David M. Stubbs

High bandwidth fast steering mirror

A high bandwidth, gimbaled, fast steering mirror (FSM) assembly has been designed and tested at the Lockheed Martin Space Systems Company (LMSSC) Advanced Technology Center (ATC). The design requirements were to gimbal a 5 cm diameter mirror about its reflective surface, and provide 1 KHz tip/tilt/piston control while maintaining λ/900 flatness of the mirror. The simple, yet very compact and rugged device also has manual tip/tilt/piston alignment capability. The off-the-shelf Piezo translators (PZT) actuators enable reliable and repeatable closed loop control. The adopted solution achieves a good mass balance and gimbaled motion about the center of the mirror front surface. Special care was taken to insure the best positioning means with the mounted mirror assembly held kinematically in place. The manual adjusters have very good resolution, with the capability to be locked in place. All solutions were thoroughly modeled and analyzed. This paper covers the design, analysis, fabrication, assembly, and testing of this device. The FSM was designed for ground test only.

Sensitivity Evaluation of Mounting Optics Using Elastomer and Bipod Flexures

A sensitivity evaluation of mounting 100mm optics using elastomer or bipod flexures was completed to determine the relative effects of geometry, structure, material, thermal and vibration environment as they relate to optical distortion. Detailed analysis was conducted using various finite element-modeling methods. Parts were built and the results were verified by conducting brassboard tests. What makes this evaluation noteworthy is the two vastly different approaches, and how they both exhibited athermal properties and minimized optical distortion. Materials were carefully selected while the geometry and structure were optimized through analytical iteration. The elastomeric optical mount consists of 12 equally spaced pads of RTV placed around the circumference of the optic. These pads were sized to maximize stiffness and minimize surface deformations. The surrounding material was appropriately selected in order to contribute to an athermal design. The bipod flexure optical mount uses three flexures cut from a single piece of material. Each flexure is a bipod oriented to comply radially with changes in temperature. This design is monolithic and uses conventional epoxy at the optical interface. The result is a very stiff athermal design. This paper covers both opto-mechanical designs, as well as analytical results from computer modeling and brassboard tests.

Space Interferometry Mission starlight and metrology subsystems

The Space Interferometry Mission (SIM), planned for launch in 2009, will measure the positions of celestial objects to an unprecedented accuracy of 4.0 microarcseconds. In order to achieve this accuracy, which represents an improvement of almost two orders of magnitude over previous astrometric measurements, a ten-meter baseline interferometer will be flown in space. NASA challenges JPL and its industrial partners, Lockheed Martin and TRW, to develop an affordable mission. This challenge will be met using a combination of existing designs and new technology. Performance and affordability must be balanced with a cost-conscious Systems Engineering approach to design and implementation trades. This paper focuses on the Lockheed Martin-led Starlight (STL) and Metrology (MET) subsystems within the main instrument of SIM. Starlight is collected by 35cm diameter telescopes to form fringes on detectors. To achieve the stated accuracy, the position of these white-light fringes must be measured to 10-9 of a wavelength of visible light. The STL Subsystem consists of siderostats, telescopes, fast steering mirrors, roof mirrors, optical delay lines and beam combiners. The MET Subsystem is used to measure very precisely the locations of the siderostats with respect to one another as well as to measure the distance traveled by starlight from the siderostat mirrors and reference corner cubes through the system to a point very close to the detectors inside the beam combiners. The MET subsystem consists of beam launchers, double and triple corner cubes, and a laser distribution system.

Multiple-aperture telescope array with a high fill factor

Traditionally a telescope system consists of a large collecting element, usually called the primary, located at the entrance pupil and some smaller elements to relay or convey the light to an image plane. As telescope systems become larger and larger, in order to achieve higher resolution and collect more light, a point is reached where the size of the required elements exceeds the current state of the art in fabrication and support. For telescopes larger than this, the entrance pupil must either be divided into manageable segments, or the entrance pupil is divided into an array of separate telescopes. A multiple telescope array consists of afocal collector telescopes distributed in the entrance pupil, relay optics to bring the light to the center and control tilt and piston errors, and a focal combiner telescope to form the image. Sparse telescope arrays have been designed for various applications. This paper addresses the issues and design constraints leading to a multiple telescope array with a high fill factor.

Adhesive bond cryogenic lens cell margin of safety test

The Near Infrared Camera (NIRCam) instrument for NASAs James Webb Space Telescope (JWST) has an optical prescription which employs four triplet lens cells. The instrument will operate at 35K after experiencing launch loads at approximately 295K and the optic mounts must accommodate all associated thermal and mechanical stresses, plus maintain an exceptional wavefront during operation. Lockheed Martin Space Systems Company (LMSSC) was tasked to design and qualify the bonded cryogenic lens assemblies for room temperature launch, cryogenic operation, and thermal survival (25K) environments. The triplet lens cell designs incorporated coefficient of thermal expansion (CTE) matched bond pad-to-optic interfaces, in concert with flexures to minimize bond line stress and induced optical distortion. A companion finite element study determined the bonded systems sensitivity to bond line thickness, adhesive modulus, and adhesive CTE. The design team used those results to tailor the bond line parameters, minimizing stress transmitted into the optic. The challenge for the Margin of Safety (MOS) team was to design and execute a test that verified all bond pad/adhesive/ optic substrate combinations had the required safety factor to generate confidence in a very low probability optic bond failure during the warm launch and cryogenic survival conditions. Because the survival temperature was specified to be 25K, merely dropping the test temperature to verify margin was not possible. A shear/moment loading device was conceived that simultaneously loaded the test coupons at 25K to verify margin. This paper covers the design/fab/SEM measurement/thermal conditioning of the MOS test articles, the thermal/structural analysis, the test apparatus, and the test execution/results.

Multiple instrument distributed aperture sensor (MIDAS) science payload concept

We describe the Multiple Instrument Distributed Aperture Sensor (MIDAS) concept, an innovative approach to future planetary science mission remote sensing that enables order of magnitude increased science return. MIDAS provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical interferometry technologies. All telescope optical assemblies are integrated into MIDAS as the primary remote sensing science payload, thereby reducing the cost, resources, complexity, I&T and risks of a set of back-end science instruments (SIs) tailored to a specific mission. MIDAS interfaces to multiple science instruments, enabling sequential and concurrent functional modes, thereby expanding the potential planetary science return many fold. Passive imaging modes with MIDAS enable remote sensing at diffraction-limited resolution sequentially by each science instrument, or at lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. For planetary science missions, the MIDAS optical design provides high-resolution imaging for long dwell times at high altitudes, thereby enabling real-time, wide-area remote sensing of dynamic surface characteristics. In its active remote sensing modes, using an integrated solid-state laser source, MIDAS enables LIDAR, vibrometry, surface illumination, and various active or ablative spectroscopies. Our concept is scalable to apertures well over 10m, achieved by autonomous deployments or manned assembly in space. MIDAS is a proven candidate for future planetary science missions, enabled by our continued investments in focused MIDAS technology development areas. In this paper we present the opto-mechanical design for a 1.5m MIDAS point design, including its accommodation of back-end science instruments.

Cryogenic bonding for lens mounts

Lockheed Martin Space Systems Company (LMSSC) has performed a feasibility study for bonded cryogenic optical mounts. That investigation represents a combined effort of design, experiments and analysis with the goal to develop and validate a working cryogenic mount system for refractive lens elements. The mount design incorporates thermal expansion matched bond pads and radial flexures to reduce bondline stress and induced optical distortion. Test coupons were constructed from lens and selected mount materials and bonded with candidate adhesives to simulate the designs bond pads. Thermal cycling of those coupons to 35K demonstrated both the systems survivability and the bonds structural integrity. Finally, a companion finite element study determined the bonded systems sensitivity to bondline thickness, adhesive modulus and adhesive CTE. The design team used those results to tailor the bondline parameters to minimize stress transmitted into the optic.

Remote sensing space science enabled by the multiple instrument distributed aperture sensor (MIDAS) concept

The science capabilities and features of an innovative and revolutionary approach to remote sensing imaging systems aimed at increasing the return on future planetary science missions many fold are described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field, diffraction-limited telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical imaging interferometer technologies into a multi-functional remote sensing science payload. MIDAS acts as a single front-end actively controlled telescope array for use on common missions, reducing the cost, resources, complexity, and risks of developing a set of back-end science instruments (SIs) tailored to each specific mission. By interfacing to multiple science instruments, MIDAS enables either sequential or concurrent SI operations in all functional modes. Passive imaging modes enable remote sensing at diffraction-limited resolution sequentially by each SI, as well as at somewhat lower resolution by multiple SIs acting concurrently on the image, such as in different wavebands. MIDAS inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the SIs. Our optical design features high-resolution imaging for long dwell times at high altitudes, <1m GSD from the 5000km extent of spiral orbits, thereby enabling regional remote sensing of dynamic planet surface processes, as well as ultra-high resolution of 2cm GSD from a 100km science orbit that enable orbital searches for signs of life processes on the planet surface. In its active remote sensing modes, using an integrated solid-state laser source, MIDAS enables LIDAR, vibrometry, surface illumination, ablation, laser spectroscopy and optical laser communications. The powerful combination of MIDAS passive and active modes, each with sequential or concurrent SI operations, increases potential science return for space science missions many fold. For example, on a mission to the icy moons of Jupiter, MIDAS enhances detailed imaging of the geology and glaciology of the surface, determining the geochemistry of surface materials, and conducting seismic and tidal studies.

Multiple Instrument Distributed Aperture Sensor (MIDAS) For Planetary Remote Sensing

An innovative approach that enables greatly increased return from planetary science remote sensing missions is described. Our concept, called Multiple Instrument Distributed Aperture Sensor (MIDAS), provides a large-aperture, wide-field telescope at a fraction of the cost, mass and volume of conventional space telescopes, by integrating advanced optical interferometry technologies. All optical assemblies are integrated into MIDAS as the primary remote sensing science payload, thereby reducing the cost, resources, complexity, integration and risks of a set of back-end science instruments (SI’s) tailored to a specific mission, such as advanced SI’s now in development for future planetary remote sensing missions. MIDAS interfaces to multiple SI’s for redundancy and to enable synchronized concurrent science investigations, such as with multiple highly sensitive spectrometers. Passive imaging modes with MIDAS enable high resolution remote sensing at the diffraction limit of the overall synthetic aperture, sequentially by each science instrument as well as in somewhat lower resolution by multiple science instruments acting concurrently on the image, such as in different wavebands. Our MIDAS concept inherently provides nanometer-resolution hyperspectral passive imaging without the need for any moving parts in the science instruments. In its active remote sensing modes using an integrated laser subsystem, MIDAS enables LIDAR, vibrometry, illumination, various active laser spectroscopies such as ablative, breakdown, fluorescence, Raman and time-resolved spectroscopy. The MIDAS optical design also provides high-resolution imaging for long dwell times at high altitudes, thereby enabling real-time, wide-area remote sensing of dynamic changes in planet surface processes. These remote sensing capabilities significantly enhance astrobiologic, geologic, atmospheric, and similar scientific objectives for planetary exploration missions.

Optomechanical performance of 3D-printed mirrors with embedded cooling channels and substructures

Advances in 3D printing technology allow for the manufacture of topologically complex parts not otherwise feasible through conventional manufacturing methods. Maturing metal and ceramic 3D printing technologies are becoming more adept at printing complex shapes, enabling topologically intricate mirror substrates. One application area that can benefit from additive manufacturing is reflective optics used in high energy laser (HEL) systems that require materials with a low coefficient of thermal expansion (CTE), high specific stiffness, and (most importantly) high thermal conductivity to effectively dissipate heat from the optical surface. Currently, the limits of conventional manufacturing dictate the topology of HEL optics to be monolithic structures that rely on passive cooling mechanisms and high reflectivity coatings to withstand laser damage. 3D printing enables the manufacture of embedded cooling channels in metallic mirror substrates to allow for (1) active cooling and (2) tunable structures. This paper describes the engineering and analysis of an actively cooled composite optical structure to demonstrate the potential of 3D printing on the improvement of optomechanical systems.