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
Magellan Project - Las Campanas Observatory
November 2000
LBT Enclosure on Mt. Graham
December 2000
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
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 Secondary
being assembled in Italy Integration in Tucson January 2001
R. Biasi
D. Gallieni
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
High authority concept for space mirrors with quasistatic actuators
Ideal shape
Structure deforms,
taking membrane with it
Actuators are driven
to compensate
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
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
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
5m
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
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.
General layout of deployed telescope
Large diffraction limited field (Burge)
4.3 x 4.3
arcmin IFOV
Annular Field from 3-mirror telescope
Fully corrected, unvignetted
24 arcmin OD
12 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
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 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
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
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
2.2
15
1.1
12.3
1W
150 nm
.55
9.6
10 W
75 nm
.3
7
100 W
38 nm
-
4.3
300 nm
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)
Nulling Interferometry: Hinz Lab Verification
Constructive image
2% of peak
0.5% of peak
Scanning pathlength
White=5% of peak
Magellan Results: a protoplanetary disk
Constructive
Null
å Mus
(calibration star)
HD 100546
(possible young solar system)
remaining flux in nulled frame is a direct image
of 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 n 2
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
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.
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