAOThis is “architecture” neutral – doesn’t depend whether it’s MCAO or MOAO It isn’t cost or risk neutralIt isn’t cost or risk neutral MCAO DMs are either not cheap or force more optical spacesMCAO DMs are either not cheap or force more optical spaces MOAO DMs need to run in open loopMOAO DMs need to run in open loop “Cheap(er)” solutions don’t work either, i.e. one DM in a wide field relay will force open loop WF sensing“Cheap(er)” solutions don’t work either, i.e. one DM in a wide field relay will force open loop WF sensing AND NOTE: throughput/emissivity is not architecture neutral eitherAND NOTE: throughput/emissivity is not architecture neutral either

4. Don Gavel: Keck NGAO meeting April 25, 2007 4 Case in point: thinking outside the box for the solutions driven purely by technical considerations There are no science cases, save hi-Z galaxies/AGNs (which want dIFUs), that want a large field - at most 20 arcsec for galactic centerThere are no science cases, save hi-Z galaxies/AGNs (which want dIFUs), that want a large field - at most 20 arcsec for galactic center So: Here is a suggested improvement (credit: Brian Bauman) for narrow field AO that might improve sky coverage at lower costSo: Here is a suggested improvement (credit: Brian Bauman) for narrow field AO that might improve sky coverage at lower cost Science Field Tip/Tilt Star LGS Technical Field Movable LGS Fixed LGS asterism