Starting a UV Mission Building from NGST Technology Strategy

better in UV-optical than HST and future ground telescopes with AO n UV detectors should be improved first n Larger mirrors with UV accuracy not currently feasible " Try an IR telescope first -sounds easier n Contamination control terrifying to many engineers April 5, 2002JCM 5 Hypothetical Design Objectives: UV NGST l Diffraction limited imaging at 0.2 µm l 50 m 2 aperture (8 m circle, or 4 x 16 m off-axis ellipse) l Coronagraphic capability for planet searching and analysis, with extremely low stray light l Huge field of view of telescope l Huge detector arrays with extremely high efficiency l Choice of spectrometers n High spectral resolution n Moderate resolution, highly multiplexed April 5, 2002JCM 6 Mirror configurations considered for NGST l Glass on glass (Kodak concept) n Traditional material, easily polished, several meters size possible n Sandwich for rigidity n Actuators for shape control on large scale l Ribbed Glass on actuators (Goodrich concept) n Many actuators for medium scale shape control l Ribbed glass bonded to carbon fiber ribs (Composite Optics Inc. concept) n Can match total thermal expansion at 2 temperatures (room and cold) l Thin uniform glass on many actuators (U of Azconcept) n Fragile, needs cheap lightweight reliable actuators l Beryllium pieces, ~ 1.3 m size, individually rigid, with actuators for position and radius of curvature control; CTE not zero l Silicon Carbide -near net shape molding processes See Phil Stahl at MSFC!<br><br> April 5, 2002JCM 7 Mirror technology program demonstrates that NGST goals are within reach Manufacturing Time/Unit Area Manufacturing Time/Unit Area HST (2.4 m) 1 year/m 2 SIRTF (0.9 m) 3 years/m 2 NGST (6 m) 1 month/m 2 Areal Density (Kg/m 2 ) 300 200 100 19801990 2000 2010 240 30 NGST Requirement 60 15 HST ALOT SIRTF NGST April 5, 2002JCM 8 Mirror Technology Development Program l NASA andDoDPartners have invested $30M in mirror technology development: April 5, 2002JCM 9 Larger mirrors l NGST aperture limited by polishing time and cost to about 6 m l Further development requires time and investment, but there are many possibilities and NASA is not the only customer l Competitions held by NGST were an excellent strategy - companies invested their own money and put good people on the job l Conclusion: size not a real limiting factor in the 6 -10 m range, no hard boundaries of the feasible l Experience, money, mass, and packaging are limiting factors -let 9s do the homework April 5, 2002JCM 10 More accurate mirrors l Need stability between fabrication and test to converge to desired shape l Prefer to operate at same temperature as polishing step, as support ribs and structures print through on cooling l Need low polishing force, deterministic polisher to avoid distortion and converge quickly n Vacuum laps, ion polishing, chemical polishing, mouse milk l Allow for correction by high order small adaptive optical part at image of primary -studied by J.Traugerfor NGST coronagraph n Much easier at room temperature -piezoelectric materials n Limited field of view if device is small and errors are large April 5, 2002JCM 11 NGST Wavefront Sensing and Control Laboratory testbedsand computer simulations are used to develop WF sensing and control systems Typical defocussedimages used for WF sensing and control WF error = 150 nm RMS Retrieved WF Data Image 20 40 60 10 20 30 40 50 60 Model Image 20 40 60 10 20 30 40 50 60 20 40 60 0 5 10 15 x 10 4 Horizontal Slice 20 40 60 2 4 6 8 10 12 14 16 18 x 10 4 Vertical Slice Model is green Data is blue In-focus images prove excellent broad-band phasing In-focus data and model image at 633 nm Model is green Data is blue Testbed has 3 separate segments Computer processes images to determine new controls defocus = 25 mm defocus = -25 mmdefocus = 0.8 mm Data is compared to computer models to validate simulations and system design methods Control applied by moving segments and deforming mirrors NGST April 5, 2002JCM 12 Off-axis ellipse, monolithic mirror l Launched standing on end in rocket shroud, proposed for NGST l Asymmetric point spread function not popular with NGST science team l Avoids discontinuities and gaps in mirror l Simplified mirror figure control -continuity not an issue l Unobscuredtelescope, far superior scattered light for coronagraph l Secondary mirror tower need not be wimpy like NGST 9s l Compatible with all NGST mirror technologies -can join smaller segments before polishing to final surface April 5, 2002JCM 13 Contamination control l A serious cost challenge if not approached right l Major hazard: polymerization of hydrocarbons by solar UV l Solution approach: n Build it clean, bake it out, bag it until launch, maybe even inside rocket shroud n Keep mirror warm compared with condensation temperature of organic vapors n Don 9t deploy in sunshine or in Earth albedo-need aperture cover? n Go to deep space orbit to avoid Earth albedo n Keep the astronauts (if any) and Shuttle very clean, or don 9t use them l Risk reduction approaches n Develop on-orbit cleaning with ion beams or liquid CO 2 (shuttletest) n Develop on-orbit recoating (but only works for one or two surfaces) April 5, 2002JCM 14 Mirror coatings l Coatings limit short wavelength coverage l Does the science drive us to bare metal applied in space? l Is there progress to be made?<br><br> April 5, 2002JCM 15 Better pointing l NGST and HST have same stability requirements, but future UV telescope must do better n Finer pointing than HST, but more photons from guide star, should be easier l NGST has much relaxed absolute pointing -no astrometry needed l NGST is in a quiet orbit: no outside disturbances l To achieve stability, need: n Low internal vibration -e.g. active and passive isolators for momentum wheels, and active and passive isolators between spacecraft bus and telescope n Fine star tracker -limited by photon statistics and numbers of stars, must see about 16th magnitude to get 4 per FOV April 5, 2002JCM 16 Configuration issues -orbit l Low Earth orbit (Space Station) -build like HST, penalties for Earth -Sun geometry, Earthalbedoan issue, but servicing possible l Low Earth polar orbit (COBE) -much more stable thermal environment, avoids Earth eclipse much of year (depending on altitude) -servicing not currently feasible, but may be in future l High Earth orbit -(IUE, ISO) -thermally stable, avoids mostalbedo, return or servicing relatively easy, but takes more rocket energy than Station orbit l Deep Space orbit (NGST, MAP, Planck, Herschel, SIRTF) - thermally stable, much harder to retrieve or service; data rate adequate with L2 orbit or higher gain antenna (SIRTF) April 5, 2002JCM 17 Configuration issues -Tube or Naked? l NGST mirror will get ~ 0.2% pits from micrometeoroids in 10 years l NGST will be cold -can 9t stay warm looking at deep space behind a shield l To keep primary mirror warm, need a tube, or lots of heat n 50 m 2 at 300 K and 0.05 emissivityradiates 1250 W (readily available with heat pipes or electrically) l To control thermal gradients, need a tube or lots of local temperature controllers or very high passive conductance l To achieve stability, need independence from orientation with respect to Sun l Solar flux changes 7% pk-pk over 1 year l Baffle tube could be deployed by inflation of plastic film with included, or erected like NGST with arms and cables April 5, 2002JCM 18 Comprehensive and Systematic Trade Studies Have Produced Two Viable Observatory Designs April 5, 2002JCM 19 Configuration issues -servicing l Only currently available servicing capability is with Shuttle at low altitude l Forecast: expect rapid progress in tele-operation of robotics n Defense n Surgery n Inspection and repairs of pipes -e.g.<br><br> N Y City water supply n Hazardous materials handling -chemical, biological, nuclear n Space station assembly robots n Underwater n Mining n Farming n Inventory management April 5, 2002JCM 20 Special Telescope Assembly Robotics l NGST assembly robot concept developed by R. Muller l Central hub, 2 arms, each with manipulators, grabbers, and electrical connections l Robot can walk around spacecraft attaching to footholds and doing work l Redundant robots for reliability l Allows more efficient use of rocket shroud volume l Robotic servicing at L2 possible l Robots can be clean and even cryogenic l Can avoid need for astronaut-safe hardware April 5, 2002JCM 21 Benefits of Remote/Robotic Assembly n Larger instruments, more efficiently packed n Wide range of possible configurations, not limited to simple hinges n Can stow optics in contamination protection chambers n Flexible response to failure, if spare parts carried, or provided on servicing mission n Flexible adjustment and configuration of future very complex systems, e.g. interferometers n Robots can be contamination-free and can reach cryogenically cooled regions n Robots can operate near delicate optics, because they don 9t get tired or impatient, and can follow pre-programmed sequences April 5, 2002JCM 22 l 10 Meter NGST Stowed in Atlas Large Fairing Stowed Secondary Half Atlas Large Fairing Optical Bench Isolated Science Instrument Module Stowed Support For Secondary -3 Places Stowed 10 Meter Primary -12 Pieces 3 Groups of 4 Mirror Sections April 5, 2002JCM 23 Figure 1 -20-Meter Telescope Stowed in Delta IV Shroud Secondary Support Structure Adjustment Mechanism (6) Secondary Mirror (baffle not shown) Folded Tripod Legs (3) Central Baffle "Fixed Part "Movable Part Primary Mirror Segments " 9-Pack 1 "9-Pack 2 "9-Pack 3 (behind) Optical Bench 2.1-m Long x 5-m Diameter Volume for Instrument Module, Spacecraft Module and Robot 6559 Payload Attach Assembly 5-mDiam.<br><br> x 14.6-m Long Fairing (usable payload envelope shown) April 5, 2002JCM 24 Figure 3A -Spar 6-Meter Robot Shoulder Joint 3 Degree-of-Freedom Wrist (2) End Effector (2) 3 m TV Target (below baseplate) Grapple with captive screw Baseplate Connector (recessed) Grapple FootholdHandholdShort GrappleGrappleGrapple 45 38 (All dimensions in millimeters) 200 25 150 . . .<br><br> . . ..<br><br> . . ..<br><br> . .. .<br><br> . 55 Figure 3B -Grapple Configurations April 5, 2002JCM 25 Better detectors l Detector requirements drive the mission configuration -are they cold? Do they need a cold telescope?<br><br> Do they provide energy resolution? What kind of electronics do they need? What data rate do they produce?<br><br> Do they inspire new scientific goals? l Detector investments can have very high benefit per unit cost, much more than mirror area, e.g, but need tens of $M to make progress -Hold competitions with real money and real payoff in flight data l Moore 9slaw works, but only if there 9s strong demand and reward for progress n Doubling time for IR detector speed over past 50 years is even faster than 18 months n Let 9s have a new shot at the UV! April 5, 2002JCM 26 Instrument technology l Coronagraphs: n Rotation shearing interferometer -suppresses on-axis radiation, good for looking close to star n Lyotstops with wavefront corrector -J.Traugerfor NGST " Can cope with segmented primary, at penalty in performance n Specialapodizingmasks " Circular graded near-Gaussiantransmission " Square version of near-Gaussianmask " Shaped aperture (binary) mask l Multiplexed spectroscopy -can use NGST concept of microshuttersand gratings April 5, 2002JCM 27 GSFC MEMS Programmable Aperture Maskfor NGST Multi-Object Spectroscopy l Enables first orbital multi-object spectrometer l Simultaneous spectra of up to 1000 targets per exposure l Key capability for discovery of the first stars and galaxies n must observe may targets to find these rare fossil remnants of the early universe l Fusion of NASA andDoDtechnologies n Magnetic Film technology provided by Naval Research Lab Micro Shutters 100 x 100 µ µ m Multi-Object Spectroscopy Simulation Magnetic Actuation 4 million micro-shutters April 5, 2002JCM 28 Conclusions and Recommendations l Be ambitious: the time frame is long and the competition (for funds and scientific discoveries) is strong l Be poetic: scientific goals must be as inspiring as the technical challenges are difficult to meet them l Start with the top payoff items: detector development l HELP FINISH NGST -APPLY FOR THE OBSERVATORY PROJECT SCIENTIST POSITION!!<br><br>

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