This paper is the third in the series on the technology development for the EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission concept, which in 2011 was selected by NASA's Explorer program for technology development (Category III). EXCEDE is a 0.7m space telescope concept designed to achieve raw contrasts of 1e6 at an inner working angle of 1.2 l/D and 1e7 at 2 l/D and beyond. This will allow it to directly detect and spatially resolve low surface brightness circumstellar debris disks as well as image giant planets as close as in the habitable zones of their host stars. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and scientific precursor for any future exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the PIAA coronagraph, enabling aggressive performance. Previously, we reported on the achievement of our first milestone (demonstration of EXCEDE IWA and contrast in monochromatic light) in air. In this presentation, we report on our continuing progress of developing the SSS for EXCEDE, and in particular (a) the reconfiguration of our system into a more flight-like layout, with an upstream deformable mirror and an inverse PIAA system, and (b) testing this system in a vacuum chamber, including IWA, contrast, and stability performance. Even though this technology development is primarily targeted towards EXCEDE, it is also germane to any exoplanet direct imaging space-based telescopes because of the many challenges common to different coronagraph architectures and mission requirements. This work was supported in part by the NASA Explorer program and Ames Research Center, University of Arizona, and Lockheed Martin SSC.
The Interface Region Imaging Spectrograph (IRIS) is a NASA SMall Explorer (SMEX) mission launched onboard a Pegasus™ booster on June 27, 2013. The spacecraft and instrument were designed and built at the Lockheed Martin Space Systems Company. The primary mission goal is to learn how the solar atmosphere is energized. IRIS will obtain high-resolution UV spectra and images in space (0.4 arcsec) and time (1s), focusing on the chromosphere and transition region of our sun, which is a complex interface region between the photosphere and corona. The IRIS instrument uses a Cassegrain telescope to feed a dual spectrograph and slit-jaw imager, which operate in the 133-141 nm and 278-283 nm wavelengths, respectively. Within the spectrograph there are sixteen optics, each requiring subtle mounting features to meet exacting surface figure and stability requirements. This paper covers the opto-mechanical design for the most challenging optic mounts, which include the Collimator, UV Fold Mirrors, and UV Gratings. Although all mounts are unique in size and shape, the fundamental design remains the same. The mounts are highly kinematic, thermally matched, and independent of friction. Their development will be described in detail, starting with the driving requirements and an explanation of the underlying design philosophy.
The Near Infrared Camera (NIRCam) instrument for NASA's 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 system's 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.
Experimental demonstrations of optical synthetic aperture imaging using spatial heterodyne interferometry have been
achieved at the Lockheed Martin Advanced Technology Center in Palo Alto, CA. In laboratory experiments, a reflective
binary star scene and an Air Force resolution bar target were illuminated and imaged by a 532 nm laser and an afocal
telescope. The real aperture diffraction limit in the horizontal direction was 65 microRadians. Complex pupil
information was obtained by mixing the scattered return light from the target with light from an off-axis local oscillator,
thus forming a linear fringe pattern on a CCD array placed at the pupil plane. Fourier transform methods were used to
extract pupil amplitude and phase. By translating the real aperture pupil, collecting data at different locations, and
extracting and combining the pupil data, a synthetic aperture twice the real aperture size was created. In the
reconstructed image resulting from the synthetic aperture pupil data, features down to 32 microRadians were clearly
High spectral resolution Fourier transform imaging spectroscopy has been demonstrated at the Lockheed Martin
Advanced Technology Center. A testbed was built using a Michelson interferometer with a two-stage end-mirror control
system. Homodyne laser metrology was used to sense relative tip, tilt and piston in the interferometer, and a 3-degree of
freedom fast steering mirror in conjunction with a linear actuator stage provided sub-nanometer actuation control over
20 millimeters of piston range. The range of piston over which signal was present allowed for spectral resolution at the
nanometer level in the visible / near infrared (VNIR) band for every pixel in the reconstructed image.