How does optical astronomy fit into the big picture for future ground and space telescopes, including infrared?

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1 How does optical astronomy fit into the big picture for future ground and space telescopes, including infrared? Roger Angel University of Arizona April 5, 2002 After all, we have all agreed that cryo infrared is the most important thing for space

2 What kinds of new optical/infrared telescopes and instruments are needed? Bigger - more photons for: less noisy data sharper images in the diffraction limit Space - much better for thermal infrared, distortion free in optical above the crippling thermal emission of atmosphere, but expensive and risky Ground - potentially better for optical and near infrared requires improved adaptive optics to correct for atmospheric blurring and recover high resolution, diffraction limited images

3 Part 1. Some Mirror Lab technology 6 mirrors m

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6 Magellan Project - Las Campanas Observatory November 2000

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8 LBT Enclosure on Mt. Graham December 2000

9 Magellan 2 Interferograms HST quality in a 6.5 m mirror at f/1.25 already exists. Just need to get one in space Mag 2, astigmatism subtracted 14 nm rms surface

10 Important extension to 2 mm thick and secondary mirrors 1) The back of an accurately figured rigid mirror can be ground away to leave a very thin shell, whose figure when properly supported is still good. (Low internal stresses in the best available glasses allow this) useful to lighten mirrors for space and to make large, figured adaptive mirrors. 2) full aperture tests of convex secondaries during manufacture with computer generated holograms allow for very large and very aspheric secondaries, eg 1/7 m f/5 secondary for the 6.5 m MMT

11 MMT336 Adaptive Secondary being assembled in Italy Integration in Tucson - January 2001 R. Biasi D. Gallieni

12 MMT adaptive secondary mirror, 64 cm diameter, 2 mm thick, highly aspheric secondary for f/1.25 primary. Collaboration with Arcetri Observatory Lab data showing image improvement at 1.5 µm when adaptive mirror is turned on to correct blurring. (Wildi, Lloyd Hart, Martin et al). Telescope test at the new 6.5 m MMT scheduled for April 2002

13 High authority concept for space mirrors with quasistatic actuators Ideal shape Structure deforms, taking membrane with it Actuators are driven to compensate

14 2 mm thick NMSD mirror for lightweight space prototype

15 Current progress with 2-m NMSD, 13 kg/m 2 system Technician attaching sub-loadspreaders to back of 2-mm thick facesheet on temporary support

16 Fully assembled NRO prototype, June 2001 (Jim Burge) 1 kg total mass (including glass, support structure, actuators and hardware, cabling, 50 cm diameter

17 Results of interferometric metrology, NRO prototype at 633 nm wavelength Single HeNe Interferogram (633 nm) Gray scale version from phase-shifting interferometer 165 nm rms - limited by gravity deflection over supports

18 Part 2 taking this technology to space NGST (cold deployed 6 m telescope at L2) TPF (major mission to detect Earthlike planets to 15 pc) cautions! Trying to run before you can walk! NASA s current program driven by Goldin s Earth image vision Getting to dangerous point where a big technical leap is required in space to accomplish significant scientific advances. Example: while it is standard for big ground telecopes to make autonomous correction of optical figure based on wavefront measurements, this has never been accomplished on even a very small space telescope. Can we afford to skip the NNTT for space?

19 Present surveillance telescopes: Titan rocket payload fairing 3 m diameter 5 m telescope inside ~ 4 m cost of copy $1.4 billion A sensible next step for astronomy: modify optics assembly to operate at 50K add deployed sunshield to allow passive cooling orbit to L2 add infrared array detectors cost $2 billion?

20 A longer term goal for space astronomy - a 15 m telescope in 6 segments with 6 m fairing The only thing that prevents adoption of 6 m building blocks as the next standard is not lift capability, but the current absence of a large enough launch fairing.a study by vehicle and aerodynamics experts could remedy this deficiency, with an expanded fairing on an EELV. The questions are: * how does load to L2 depend on fairing size - effect of increased drag * cost to design and validate, including mods to launch facility As an example of EELV configurations, the above illustrations show the basic 5 m diameter Delta IV with the addition of a 7 m fairing to take 6 m optics. The medium, left, would be appropriate for a complete, lightweight 6 m telescope. The heavy, center, could launch the stack of seven 6 m segments needed for a 15 m telescope. The enlarged fairing still presents less drag than the STS-Shuttle system with its 8-m external fuel tank and orbiter, shown to scale on the right.

21 General layout of deployed telescope

22 Large diffraction limited field (Burge) Annular Field from 3-mirror telescope Fully corrected, unvignetted 24 arcmin OD 12 arcmin ID 4.3 x 4.3 arcmin IFOV Instantaneous field of view (IFOV) of 4.3 x 4.3 arcmin (1 km at 800 km) Steer the line of sight over the field of regard using the steering mirror Field accessible with steering mirror

23 Davison deployment sequence

24 The first radial support swings out

25 Then the top segment on the stack is swung out anticlockwise

26 Suggested program for post-hubble cryo-telescopes 1. Get on fast with cryo modified 4 m class spy satellite 2. Develop (with DoD) 7 m fairing for current 5 m EELVs for 6 m successor 3. Test Davison deployment scheme with 4 m segments for 10 m equivalent telescope 4. Full 15 m telescope with 6 x 6m segments Technically, an optical HST successor fits in as part of such a broader development scheme. Note that we could make today a rigid 6.5 m honeycomb mirror weighing 5 tons by the existing method. It could likely be flown in a HST mass (11 ton) telescope with EELV

27 Part 3. Optical role in space The sky is almost as dark on the ground in the u - r bands So the only real advantage for space is the freedom from atmospheric distortion This is where HST really has scored. In the future, though, will adaptive optics allow ground telescopes to win out, with their bigger apertures made diffraction limited? Two domains: 1) faint objects, normal diffraction limit 2) exoplanets, super diffracation limited for high contrast

28 wavefront accuracy limited by photon noise of wavefront sensor, Kolmogorov turbulence Photon flux needed for reasonable Strehl scales as λ -3.6 lambda magnitude laser power rms wave error nm W 150 nm W 75 nm W 38 nm nm

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30 MCAO with multiple Rayleigh guide stars (Lloyd Hart and Angel) Current lasers, 532 nm, 10W, $65,000 a pop Rayleigh made useful with dynamic refocus of rising pulse 5 lasers give decent Strehl over 1 arcminute in H band for m telescope with 2-3 deformable mirrors total guide stars diffraction limited in same band can be very faint, good sky cover With more lasers and dms with higher resolution, cover to 0.5 micron wavelength should be possible over 1 arcmin natural guide star needs photons in time r 0 /v wind from whole telescope. 400/m 2 /sec for 8 m telescope = 18.5 mag. Near all sky cover, given corrected 1 arcminute field

31 Optical detection and spectroscopy of Earth-like planets

32 The sun is a million to 10 billion times brighter than the Earth in reflected light, only a 10 million times in thermal infrared

33 Original Bracewell nulling interferometer concept Bracewell proposed space infrared nulling interferometer to detect thermal emission of giant exo-planets (Nature, 1978)

34 Nulling Interferometry: Hinz Lab Verification Constructive image 2% of peak 0.5% of peak Scanning pathlength White=5% of peak

35 Magellan Results: a protoplanetary disk Constructive Null å Mus (calibration star) HD (possible young solar system) remaining flux in nulled frame is a direct image of a protoplanetary disk surrounding the star (Hinz)

36 TPF built as free flyers

37 How TPF would see the spectrum of an Earth twin at 10 pc Simulated spectrum by Angel and Woolf

38 An achievable next step Simple Bracewell system, 9 m long (Woolf, Lockheed Martin)

39 Optical detection of earthlike planets. How well can we do from the ground? More generally, what is the proper ground/space balance for all astronomy in the optical, where sky background is not serious?

40 N little arrows in line add up to amplitude n and intensity n 2

41 Reverse problem - how to get the star arrows to add to zero at the image of the planet The problem - contrast ratio 10 10, angular separation 0.1 arcsec (solar system twin at 10 pc)

42 Average halo strength of scattered starlight, given wavefront correction accuracy limited by measurement photon noise. Intensity at any point in halo given by square of sum of little arrows. Suppose wavefront correction is made over n subapertures each contributing unit amplitude from star. The correction errors are << 1 radian, spatially uncorrelated. At the star image, with all in arrows in phase, a* total = n, I*~ n 2 (whopping arrow) For the field around the star, suppose the aperture of diameter D is apodized, so that if there were no wavefront errors, the stellar amplitude is zero in the halo beyond some radius ( 5-10 λ/d). In fact, errors in wavefront correction turn the arrows by small angles to the left or right, equivalent to adding small arrows of length dφ. The rms total sum amplitude is now 0+the drunkards walk sum of the added small arrows, ie a halo = n dφ, and I halo = n dφ 2. If the phase is measured with k photons per subaperture, dφ = 1/ k, and I halo = n/k, and I halo /I* = 1/kn = 1/(total number of photons measured across the full aperture).

43 Signal/noise ratio for detection of Earth-like planet in a star halo Let fluxes for planet and star be F p and F* photons/m 2 /sec Suppose the halo noise is uncorrelated for successive correction cycles of length τ the S/N ratio for one measurement cycle is F p /F*. τ F s A = F p τ A in integration time T we improve the S/N ratio as root T/τ S/N = F p A (Tτ) (proportional to the planet flux but independent of the star flux) Example: For solar system twin at 10 pc, F earth =10-2 /m 2 /sec A=700 m 2 (D=30 m), τ = 1 msec =>S/N = 12 in 1 hour

44 Details Potentially reachable limits with 30 m class aperture λ/d ~ 7 mas, planet at radius 14 λ/d. Apodization requirement will not cost much light technical needs: high res, high speed correction knowledge of the wavefront evolution so reconstructor can be continuously updated and errors assessed. Forward prediction to avoid repetition of same speckle pattern. (Sandler and Stahl have modeled this numerically) Mirror smoothness and diffraction, continuity Space telescopes must be smaller, and lose much of their aperture to apodization. To do the same job they require very high surface accuracy.

45 Conclusions 6 m optical fits vision of 6m launch capability uniquely powerful for lambda < 0.5 microns and fields > 1 arcminute at diffraction limit May be useful for exoplanet studies, but requires exraordinary new technology for space with absolute tolerance held to picometer levels