Optical Design of Giant Telescopes for Space Jim Burge, Erin Sabatke Optical Sciences Center Roger Angel, Neville Woolf Steward Observatory University of Arizona
Jan 12, 2016
Optical Design of Giant Telescopes for Space
Jim Burge, Erin SabatkeOptical Sciences Center
Roger Angel, Neville Woolf
Steward Observatory
University of Arizona
The need for large telescopes
• Push back the frontier for astrophysics– We want to study what we can barely detect– We know that increased technology will detect new things
• Imaging planets around other stars– Requires blocking or nulling the star light
• Laser projectors for interstellar vehicles– Use light momentum to push the sail for interstellar travel
• Earth observations from geosynchronous
Telescopes in space
Hubble’s telescope
Mt. Wilson 100-in
1917
Hubble Space Telescope
1990
Natural evolution to large telescopes
• Make the primary larger– keep it in the shade– make the f/number faster to
limit length– effective optical surface using
smaller segments that can be launched and deployed
– maintain weight while increasing area
– Requires primary mirror with density ~15 kg/m2
Next Generation Space Telescope 2009
8-m aperture
Optical design issues for NGST
• Three mirror anastigmat, 10 arc min FOV• Fine steering mirror at a pupil
– image stabilization limited by field rotation, distortion
• Fast primary is highly aspheric and difficult to fabricate and test
Science Instruments
Tertiary mirror
Secondary mirror
8-m primary mirror
Fine Steering Mirror
Multiple Aperture Systems
• Increase baseline and collecting area by combining multiple apertures
Terrestrial Planet Finder
• 100-m array• Use nulling (destructive
interference) to cancel star light
• Detect planets and obtain low resolution spectra, looking for familiar atmospheric constituents
• Special PurposeVery small field of viewoptimized for exoplanets
• Solar orbit, benign thermal and gravity environment
TPF as free flying array of 3.5-m telescopes
Wavelength (µm)
Inte
nsity
7 8 9 10 11 12 13 14 1615
What about giant telescopes
• Size is limited by mass from mirror technology• NGST mirror technology could get to 5 kg/m2
• For economical launch with existing technology, need mass << 1 kg/m2
50 cm diameter mirror under construction
1 mm thick glass
7 gram actuators
1 kg/m2 composite support
Ultralight mirrors for space optics
• Lower mass mirrors require thinner substrates(<< 1 mm)
• The difficulty is support and control• Curved optics intrinsically require shape control• Flat optics can be made by simply stretching a
thin membrane
Error in reflective surface = half of thickness variation
Control for flat membrane mirrors
• Start with thin, reflective membrane of uniform thickness
• Hold it in tension from a plane at the perimeter
• Define the perimeter with multiple points, each one under active control.
• Reduces shape control to 1 dimension - perimeter
Membrane with reflective coating
Shape control with actuators at nodes
Tension control
Rigid frame
Membrane mirror technology
• Numerous developments underway at University of Arizona
(Stamper et al. In Imaging Technology and Telescopes, presented Sunday).
What good are flats?
• Collect light using diffraction– from Rod Hyde, Livermore
– limited bandwidth, contrast
• Or use an array of flats to approximate a paraboloidal reflector– like solar collectors– downstream optics compensate for non-curvature
Primary made from flat segments
Optical design issues for primary made from flats
• On axis - easy– make different segments come to focus at the same place
with the same path length
• For field of view - tricky. For each subaperture system, must also– meet sine condition (constant mapping of entrance pupil to
exit pupil)– match image scale and distortion – match field curvatures
• The general solution is to make the effective focal ratio of the primary as long as possible
Telescope with free flying elements
5 0 m p r i m a r y
1 0 m c o m b i n i n gt e l e s c o p e
S u n s h i e l d
S u n s h i e l d
1 k m
Faster telescopes for “conventional” rigid systems
Slower designs for telescope with free flying elements
Transition to Membranes
F/1 systems F/20 systems
Telescope diameter 2.6m 8.0m 14m 25m 100m
surface density 150Kg/m2 16Kg/m2 5Kg/m2 1.6Kg/m2 0.1Kg/m2
Mass 800Kg 800Kg 800Kg 800Kg 800Kg
Moment of Inertia 1 unit 10 30 36,000 600,000
Rotation period for samethruster expended 1 3 5 190 800
Rotation period for samereaction wheel use 1 10 30 36,000 600,000
Membrane telescopes are for long observations of ultra-faint objects only. General Purpose telescopes should be restricted to rigid mirrors.
The length comes at the price of system agility
Truss for large primary mirror
(Tom Connors, Steward Observatory)
Optical design and analysis
• Simulations using Optima (from Lockheed Martin)• Test case with several flat mirrors
Analysis of flat mirror telescope
On axis
PSF
OPD
1 arc minute
Pupil mapping and phase errors
Mapping distortion of entrance pupil (h) to exit pupil (h’)couples with wavefront tilt to cause phase errors
Entrance
pupil
Exit pupilNo distortion
From Object
To image
Phase errorin wavefront
Wavefront ispreserved
Wavefront
Exit pupil withnon-linearmapping
Entrancepupil
’
h
h’
Phase fromWF ‘tilt’
Ideal mapping Distorted mapping
‘
Definition of “sine condition”
Spherical entrance pupil, coordinates of sin(U)
Spherical exit pupil, coordinates of sin(U’)
Sine condition requires linear mapping ofsin(U) -> sin(U’)
Sine condition violation
• Geometric pupil distortion causes violation of sine condition, which varies with field
• This causes images to dephase for < 1 arcmin FOV
Quantifying sine condition violation
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 0.5 1 1.5 2 2.5 3 3.5
Field angle (arcminutes)
Sin
e co
nd
itio
n v
iola
tio
n:
% d
evia
tio
n f
rom
on
-axi
s v
alu
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f si
ne
con
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ion
Optical design summary
The design with flat segments works!
However, the field of view is limited by the apertures dephasing with field, from the sine condition violation
Preliminary results indicate that a 100-m telescope of this type could be made with
6 meter segments2 km length7 m secondary (spherical relay)10 meter corrector (60 cm elements)0.3 µm rms wavefront errors for 20 arcsec FOV(40 nm rms for 6 arc seconds)
Curving the primary
The system works much better if the primary can be curvedElectrostatics can be used to do this
A two-mirror 100-m telescope can achieve 5 arc minute FOV at f/20 with
2 km length 10 m concave secondary0.4 µm rms wavefront error
The 6 m primary segments have 4 mm sag
This has 40 nm rms wavefront error at 1.6 arc minute FOV
Stretched Membrane with Electrostatic Curvature
• Primary mirrors with membrane reflectors can be made from slightly curved segments (using electrostatics)
The moon imaged at Steward Observatory with the first telescope to use a primary mirror of Stretched Membrane with Electrostatic Curvature (SMEC). The silicon nitride membrane was 0.7 um thick and curved to a 3 m focal length by a field of 2 MV/m
What about a strip mirror
Simulated performance 100m x 2m
HST
Slot,1 exposure
NGST
Slot18 exposures
(Keith Hege, Steward Observatory)
Truss modeled for strip mirror
(Tom Connors, Steward Observatory)
Summary
There is no evolutionary path from today’s systems to giant telescopes in space.
Launch constraints require low mass, leading to optics made from membranes.
Orbital mechanics allows the use of free flying elements and sunshields.
Primary mirrors, made from arrays of flat mirrors can provide corrected images.
With added weight and complexity, the membranes can be moderately curved, gaining an order of magnitude in field of view.