The function of a Lateral Transfer Retroreflector is to accurately shift a beam of light laterally, while changing its direction 180 degrees. It uses three optically-flat, reflective surfaces located in mutually perpendicular planes to return an output beam parallel, but laterally separated from the input beam. The device maintains parallelism of the two beams regardless of its own orientation. From mid-2011 to late 2015, two types of LTR were designed, developed, produced and tested at Goddard Space Flight Center in Greenbelt Maryland. Information about the development process, along with performance results is given.
This paper summarizes a comparison of pre- and post-flight optical performance on optical components (mostly
mirrors) from the Corrective Optics Space Telescope Axial Replacement (COSTAR) instrument and the Wide
Field Planetary Camera 2 (WF/PC 2) pickoff mirror. These measurements were carried out after both the
COSTAR and WF/PC 2 were retrieved from the Hubble Space Telescope in May of 2009 and returned to GSFC
in July of 2009. Both of these instruments had a highly UV-reflecting coating of Al with a MgF2 layer on
top for protection on their reflecting optics. We studied these in order to document the aging process on these
coatings while in space for more than 15.5 years. When compared to data before flight and witness coupons
kept on the ground, we find a severely degraded UV performance for the coatings that flew in space, particularly
at the Lyman-α wavelength. Based on similar observations seen earlier on the WF/PC1 POM, the current
degradation, of the latest optical components removed from HST, are a result of outgassing of substances such
as hydrocarbons and silicone from nearby hardware on the spacecraft and UV light that photo-polymerize those
materials on the mirror surfaces.
The nature of Dark Energy can by constrained by the precise determination of super-novae distance moduli in ultraviolet
to near IR pass-bands. Space-based observations are required for these moduli to be measured with the scientifically
required photometric accuracies. Consequently, robust pass-band filters operable at cryogenic temperatures (120-140K)
are needed that have challenging performance attributes including high in-band transmission, low ripple, good out-ofband
rejection, and moderate band-edge slope. We describe the requirements and performance of dielectric multi-layer
filters with spectral profiles that are suitable for both achieving the science and for accurate calibration using plausible
on-orbit measurement systems.
The James Webb Space Telescope (JWST) is an infrared, space-based telescope scheduled for launch in 2013. JWST will hold four scientific instruments, including the Near Infrared Spectrograph (NIRSpec). NIRSpec operates in the wavelength range from 0.6 to 5 microns, and will be assembled by the European Space Agency. NASA/Goddard Space Flight Center (GSFC) is responsible for two NIRSpec subsystems: the detector subsystem, with the focal plane array (FPA), and the micro-shutter subsystem, with the micro-shutter assembly (MSA). The FPA consists of two side-by-side Rockwell Scientific HgCdTe 2Kx2K detectors, with the detectors and readout electronics optimized for low noise. The MSA is a GSFC developed micro-electro-mechanical system (MEMS) that serves as a programmable slit mask, allowing NIRSpec to obtain simultaneous spectra of >100 objects in a single field of view. We present the optical characterization test plan of the FPA. The test plan is driven by many requirements: cryogenic operating temperature, a flight-like beam shape, and multi-wavelength flux from 1 to 10,000 photons per second, thus low stray light is critical. We use commercial optical modeling software to predict stray light effects at the FPA. We also present the optical contrast test plan of the MSA. Each individual shutter element operates in an on/off state, and the most important optical metric is contrast. The MSA is designed to minimize stray and scattered light, and the test setup reduces stray light such that the optical contrast is measurable.
Wide-Field Camera 3 (WFC3) has been built for installation on the Hubble Space Telescope (HST) during the next servicing mission. The WFC3 instrument consists of both a UVIS and an IR channel, each with its own complement of filters. On the UVIS side, a selectable optical filter assembly (SOFA) contains a set of 12 wheels that house 48 elements (42 full-frame filters, 5 quadrant filters, and 1 UV grism). The IR channel has one filter wheel which houses 17 elements (15 filters and 2 grisms). While the majority of UVIS filters exhibited excellent performance during ground testing, a subset of filters showed filter ghosting; improved replacements for these filters have been procured and installed. No filter ghosting was found in any of the IR filters; however, the new IR detector for WFC3 will have significantly more response blueward of 800 nm than the original detector, requiring that two filters originally constructed on a fused silica substrate be remade to block any visible light transmission. This paper summarizes the characterization of the final complement of the WFC3 UVIS and IR filters, highlighting improvements in the replacement filters and the projected benefit to science observations.
The Wide Field Camera 3 (WFC3) is a panchromatic imager that will be deployed in the Hubble Space Telescope
(HST). The mission of the WFC3 is to enhance the imaging capability of HST in the ultraviolet, visible and
near-infrared spectral regions. Together with a wavelength coverage spanning 2000 Angstrom to 1.7 μm, the WFC3
high sensitivity, high spatial resolution, and large field-of-view provide the astronomer with an unprecedented
set of tools for exploring all types of exciting astrophysical terrain and for addressing many key questions in
astronomy today. The filter complement, which includes broad, medium, and narrow band filters, naturally
reflects the diversity of astronomical programs to be targeted with WFC3. The WFC3 holds 61 UVIS filter
elements, 15 IR filters, and 3 dispersive elements. During ground testing, the majority of the UVIS filters were
found to exhibit excellent performance consistent with or exceeding expectations; however, a subset of filters
showed considerable ghost images; some with relative intensity as high as 10-15%. Replacement filters with
band-defining coatings that substantially reduce these ghost images were designed and procured. A state-of-the-art
characterization setup was developed to measure the intensity of ghost images, focal shift, wedge direction,
transmitted uniformity and surface features of filters that could affect uniformity in flat-field images. We will
report on these filter characterization methods, as well as the spectral performance measurements of the in-band
transmittance and out-of-band blocking.
The LOng-Range Reconnaissance Imager (LORRI) is a panchromatic imager for the New Horizons Pluto/Kuiper belt mission. New Horizons is being prepared for launch in January 2006 as the inaugural mission in NASA's New Frontiers program. This paper discusses the calibration and characterization of LORRI.
LORRI consists of a Ritchey-Chretien telescope and CCD detector. It provides a narrow field of view (0.29°), high resolution (pixel FOV = 5 μrad) image at f/12.6 with a 20.8~cm diameter primary mirror. The image is acquired with a 1024 x 1024 pixel CCD detector (model CCD 47-20 from E2V). LORRI was calibrated in vacuum at three temperatures covering the extremes of its operating range (-100°C to +40°C for various parts of the system) and its predicted nominal temperature in-flight. A high pressure xenon arc lamp, selected for its solar-like spectrum, provided the light source for the calibration. The lamp was fiber-optically coupled into the vacuum chamber and monitored by a calibrated photodiode. Neutral density and bandpass filters controlled source intensity and provided measurements of the wavelength dependence of LORRI's performance. This paper will describe the calibration facility and design, as well as summarize the results on point spread function, flat field, radiometric response, detector noise, and focus stability over the operating temperature range.
LORRI was designed and fabricated by a combined effort of The Johns Hopkins University Applied Physics Laboratory (APL) and SSG Precision Optronics.
Calibration was conducted at the Diffraction Grating Evaluation Facility at NASA/Goddard Space Flight Center with additional characterization measurements at APL.
KEYWORDS: Camera shutters, Electrodes, Silicon, Metals, Magnetism, James Webb Space Telescope, Optical fabrication, Microelectromechanical systems, Semiconducting wafers, Reactive ion etching
Micro Electromechanical System (MEMS) microshutter arrays are being developed at NASA Goddard Space Flight Center for use as a field selector of the Near Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST). The microshutter arrays are designed for the spontaneous selection of a large number of objects in the sky and the transmission of light to the NIRSpec detector with high contrast. The JWST environment requires cryogenic operation at 35 K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) silicon wafers. Arrays are close-packed silicon nitride membranes with a pixel size of 100 x 200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with a minimized mechanical stress concentration. Light shields are processed for blocking light from gaps between shutters and frames. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes multi-layer metal depositions, the patterning of magnetic stripes and shutter electrodes, a reactive ion etching (RIE) to form shutters out of the nitride membrane, an anisotropic back-etch for wafer thinning, followed by a deep RIE (DRIE) back-etch to form mechanical supporting grids and release shutters from the silicon substrate. An additional metal deposition is used to form back electrodes. Shutters are actuated by a magnetic force and latched using an electrostatic force. Optical tests, addressing tests, and life tests are conducted to evaluate the performance and the reliability of microshutter arrays.
KEYWORDS: Wavefront sensors, Data modeling, Diffraction, James Webb Space Telescope, Phase retrieval, Spectrographs, Mirrors, Cameras, Point spread functions, Astronomical imaging
An analysis is presented that illustrates how the James Webb Space Telescope (JWST) fine-phasing process can be carried out using the Near-Infrared Spectrograph (NIRSpec) data collected at the science focal plane. The analysis considers a multi-plane diffraction model which properly accounts for the microshutter diffractive element placed at the first relay position of the spectrograph. Wavefront sensing results are presented based on data collected from the NASA Goddard Microshutter Optical Testbed.
KEYWORDS: Camera shutters, James Webb Space Telescope, Electrodes, Silicon, Spectrographs, Magnetism, Prototyping, Failure analysis, Near infrared, Galactic astronomy
The Near Infrared Spectrograph (NIRSpec) for the James Webb Space Telescope (JWST) is a multi-object spectrograph operating in the 0.6-5.0 μm spectral range. One of the primary scientific objectives of this instrument is to measure the number and density evolution of galaxies following the epoch of initial formation. NIRSpec is designed to allow simultaneous observation of a large number of sources, vastly increasing the capability of JWST to carry out its objectives. A critical element of the instrument is the programmable field selector, the Microshutter Array. The system consists of four 175 x 384 close packed arrays of individually operable shutters, each element subtending 0.2” x 0.4”on the sky. This device allows simultaneous selection of over 200 candidates for study over the 3.6’ x 3.6’ field of the NIRSpec, dramatically increasing its efficiency for a wide range of investigations. Here, we describe the development, production, and test of this critical element of the NIRSpec.
The Ultraviolet and Optical Telescope (UVOT) is one of the three
astronomical instruments onboard the SWIFT spacecraft. The optical
calibration of this instrument, which was done prior to integration
to the SWIFT spacecraft optical bench, is key to determine if UVOT
will meet its science objectives. In this paper, we describe
the optical ground support equipment (GSE) used for the
calibration of UVOT. These tests, which were carried out in the
Diffraction Grating Evaluation Facility (DGEF), at NASA Goddard
Space Flight Center, required building an optical stimulus. We
report the radiometric measurements of all the optical components
used in putting together this stimulus. This includes a vacuum
collimator with a Cassegrain design, a Pt/Cr-Ne light source, a
complete set of neutral density filters spanning 6 orders of
magnitude in transmission levels, a set of narrow-band filters
matching the center of each of the six bands of UVOT, a set of pinholes of various sizes, flat fielding diffusers, and a set of parabolic mirrors.
The complex study of a ultra-fine (4000 to 5870 grooves/mm) holographic gratings designed for Near Ultra Violet (NUV) channel of Cosmic Origin Spectrograph (COS) are presented. The gratings underwent a comprehensive program of efficiency and scatter characterization at flight-like environment conditions. Initial tests revealed significant departures of grating efficiencies from the values predicted. Effects of profile non-replication (layers nonconformity) for multi-layer coated grating surfaces are investigated. The rigorous efficiency modeling based on groove profiling atomic force microscopy (AFM) data for both multi-layer conformal and non-conformal coated gratings in both polarizations
allowed to identify the problem as a leaky mode anomalies for dielectrically coated gratings. Both shift in peak efficiencies and wide absorption band are observed to be critically dependant of grating groove shape and reflective coating thickness. Grating depth/profile topography changes induced by coating process are observed and called for groove profile measurements at all stages of the gratings production line. Grating scatter characterizations data for COS and SORCE/SOLSTICE gratings at UV-VUV wavelength are presented. The wavelength scaling at VUV-UV waveband (140 nm to 442 nm) for holographically ruled 3600 gr/mm grating are also reported.
Design and performance of Fully Automated Ultraviolet Spectrographic Tester (FAUST) providing Bidirectional Scatter Distribution Function (BSDF) measurements at wavelengths ranging from the vacuum ultraviolet to the infrared has been described in details. The instrument is capable of measuring both very near (3 arcseconds) and very wide angle (over 120 degrees) scatter off both highly specular (mirrors and gratings) and highly diffuse surfaces for +/- 90 degrees incidence angle variations. Scatterometer dynamic range of over 11 orders of magnitude has been demonstrated. Stray light reduction techniques practicing for instrument signature improvement are discussed. Principles of light sources and detectors choice, instrument automation and calibration are explained. Instrument signature data along with some examples of a list of scatter measurements and gratings efficiency measurements performed by the use of FAUST are also presented here.
The Wide Field Camera 3 (WFC3) is a panchromatic imager that will be deployed in the Hubble Space Telescope (HST) in 2004. The mission of the WFC3 is to enhance HST's imaging capability in the ultraviolet, visible and near-infrared spectral regions. Together with a wavelength coverage spanning 2000A to 1.7 microns, the WFC3 high sensitivity, high spatial resolution, and large field-of-view provide the astronomer with an unprecedented set of tools for exploring all types of exciting astrophysical terrain and for addressing many key questions is astronomy today. The filter compliment, which includes broad, medium, and narrow band filters, naturally reflects the diversity of astronomical programs to be targeted with WFC3. The WFC3 holds 61 UVIS filters elements, 14 IR filters, and 3 dispersive elements. Accurate and comprehensive knowledge of the optical performance of these components including its pass-band and out-of-band rejection behavior are necessary to verify that the instrument will meet its scientific objectives. The measured throughput curves are essential components in instrument performance models used to plan observations, and in calibration algorithms for removing the instrument signature from in-flight data. We will report on the normal incidence in-band and out-of-band transmittance of the IR filters measured near the operating temperature of -30 degree(s)C and additional tests used to characterizes the filter's performance. Details of the characterization apparatus, that include an optical cryostat, and a grating spectrometer are discussed.
Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.
The Hubble Space Telescope (HST) advanced camera for surveys (ACS) employs a wide variety of spectral filtration components including narrow band, medium band, wide band, and far UV (FUV) long pass filters, spatially-variable filters, VIS/IR polarizers, NUV polarizers, FUV prisms, and a grism. These components are spread across ACS's wide field, high resolution, and solar blind channels which provide diffraction-limited imaging of astronomical targets using aberration-correcting optics which remove most aberrations form HST's optical telescope assembly. In order for ACS to be truly advanced, these filters must push the state-of-the-art in performance in a number of key areas at the same time. Important requirements which these filters must meet include outstanding transmitted wavefront, high transmittance, uniform transmittance across each filter, spectrally structure-free bandpasses, exceptionally high out of band rejection, and a high degree of parfocality. These constitute a very stringent set of requirements indeed, especially for filters which are up to 90 mm in diameter. The development of unique optical metrology stations used to demonstrate that each ACS filter will meet its design specifications is discussed. Of particular note are specially-designed spectral transmissometers and interferometers.
In the summer of 1996, three study teams developed conceptual designs and mission architectures for the NGST. All three conceptual designs provided scientific capabilities that met or surpassed those envisioned by the Hubble Space Telescope and Beyond Committee. Each group highlighted areas of technology study included: deployable structures, lightweight optics, cryogenic optics and mechanisms, passive cooling, a non-orbit closed loop wavefront sensing and control. NASA and industry are currently planning to develop a series of ground testbeds and validation flights to demonstrate many of these technologies. The developmental cryogenic active telescope testbed (DCATT) is a system level testbed to be developed at Goddard Space Flight Center in three phases over an extended period of time. This testbed will combine an actively controlled telescope with the hardware and software elements of a closed loop wavefront sensing and control system to achieve diffraction limited imaging at 2 microns. We will present an overview of the system level requirements, a discussion of the optical design, and results of performance analyses for the Phase 1 ambient concept for DCATT.
The transmitted wavefronts of optical filters for he Hubble Space Telescope (HST) advanced camera for surveys (ACS) are characterized using the Wildly and Openly Modified Broadband Achromatic Twyman Green (WOMBAT) Interferometer developed in the NASA/GSFC Optics Branch's Diffraction Grating Evaluation Facility. Because only four of thirty-three of ACS's optical bandpass filters transmit the 633 nm light of most commercial interferometers, a broadband interferometer is required to verify specified transmitted wavefront of ACS filters. WOMBAT's design is a hybrid of the BAT2 interferometer developed for JPL used for HST wide field and planetary camera II filters and a WYKO 400 phase shifting interferometer. It includes a broadband light source, monochromator, off-axis, parabolic collimating and camera mirrors, an aluminum-coated fused silica beamsplitter, flat retroreflecting mirrors for the test and reference arms, and a UV-sensitive CCD camera. An outboarded, piezo-electric phase shifter holds the flat mirror in the interferometer's reference arm. The interferometer is calibrated through interaction between the WYKO system's software and WOMBAT hardware for the test wavelength of light entering the beamsplitter. Phase-shifted interferograms of the filter mounted in the tests arm are analyzed using WYKO's vision for optical testing software. Filters as large as 90 mm in diameter have been measured over a wavelength range from 200 to 1100 nm with a ACS fixed bandpass and spatially-variable bandpass filters for a variety of wavelengths.
The second servicing mission for the Hubble Space Telescope (HST), scheduled for early 1997, will be the first change in the spectroscopic capabilities of HST since its initial deployment. The Space Telescope Imaging Spectrograph (STIS) is a multipurpose instrument covering the far ultraviolet (FUV) through near infrared spectral range. It acquires spectra at several spectral resolutions, which facilitates observations at many distances and brightnesses. STIS will replace both of the first generation spectrographs, the Goddard High Resolution Spectrograph and the Faint Object Spectrograph. This will allow the addition of a Near- Infrared Camera. STIS required the development and testing of many high quality diffraction gratings, including several very difficult echelles for the FUV. The methods and results of this grating development program are presented. The results serve as a snapshot of industry capabilities for producing high quality spaceborne diffraction gratings.
Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet (EUV). For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. CVD-SiC is a highly polishable material yielding low scatter surfaces. High UV reflectivity and desirable mechanical and thermal properties make CVD-SiC an attractive mirror and/or coating material for EUV applications. The EUV performance of SiC mirrors as well as some strengths and problem areas are discussed.
Originally developed as an ion source for ion accelerators, the Duoplasmatron has also served as a VUV line source in monochromators. We have adapted and modified the Duoplasmatron, historically used at ambient conditions, for use in the vacuum chamber of the Diffraction Grating Evaluation Facility at NASA/GSFC. Construction, operational parameters and maintenance of the light source will be discussed.
Verification of component specification and EUV optical performance for the diffraction gratings of the normal incidence EUV spectrograph of SOHO/CDS is discussed. This discussion includes: a comparison of experimentally obtained EUV spectral images of a point source with corresponding ray trace analysis, in-band Lyman alpha leak determination, verification of toroidal radii of curvature, verification of blaze direction by calculation and VUV measurement, and zero order efficiency at 124 nm.
This paper will present transmittance data, in the 1- to 1000-micron wavelength region, at temperatures from 300K down to near 4K, for a selection of filters composed of multilayer thin films on transmitting substrates, reststrahlen crystals, mesh-grid elements, and hybrids of these types. Polyethylene laminates and vapor deposited parylene will be compared as antireflection layers for high refractive index infrared crystals at long wavelengths.
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