The Line Emission Mapper (LEM) is a Probe mission concept developed in response to NASA’s Astrophysics Probe Explorer (APEX) Announcement of Opportunity. LEM has a single science instrument composed of a large-area, wide-field X-ray optic and a microcalorimeter X-ray imaging spectrometer in the focal plane. LEM is optimized to observe low-surface-brightness diffuse X-ray emission over a 30′ equivalent diameter field of view with 1.3 and 2.5 eV spectral resolution in the 0.2−2.0 keV band. Our primary scientific objective is to map the thermal, kinetic, and elemental properties of the diffuse gas in the extended X-ray halos of galaxies, the outskirts of galaxy clusters, the filamentary structures between these clusters, the Milky Way star-formation regions, the Galactic halo, and supernova remnants in the Milky Way and Local Group. The combination of a wide-field optic with 18′′ angular resolution end-to-end and a microcalorimeter array with 1.3 eV spectral resolution in a 5′ × 5′ inner array (2.5 eV outside of that) offers unprecedented sensitivity to extended low-surface-brightness X-ray emission. This allows us to study feedback processes, gas dynamics, and metal enrichment over seven orders of magnitude in spatial scales, from parsecs to tens of megaparsecs. LEM will spend approximately 11% of its five-year prime science mission performing an All-Sky Survey, the first all-sky X-ray survey at high spectral resolution. The remainder of the five-year science mission will be divided between directed science (30%) and competed General Observer science (70%). LEM and the NewAthena/XIFU are highly complementary, with LEM’s optimization for soft X-rays, large FOV, 1.3 eV spectral resolution, and large grasp balancing the NewAthena/X-IFU’s broadband sensitivity, large effective area, and unprecedented spectral resolving power at 6 keV. In this presentation, we will provide an overview of the mission architecture, the directed science driving the mission design, and the broad scope these capabilities offer to the entire astrophysics community.
Directly imaging Earth-like exoplanets (“exoEarths”) with a coronagraph instrument on a space telescope requires a stable wavefront with optical path differences limited to tens of picometers RMS during exposure times of a few hours. Although the structural dynamics of a segmented mirror can be directly stabilized with telescope metrology, another possibility is to use a closed-loop wavefront sensing and control system in the coronagraph instrument that operates during the science exposures to actively correct the wavefront and relax the constraints on the stability of the telescope. We present simulations of the temporal filtering provided using the example of LUVOIR-A, a 15-m segmented telescope concept. Assuming steady-state aberrations based on a finite-element model of the telescope structure, we (1) optimize the system to minimize the wavefront residuals, (2) use an end-to-end numerical propagation model to estimate the residual starlight intensity at the science detector, and (3) predict the number of exoEarth candidates detected during the mission. We show that telescope dynamic errors of 100 pm RMS can be reduced down to 30 pm RMS with a magnitude 0 star, improving the contrast performance by a factor of 15. In scenarios where vibration frequencies are too fast for a system that uses natural guide stars, laser sources can increase the flux at the wavefront sensor to increase the servo-loop frequency and mitigate the high temporal frequency wavefront errors. For example, an external laser with an effective magnitude of −4 allows the wavefront from a telescope with 100 pm RMS dynamic errors and strong vibrations as fast as 16 Hz to be stabilized with residual errors of 10 pm RMS thereby increasing the number of detected planets by at least a factor of 4.
The Polstar Mission uses time domain FUV and NUV spectropolarimetric observations to provide insight into how our galaxy became able to support a solar system like ours. Polstar will study the evolution of massive stars, including the inter-stellar dust and gas that they interact with and the dynamics of protoplanetary disks and lower resolution near UV (180nm – 320 nm) to study interstellar reddening mechanism. Polstar will simultaneously measure at high spectral resolving power all four Stokes parameters (I, Q, U, V)T to high accuracy (0.001) with a precision better than 0.0001 using an instrument comprising a modulator with rotating MgF2 retarders and a fixed MgF2 Wollaston prism analyzer that concurrently measures the two orthogonal polarizations of the retarder modulated signal at six optimized rotational angles on a common detector. The output of the Wollaston prism either directly enters an echelle spectrograph, which provides spectral resolution of ~30K with a wavelength range from 122 nm to 200 nm, or is reflected to a prism spectrograph, which provides spectral resolution ranging from 140– 4000 with a wavelength range from 180 nm to 320 nm. In this paper we outline the instrument calibration approach to obtain the Mueller matrices over the full wavelength range that permit us to determine the Stokes parameters, and to spectrally calibrate the instrument. We also describe the ground support equipment for calibration.
Polstar combines, for the first time, the complementary benefits of spectroscopy and polarimetry to probe the complex interface between massive stars and the interstellar medium. Furthermore, it leverages an innovative combination of effective area and time coverage, to reach the diversity of targets necessary to transform our understanding of the ecology of star and planet creation. Detailed knowledge of these bright, yet distant objects, is crucial for understanding the transformation of our galaxy, from the barren landscape of the early Big Bang, into the chemically enriched environment that produced the solar system we call home. Polstar will map stellar wind and magnetospheric structures by uniting time domain, polarimetry and spectroscopy capability in the near- and far-UV (NUV and FUV), which are densely populated with high-opacity resonance lines encoding a rich array of diagnostic information. UV spectropolarimetry is equally important for probing interstellar dust and protoplanetary disks. The instrument combines advances in high reflectivity UV coatings and delta-doped CCDs with high quantum efficiencies to provide dedicated FUV spectropolarimetry for the first time in 25 years. The FUV channel (Ch1), covers 122-200nm at resolution R>30k, while the NUV channel (Ch2) covers 122-320nm at R~140-4,000. The instrumental polarization stability is built to provide signal-to-noise ratios (SNR) for UV polarimetry precision of 1x10-3 per exposure per resolution element (resel). Precision can be further improved with spectral binning and/or stacking multiple exposures. Polstar spectral resolution in Ch1 is >30x better than WUPPE, with 10x better effective area, while reaching shorter wavelength than WUPPE to access strong lines of species like NIV and SiIV. The 3-year mission of Polstar is 100x longer than WUPPE with orders of magnitude gains in stellar and interstellar observations.
The Ultraviolet/Optical/Infrared (UVOIR) flagship astrophysics architectures proposed by the Astro2020 Decadal Survey fundamentally challenge the current test-like-you-fly approach to space systems, because of their physical scale, multiple stages of on-orbit deployment, and extremely stringent optical performance requirements unique to visible-light coronagraphy. These limitations elevate the importance of integrated control, structural dynamics, and optical modeling, particularly in early system architecture studies. A unique non-contact observatory control architecture called Disturbance Free Payload (DFP) for next-generation large astrophysics observatories involves physically isolating the segmented telescope structure from the supporting spacecraft by means of a non-contact interface. In this control architecture, rigidbody telescope pointing is achieved by actuating the payload with non-contact voice coil actuators and maintaining positive interface gaps using spacecraft inertial actuators and interface non-contact sensors. This architecture presents distinct advantages over current state-of-the-art spacecraft vibration isolation approaches, particularly for large flexible spacecraft, but also introduces unique disturbance and coupling mechanisms that must be analyzed. In this paper, development of an integrated model is described, consisting of a 6.7-meter inscribed segmented optical system, and an unobscured telescope with 55 primary mirror segments. The paper starts with an overview of the models that directly predict time-domain lineof-sight and wavefront error dynamic stability (optics, dynamics, control system, error sources). Next, key dynamic stability performance metrics for coronagraph contrast performance are described and a systematic methodology for realizing an accurate but computationally feasible truncated modal model is presented. Finally, an exemplar point design that is compliant to 10-picometer RMS wavefront error is developed, and the necessary component errors to achieve this performance are presented.
Future large space telescope missions demand extreme stability to enable high contrast coronagraphy for exoplanet observation. The wavefront control systems needed to achieve and maintain the required wavefront quality of the imaging system requires high-performance metrology sensors capable of picometer class sensitivity over long duration exposures, as well as for ground-based verification of build performance. For nearly two decades, Lockheed Martin has invested in developing laser metrology gauge technologies implemented in Photonic Integrated Circuits (PICs). We describe a high precision displacement metrology system currently under development and in test which has a path to flight for these future systems. Recent implementations have demonstrated picometer-class sensitivity at high (< 1 Hz) frequencies using largely commercial-off-the-shelf hardware. The current work aims to improve performance at longer timescales.
The Astro2020 decadal survey recommended a ~6m IR/O/UV telescope equipped with a coronagraph instrument to directly image exoEarths in the habitable zone of their host star. A telescope of such size may need to be segmented to be folded and then carried in current launch vehicles. However, a segmented primary mirror introduces the potential for mid spatial frequency optical wavefront instabilities during the science operations that would degrade the coronagraph performance. A coronagraph instrument with a wavefront sensing and control (WS&C) system can stabilize the wavefront with a picometer precision at high temporal frequencies (<1Hz). In this work, we study a realistic set of aberrations based on a finite element model of a slightly bigger (8m circumscribed, 6.7m inscribed diameter) segmented telescope with its payload. We model an adaptive optics (AO) system numerically to compute the post-AO residuals. The residuals then feed an end-to-end model of a vector vortex coronagraph instrument. The long exposure contrast thus obtained is finally used in an ExoEarth yield method calculation to understand the overall benefits of the adaptive optics system in the flagship mission success.
One of the primary science goals of the Large UV/Optical/Infrared Surveyor (LUVOIR) mission concept is to detect and characterize Earth-like exoplanets around nearby stars with direct imaging. The success of its integrated instrument ECLIPS (Extreme Coronagraph for Living Planetary Systems) depends on the ability to stabilize the wavefront from a large segmented mirror at a level of a few picometers during an exposure time of a few hours. To relax the constraints on the mechanical stability, ECLIPS will be equipped with a wavefront sensing and control (WS&C) architecture to correct wavefront errors at high temporal frequencies (<~1 Hz). These errors are expected to be dominated by spacecraft structural dynamics exciting vibrations at the segmented primary mirror. In this work, we present detailed simulations of the WS&C system within the ECLIPS instrument and the resulting contrast performance. This study assumes realistic wavefront aberrations based on a Finite Element Model of the telescope and the spacecraft structural dynamics. Wavefront residuals are then computed according to a model of the adaptive optics system that includes numerical propagation to simulate realistic images on the wavefront sensor and an analytical model of the temporal performance. An end-to-end numerical propagation model of ECLIPS is then used to estimate the residual starlight intensity distribution on the science detector. We show that the contrast performance depends strongly on the target star magnitude and advocate for the use of laser metrology to mitigate high temporal frequency wavefront errors and increase the mission yield.
The Polstar Mission seeks to study the evolution of massive stars including their effect on the interstellar medium and their behavior in binary systems using a 60 cm telescope with a UV Spectropolarimeter within MIDEX mission constraints on cost cap, throughput, coating requirements, and system-level dimensional stability. The mission is in a high-earth orbit and must ensure precise and repeatable polarimetric observations. Design-to-cost paradigms are exercised throughout all design phases and heritage approaches to structure and mirrors are evoked. In terms of classical error budgets, designing for diffraction-limited performance at 1.2 μm is sufficient, however, there are special design concerns at these wavelengths which require maximizing throughput of photons. Special coatings and minimum reflections are mandatory with meticulous attention to cleanliness throughout the entire mission life cycle. Decontamination heaters must be employed shortly after launch, prior to opening the door, and periodically throughout the mission lifetime. Additionally, spectropolarimetry requirements impose constraints on symmetry and control of phase and amplitude. The secondary mirror must have adjustment capability in three degrees of freedom (tip, tilt, and focus) to address drifts from thermal perturbations, aging, and possibly even spacecraft jitter. We present in-process design approach and analyses to meet the challenges of ultraviolet wavelengths and polarization stability..
The Polstar mission will provide for a space-borne 60cm telescope operating at UV wavelengths with spectropolarimetric capability capturing all four Stokes parameters (intensity, two linear polarization components, and circular polarization). Polstar’s capabilities are designed to meet its goal of determining how circumstellar gas flows alter massive stars' evolution, and finding the consequences for the stellar remnant population and the stirring and enrichment of the interstellar medium, by addressing four key science objectives. In addition, Polstar will determine drivers for the alignment of the smallest interstellar grains, and probe the dust, magnetic fields, and environments in the hot diffuse interstellar medium, including for the first time a direct measurement of the polarized and energized properties of intergalactic dust. Polstar will also characterize processes that lead to the assembly of exoplanetary systems and that affect exoplanetary atmospheres and habitability. Science driven design requirements include: access to ultraviolet bands: where hot massive stars are brightest and circumstellar opacity is highest; high spectral resolution: accessing diagnostics of circumstellar gas flows and stellar composition in the far-UV at 122-200nm, including the NV, SiIV, and CIV resonance doublets and other transitions such as NIV, AlIII, HeII, and CIII; polarimetry: accessing diagnostics of circumstellar magnetic field shape and strength when combined with high FUV spectral resolution and diagnostics of stellar rotation and distribution of circumstellar gas when combined with low near-UV spectral resolution; sufficient signal-to-noise ratios: ~103 for spectropolarimetric precisions of 0.1% per exposure; ~102 for detailed spectroscopic studies; ~10 for exploring dimmer sources; and cadence: ranging from 1-10 minutes for most wind variability studies, to hours for sampling rotational phase, to days or weeks for sampling orbital phase. The ISM and exoplanet science program will be enabled by these capabilities driven by the massive star science.
Future large space telescope missions demand extreme stability to enable high contrast coronagraphy for exoplanet observation. The wavefront control systems needed to achieve and maintain the required wavefront quality of the imaging system requires high-performance metrology sensors capable of picometer class sensitivity over long duration exposures, as well as for ground-based verification of build performance. For nearly two decades, Lockheed Martin has invested in developing laser metrology gauge technologies implemented in Photonic Integrated Circuits (PICs). We describe a high precision displacement metrology system currently under development and in test which has a path to flight for these future systems. Recent implementations have demonstrated picometer-class sensitivity at high (< 1 Hz) frequencies using largely commercial-off-the-shelf hardware. The current work aims to improve performance at longer timescales.
KEYWORDS: Space telescopes, Telescopes, James Webb Space Telescope, Mirrors, Optical instrument design, Astronomy, Space operations, Cryogenics, Aerospace engineering, Cryocoolers
The Origins Space Telescope will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. How did galaxies evolve from the earliest galactic systems to those found in the universe today? How do habitable planets form? How common are life-bearing worlds? We describe how Origins was designed to answer these alluring questions. We discuss the key decisions taken by the Origins mission concept study team, the rationale for those choices, and how they led through an exploratory design process to the Origins baseline mission concept. To understand the concept solution space, we studied two distinct mission concepts and descoped the second concept, aiming to maximize science per dollar and hit a self-imposed cost target. We report on the study approach and describe the concept evolution. The resulting baseline design includes a 5.9-m diameter telescope cryocooled to 4.5 K and equipped with three scientific instruments. The chosen architecture is similar to that of the Spitzer Space Telescope and requires very few deployments after launch. The cryo-thermal system design leverages James Webb Space Telescope technology and experience.
The Origins Space Telescope (Origins) study team prepared and submitted a Mission Concept Study Report for the 2020 Decadal Survey in Astrophysics. During the study, a Materials Working Group was formed to evaluate materials for Origins. The Materials Working Group identified material candidates and evaluated the candidates using driving requirements and key material considerations. The evaluation resulted in several options to aid the study team in making a materials selection for the mission concept. Our paper details the approach to the materials evaluation and the results.
The Origins Space Telescope will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. How did galaxies evolve from the earliest galactic systems to those found in the Universe today? How do habitable planets form? How common are life-bearing worlds? To answer these alluring questions, Origins will operate at mid- and far-infrared (IR) wavelengths and offer powerful spectroscopic instruments and sensitivity three orders of magnitude better than that of the Herschel Space Observatory, the largest telescope flown in space to date. We describe the baseline concept for Origins recommended to the 2020 US Decadal Survey in Astronomy and Astrophysics. The baseline design includes a 5.9-m diameter telescope cryocooled to 4.5 K and equipped with three scientific instruments. A mid-infrared instrument (Mid-Infrared Spectrometer and Camera Transit spectrometer) will measure the spectra of transiting exoplanets in the 2.8 to 20 μm wavelength range and offer unprecedented spectrophotometric precision, enabling definitive exoplanet biosignature detections. The far-IR imager polarimeter will be able to survey thousands of square degrees with broadband imaging at 50 and 250 μm. The Origins Survey Spectrometer will cover wavelengths from 25 to 588 μm, making wide-area and deep spectroscopic surveys with spectral resolving power R ∼ 300, and pointed observations at R ∼ 40,000 and 300,000 with selectable instrument modes. Origins was designed to minimize complexity. The architecture is similar to that of the Spitzer Space Telescope and requires very few deployments after launch, while the cryothermal system design leverages James Webb Space Telescope technology and experience. A combination of current-state-of-the-art cryocoolers and next-generation detector technology will enable Origins’ natural background-limited sensitivity.
In the next decade, NASA envisions a large space-based telescope that will perform unprecedented astronomy focused on the detection and characterization of Earth-like exoplanets. Recent advances in optical coronography enable this mission, but the technology imposes challenging requirements on telescope dynamic stability and vibration isolation. An integrated non-contact pointing and vibration isolation system called the Disturbance Free Payload (DFP) provides a means to achieve this stability. This system provides an ideal non-contact state (with only residual coupling from power and data cables and actuator effects) while allowing for the necessary degree of rigid-body payload control to meet required telescope pointing and system line-of-sight (LOS) agility. A subscale demonstration of the DFP technology on a CubeSat operating in 6 degrees of freedom in the space environment is one of several developments needed to advance the DFP architecture to TRL 6. This paper describes the mission goals and the preliminary payload and experiment design.
For the Large Ultraviolet/Optical/Infrared Surveyor (LUVOIR) to perform high-contrast direct imaging of habitable exoplanets using a coronagraph instrument, the system must maintain extremely low system dynamic wavefront error (on the order of 10 picometers RMS over the spatial frequencies corresponding to the dark-hole region of the coronagraph) over a long time wavefront control sampling interval (typically 10 or more minutes). Meeting this level of performance requires a telescope vibration isolation system that delivers a high degree of dynamic isolation over a broad frequency range. A non-contact pointing and isolation system called the Vibration Isolation and Precision Pointing System (VIPPS) has been baselined for the LUVOIR architecture. Lockheed Martin has partnered with NASA to predict the dynamic wavefront error (WFE) performance of such a system, and mature the technology through integrated modeling, subsystem test and subscale hardware demonstration. Previous published results on LUVOIR dynamic WFE stability performance have relied on preliminary models that do not explicitly include the effects of a segmented Primary Mirror. This paper presents a study of predicted dynamic WFE performance of the LUVOIR-A architecture during steady-state operation of the coronagraph instrument, using an integrated model consisting of a segmented primary mirror, optical sensitivities, steering mirror and non-contact isolation, and control systems. The design assumptions and stability properties of the control system are summarized. Principal observatory disturbance sources included are control moment gyroscope and steering mirror exported loads. Finally, observatory architecture trades are discussed that explore tradeoffs between system performance, concept of operation and technology readiness.
The Origins Space Telescope will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. How did galaxies evolve from the earliest galactic systems to those found in the universe today? How do habitable planets form? How common are life-bearing worlds? To answer these alluring questions, Origins will operate at mid- and far-infrared wavelengths and offer powerful spectroscopic instruments and sensitivity three orders of magnitude better than that of Herschel, the largest telescope flown in space to date. After a 3 ½ year study, the Origins Science and Technology Definition Team will recommend to the Decadal Survey a concept for Origins with a 5.9-m diameter telescope cryocooled to 4.5 K and equipped with three scientific instruments. A mid-infrared instrument (MISC-T) will measure the spectra of transiting exoplanets in the 2.8 – 20 μm wavelength range and offer unprecedented sensitivity, enabling definitive biosignature detections. The Far-IR Imager Polarimeter (FIP) will be able to survey thousands of square degrees with broadband imaging at 50 and 250 μm. The Origins Survey Spectrometer (OSS) will cover wavelengths from 25 – 588 μm, make wide-area and deep spectroscopic surveys with spectral resolving power R ~ 300, and pointed observations at R ~ 40,000 and 300,000 with selectable instrument modes. Origins was designed to minimize complexity. The telescope has a Spitzer-like architecture and requires very few deployments after launch. The cryo-thermal system design leverages JWST technology and experience. A combination of current-state-of-the-art cryocoolers and next-generation detector technology will enable Origins’ natural backgroundlimited sensitivity.
The Origins Space Telescope (OST) will trace the history of our origins from the time dust and heavy elements permanently altered the cosmic landscape to present-day life. How did the universe evolve in response to its changing ingredients? How common are life-bearing planets? To accomplish its scientific objectives, OST will operate at mid- and far-infrared wavelengths and offer superlative sensitivity and new spectroscopic capabilities. The OST study team will present a scientifically compelling, executable mission concept to the 2020 Decadal Survey in Astrophysics. To understand the concept solution space, our team studied two alternative mission concepts. We report on the study approach and describe both of these concepts, give the rationale for major design decisions, and briefly describe the mission-enabling technology.
The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical prescription which terminates at two focal plane arrays for each module. The instrument will operate at 37K after experiencing launch loads at 293K. The focal plane array housings (FPAHs), including stray light baffles (SLBs) must accommodate all associated thermal and mechanical stresses. In addition, the stray light baffles must be installed in situ on the previously assembled flight modules. The main purpose of the FPAH SLBs is to effectively attenuate mission limiting stray light on the focal planes. This paper will provide an overview of the NIRCam stray light baffle design, mechanical and optical analysis, hardware implementation and test results.
The Near Infrared Camera (NIRCam) for the James Webb Space Telescope (JWST) has been developed over the last
several years and during the course of development, the team of engineers has overcome several technical difficulties
and discovered many things that could be improved about the design. The instrument employs a Beryllium optical
bench, mounted transmissive and reflective optics, several mechanisms and the electronics to control them. This paper
will discuss some of the technical issues encountered and the lessons learned as a result of them. These issues involve
tapping of threads in and anodic coating of Beryllium, material interfaces within mechanisms, paints and coatings of
metals, mounting of optics and general engineering practice. The issues, root causes and resolutions for problems will
be presented in addition to suggestions and recommendations for future designs.
The Near Infrared Camera for the James Webb Space Telescope is designed to operate at a temperature of 37K. The
instrument must be assembled and aligned at room temperature. The optical design is refractive and incorporates several
different lens materials in addition to several mirrors which make an athermal design very difficult. All of the instrument
components are designed so that the instrument can come into alignment at 37K after assembly at room temperature. The
methods to predict alignment shifts are presented in this paper.
The Near Infrared Camera (NIRCam) Optical Bench Assembly (OBA) is a I-220H beryllium adhesively-bonded
structure designed to operate at 35K. To support design activities, an adhesive testing program was performed, with
particular emphasis on adhesive allowables at 35K. The geometries of the samples were designed to emulate the
structural features of the OBA. The testing program is described, test data presented, and the results applied to the NIRCam OBA.
The Near Infrared Camera (NIRCam) is the primary imaging instrument on the James Webb Space Telescope. The
primary structure for NIRCam is called the Optical Bench Assembly (OBA). The OBA is a bonded Beryllium structure
designed to operate at 35K. The structure has recently undergone thermal cycling to 35K followed by structural
qualification vibration testing. Analytical predictions were made of the structural performance during vibration. These
predictions closely matched the actual performance. This paper summarizes the build and assembly of the OBA, and
focuses on the qualification thermal and structural testing of the OBA. The qualification testing is described and pre-test
analysis is presented and compared with test results.
The Near Infrared Camera (NIRCam) instrument for NASA's James Webb Space Telescope (JWST) has an optical
prescription which employs several mirrors, some of which are powered and some of which are flats that aid in
packaging. Two distinct designs for the mirrors and their mounts have been developed such that different requirements
for mass, packaging and induced wavefront error can be met. The instrument will operate at 37K after experiencing
launch loads at ~293K and the mounts must accommodate all associated thermal and mechanical stresses. Two of the
mirrors needed to be redesigned after initial prototype testing of one of the designs. This paper will provide an update
on the design and analysis status for all the mirrors including results of the initial prototype testing.
The Near Infrared Camera is the primary imaging instrument on the James Webb Space Telescope. This instrument operates in the wavelength range of 0.6 to 5 microns and at a temperature of 35K. Two mirror-image optical paths or modules are utilized to provide two adjacent fields of view for science observations and redundancy for the purpose of wavefront sensing. All optical components are supported and aligned by an Optical Bench Assembly consisting of two benches mounted back to back. Each optical bench is a closed back Beryllium structure optimized for mass and stiffness. The closed back structure is achieved by bonding two machined parts together at the midplane of the structure. Each bench half is an open back structure consisting of a facesheet with machined ribs optimized to provide stiffness and to support along primary load paths. The two benches are integrated with optical components separately and are subsequently joined by bolts and pins to form the Optical Bench Assembly. The assembly is then mounted to interface struts, which are used to mount the instrument within the Integrated Science Instrument Module for integration into the JWST observatory. The design of the Optical Bench Assembly is describing including trade studies and analysis results.
The Near Infrared Camera (NIRCam) for NASA's James Webb Space Telescope (JWST) is one of the four science instruments to be installed into the Integrated Science Instrument Module (ISIM) on JWST. I-220H beryllium was chosen as the optical bench material for NIRCam based on its high specific stiffness, relatively high thermal conductivity, low CTE at cryogenic temperatures, and overall thermal stability at cryogenic temperatures. Beryllium has cryogenic heritage, but development of a structural bonded joint that could survive cryogenic temperatures was required. This paper will describe the trade studies performed in which bonded, I-220H beryllium was selected.
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.
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