NASA is currently developing optical communications to use with its spacecraft—both in earth-orbit and in deep space.
This may allow spacecraft to use small, pencil-beam telescopes instead of large, wide-beam microwave antennas,
potentially saving weight, reducing transmission power, and increasing communications bandwidth. The Earth side of
such communications links will require a network of low cost, ground-based telescopes.
The ground support mission mentioned above would benefit from the development of lightweight, low cost, 1 to 2 meter
aperture telescopes. The key is the development of low cost, diffraction limited mirrors that cost orders of magnitude
less than NASA’s current telescope mirrors, have a drastically reduced manufacturing time, with significant weight
reduction (low areal density).
Spin-cast epoxy mirrors do not require any grinding, polishing, or figuring and therefore have the potential for low cost,
short production time, and light weight. The specially-formulated thin epoxy described here naturally forms a parabolic
surface when spun at constant velocity and once it hardens, the mirror surface is ready for use except for a reflective
A recently produced 50cm diameter f/2 spin-cast epoxy mirror has been measured to have a 6-8 micron RMS surface
figure deviation and approximately 1 nm microroughness. Other advances include the synthesis and co-polymerization
of spiro orthocarbonate compounds (SOCs) to reduce chemical shrinkage and the engineering of a stiff mold to hold the
curing epoxy as it spins.
The nineteenth century Fraunhofer primary objective grating (POG) telescope has been redesigned with a secondary
spectrometer. The POG is embossed on a membrane and placed at an angle of grazing exodus relative to a conventional
spectrographic telescope. The result is a new type of telescope that disambiguates overlapping spectra and can capture
spectral flux from all objects over its free spectral range, nearly 40°. For space deployment, the ribbon-shaped membrane
can be stowed as a cylinder under a rocket fairing for launch and deployed in space from a cylindrical drum. Any length
up to kilometer scale could be contemplated.
Presented is a novel optical system using Cis-Trans photoisomerization where nearly every molecule of a mirror
substrate is itself an optically powered actuator. Primary mirrors require sub-wavelength figure (shape) error in order to
achieve acceptable Strehl ratios. Traditional telescopy methods require rigid and therefore heavy mirrors and reaction
structures as well as proportionally heavy and expensive spacecraft busses and launch vehicles. Areal density can be
reduced by increasing actuation density. Making every molecule of a substrate an actuator approaches the limit of the
areal density vs actuation design trade space.
Cis-Trans photoisomerization, a reversible reorganization of molecular structure induced by light, causes a change in the
shape and volume of azobenzene based molecules. Induced strain in these "photonic muscles" can be over 40%. Forces
are pico-newtons/molecule. Although this molecular limit is not typically multiplied in aggregate materials we have
made, considering the large number of molecules in a mole, future optimized systems may approach this limit
In some π-π* mixed valence azo-polymer membranes we have made photoisomerization causes a highly controllable
change in macroscopic dimension with application of light. Using different wavelengths and polarizations provides the
capability to actively reversibly and remotely control membrane mirror shape and dynamics using low power lasers,
instead of bulky actuators and wires, thus allowing the substitution of optically induced control for rigidity and mass.
Areal densities of our photonic muscle mirrors are approximately 100 g/m2. This includes the substrate and actuators
(which are of course the same). These materials are thin and flexible (similar to saran wrap) so high packing ratios are
possible, suggesting the possibility of deployable JWST size mirrors weighing 6 kilograms, and the possibility of
ultralightweight space telescopes the size of a football field. Photons weigh nothing. Why must even small space
telescopes weigh tons? Perhaps they do not.
A new class of astronomical telescope with a primary objective grating (POG) has been studied as an alternative to
mirrors. Nineteenth century POG telescopes suffered from low resolution and ambiguity of overlapping spectra as well
as background noise. The present design uses a conventional secondary spectrograph to disambiguate all objects while
enjoying a very wide instantaneous field-of-view, up to 40°. The POG competes with mirrors, in part, because
diffraction gratings provide the very chromatic dispersion that mirrors defeat. The resulting telescope deals effectively
with long-standing restrictions on multiple object spectrographs (MOS). The combination of a POG operating in the
first-order, coupled to a spectrographic astronomical telescope, isolates spectra from all objects in the free spectral range
of the primary. First disclosed as a concept in year 2002, a physical proof-of-principle is now reported. The miniature
laboratory model used a 50 mm plane grating primary and was able to disambiguate between objects appearing at
angular resolutions of 55 arcseconds and spectral spacings of 0.15 nm. Astronomical performance is a matter of
increasing instrument size. A POG configured according to our specifications has no moving parts during observations
and is extensible to any length that can be held flat to tolerances approaching float glass. The resulting telescope could
record over one million spectra per night of objects in a line of right ascension. The novel MOS does not require pre-imaging
to start acquisition of uncharted star fields. Problems are anticipated in calibration and integration time. We
propose means to ameliorate them.
Photons weigh nothing. Why must even small space telescopes weigh tons? Primary mirrors require sub-wavelength
figure (shape) error in order to achieve acceptable Strehl ratios. Traditional telescopy methods require rigid and
therefore heavy mirrors and reaction structures as well as proportionally heavy and expensive spacecraft busses and
launch vehicles. Our team's vision is to demonstrate the technology for making giant space telescopes with 1/2000 the
areal density of the Hubble. Progress on a novel actuation approach is presented. The goal is to lay groundwork to
achieve a 10 to 100 fold improvement in spatial resolution and a factor of 10 reduction in production and deployment
cost of active optics. This entailed the synthesis and incorporation of photoactive isomers into crystals and polyimides to
develop nanomachine laser controlled molecular actuators.
A large photomechanical effect is obtained in polymers 10-50 μm thick. Laser-induced figure variations include the
following: 1) reversible bi-directional bending; 2) large deformation range; 3) high speed deformation; and 4) control
with a single laser (~0.1 W/cm2). Photolyzation data presented showing reversible semi-permanence of the
photoisomerization indicates that a scanned 1 watt laser rather than a megawatt will suffice for large gossamer structure
Areal density can be reduced by increasing actuation. Making every molecule of a substrate an actuator approaches the
limit of the design trade space. Presented is a photomechanical system where nearly every molecule of a mirror
substrate is itself an optically powered actuator. Why must even small space telescopes weigh tons? Data suggests they
A ribbon-shaped primary objective grating (POG) telescope lends itself to deployment in space, because it can be stowed
for transport on a roll. Unlike mirrors which need to be segmented for sizes beyond the diameter of the fairing or
payload bay, the ribbon is a continuous integral surface transported on a drum and unfurled during deployment. A flat
POG membrane abandons a standard three dimensional figure requirement of mirrors and solves the problem of making
primary objectives from tensile structures. Moreover, POG telescopes enjoy relaxed surface dimensional tolerances
compared with mirrors. We have demonstrated mathematically and empirically that the tolerance for flatness relaxes as
the receiving angle increases toward grazing exodus where the magnification of the POG is greatest. At the same time,
the tolerance for phase error is worsened as the angle of reconstruction moves toward grazing exodus. The problem will
be aggravated by the rigors of the space deployment environment. We give a mathematical treatment for the flatness and
phase error. We mention engineering methods that could ameliorate the error.
We discuss a transmission primary objective grating (POG) telescope that is nearly flat to the ground with its secondary
components buried below ground in a protected environment that enjoys a controlled atmosphere. Temperature gradients
can be held steady by sealing this enclosure. End-to-end ray paths need not be interrupted by spiders or other structural
support elements. Unlike mirror and lens telescopes, this layout is intrinsically off-axis. Light diffracted from a POG at a
grazing angle can be collected a few meters below the POG, and the substructures do not require a deep excavation, as
would be required for buried on-axis mirrors such as a zenith tube. The POG principle can take advantage of the rotation
of the earth to acquire spectra sequentially, so active tilt and rotate axes are not necessary during observations. The POG
aperture is extensible as a ribbon optic to kilometer scale at a linear increase in cost, as compared to other choices where
infrastructure grows as the cube of the telescope size. The principle of operation was proven in miniature during bench
tests that show high resolution spectra can be obtained at angular resolutions equal to seeing. Mathematical models of
the underlying relationships show that flux collection increases with increased angles of grazing exodus even as
efficiency decreases. Zemax models show a 30° field-of-view and the capacity to take spectra of all sources within that
very wide field-of-view. The method lends itself to large apertures, because it is tolerant of POG surface unevenness.
Inside an aircraft fuselage there is little room for the mass of all the instrumentation of a ground-based observatory much
less a primary objective aperture at the scale of 10 meters. We have proposed a solution that uses a primary objective
grating (POG) which matches the considerable length of the aircraft, approximately 10 meters, and conforms to aircraft
aerodynamics. Light collected by the POG is diffracted at an angle of grazing exodus inside the aircraft where it is
disambiguated by an optical train that fits within to the interior tunnel. Inside the aircraft, light is focused by a parabolic
mirror onto a spectrograph slit. The design has a special benefit in that all objects in the field-of-view of the free spectral
range of the POG can have their spectra taken as the aircraft changes orientation. We suggest flight planes that will
improve integration times, angular resolution and spectral resolution to acquire targets of high stellar magnitudes or
alternatively increase the number of sources acquired per flight at the cost of sensitivity.
Reported is an investigation of a novel approach for producing and correcting active optical mirrors. Photoactive polymers represent a special class of "smart materials" whose electronic and physical properties such as conductivity, charge distribution, and especially shape can be changed in response to the environment (voltage, light, stress). The ability of photoactive polymers to change the structure of a polymer matrix in response to light is being studied to allow active figure control of membranes for optical element use. Photoactive substrates (mirrors) were produced. Incoherent light sources were used to effect shape control. Shack-Hartman Wavefront sensing was used to quantify the initial and optically altered figure of samples. Motion of two classes of samples was measured and is reported here. Proposed is also a new stress control technology as well as new hybrid technology combining two classes of photoactive materials.
The importance of research in optical nano-engineering today is comparable to that of research in semiconductors 60 years ago. Biologically inspired photoactive isomers are being engineered and incorporated into substrates to construct optically addressable nanomachine “laser controlled molecular actuators” which will provide non-contact active figure control, allowing a robust response of lightweight optics to pointing slewing, thermal perturbations, and misalignment. The ability of photoactive molecules to change structure within a matrix in response to light has application to minimizing optical element mass while drastically improving control authority of active optics. Ongoing experiments are providing a foundation for applications in the development of novel optically addressable light-activated shape control of deformable mirrors, as well as addressing issues like damping vibrations after re-pointing large space telescopes. Several types of systems are being studied. The goal is to design and then synthesize materials, generate a picture of molecular scale mechanical forces, bulk geometric distortions, and then to optimize the systems for active optical elements. This requires the design and synthesis of novel photoactive materials. Understanding the molecular-scale motion of photomechanical nano-machines require chemical studies, novel synthesis and fabrication techniques, spectroscopy, imaging molecular-scale mechanical responses and surfaces, optical metrology, and development of novel control techniques. After synthesizing appropriate photoactive substances, active substrates will be produced. A test apparatus is being developed to quantify control authority. Once the chemistry has evolved, a down selection will occur, and new substrates will be fabricated and tested. This talk will report results of this effort.