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

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

Part 1. Some Mirror Lab technology

6 mirrors 6.5 - 8.4 m

Magel lan Project - Las Campanas Observa tory

N o v e m b e r 2 0 0 0

LBT Enclosure on Mt. Graham

December 2000

Mag 2 , as t igmat ism subt rac ted14 nm rms sur face

Mage l l an 2 I n te r f e rog rams

HST quality in a 6.5 m mirror at f/1.25 already exists. Just need to get one in space

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

MMT336 Adaptive Secondarybeing assembled in Italy Integration in Tucson -

January 2001

R. Biasi

D. Gallieni

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

MMT adaptive secondary mirror, 64 cm diameter, 2 mm thick, highly aspheric secondary for f/1.25 primary. Collaboration with Arcetri Observatory

High authority concept for space mirrors with quasi-static actuators

Ideal shape

Actuators are drivento compensate

Structure deforms,taking membrane with it

2 mm thick NMSD mirror for lightweight space prototype

Current progress with 2-m NMSD, 13 kg/m2 system

Technician attaching sub-loadspreaders to back of 2-mm thick facesheet on temporary support

Fully assembled NRO prototype, June 2001 (Jim Burge)

1 kg total mass (including glass, support structure, actuators and hardware, cabling,

50 cm diameter

Single HeNe Interferogram (633 nm)Gray scale version from phase-shifting

interferometer

165 nm rms - limited by gravity deflection over supports

Results of interferometric metrology, NRO prototype at 633 nm wavelength

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?

Present surveillance telescopes:

Titan rocket 3 m diameter

payload fairing 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?

A longer term goal for space astronomy -a 15 m telescope in 6 segments with 6 m fairing

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.

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

General layout of deployed telescope

Large diffraction limited field (Burge)

Annular Field from 3-mirror telescope Fully corrected, unvignetted 24 arcmin OD12 arcmin ID

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

4.3 x 4.3 arcmin IFOV

Field accessible with steering mirror

Davison deployment sequence

The first radial support swings out

Then the top segment on the stack is swung out anticlockwise

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 EELVsfor 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

Part 3. Optical role in spaceThe 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

wavefront accuracy limited by photon noise of wavefront sensor, Kolmogorov

turbulencePhoton flux needed for reasonable Strehl scales as λ-3.6

lambda magnitude laser power rms wave error

2.2 15 300 nm

1.1 12.3 1W 150 nm

.55 9.6 10 W 75 nm

.3 7 100 W 38 nm

- 4.3 19 nm

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 6.5 - 8 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 - 100 photons in time r0/vwind from whole telescope. 400/m2/sec for 8 m telescope = 18.5 mag. Near all sky cover, given corrected 1 arcminute field

Optical detection and spectroscopy of Earth-like planets

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

“only” a 10 million times in thermal infrared

Original Bracewell nulling interferometer concept

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

Constructive image Scanning pathlength

0.5% of peak2% of peak

White=5% of peak

Nulling Interferometry: Hinz Lab Verification

Magellan Results: a protoplanetary diskConstructive Null

å Mus(calibration star)

HD 100546(possible young solar system)

remaining flux in nulled frame is a direct imageof a protoplanetary disk surrounding the star (Hinz)

TPF built as free flyers

How TPF would see the spectrum of an Earth twin at 10 pc

Simulated spectrum by Angel and Woolf

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

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?

N little arrows in line add up to amplitude n and intensity n2

Reverse problem - how to get the star arrows to add to zero at the

image of the planet

The problem -

contrast ratio 1010, angular separation 0.1 arcsec(solar system twin at 10 pc)

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*~ n2 (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

ahalo=√n dφ, and Ihalo = n dφ2.

If the phase is measured with k photons per subaperture, dφ = 1/√k, and

Ihalo = n/k, and Ihalo/I* = 1/kn

= 1/(total number of photons measured across the full aperture).

Signal/noise ratio for detection of Earth-like planet in a star halo

Let fluxes for planet and star be Fp and F* photons/m2/sec

Suppose the halo noise is uncorrelated for successive correction cycles of length τ

the S/N ratio for one measurement cycle is

Fp/F* . τ Fs A = Fp τ A

in integration time T we improve the S/N ratio as root T/τ

S/N = Fp A √ (Tτ)

(proportional to the planet flux but independent of the star flux)

Example: For solar system twin at 10 pc, Fearth=10-2 /m2/sec

A=700 m2 (D=30 m), τ = 1 msec =>S/N = 12 in 1 hour

DetailsPotentially 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.

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