Rene A. Boucarut

Programmable microshutter arrays for the JWST NIRSpec: optical performance

Two-dimensional microshutter arrays (MSAs) are being developed at the NASA Goddard Space Flight Center for the James Webb Space Telescope (JWST) for use as a programmable aperture mask for object selection for the Near Infrared Multiobject Spectrograph (NIRSpec). The MSAs are designed to provide high transmission efficiency for the selected objects and high on to off contrast ratio at the /spl sim/35 K operating temperature of JWST. The arrays of shutters are produced from silicon nitride membranes on a 100/spl times/200 /spl mu/m pitch. Individual shutters consist of a shutter blade of silicon nitride suspended from the shutter frame by a nitride torsion flexure. The shutters are normally closed. All shutters in the array are opened by the scanning magnetic field, and are held open by an electrostatic potential applied between the open shutters and the shutter support grid electrodes. To close the required shutters for a specific configuration, the potential between the shutter to be deselected and the support frame is set to zero, allowing the shutter to close. In this way, full random access addressing is achieved. We have produced such shutters and have demonstrated mechanical actuation and selection. Optical tests of open and closed shutters have demonstrated the required contrast for the JWST application. The MSA is a pioneering technology that provides the most capable possible multiobject spectrograph for JWST. It provides high contrast selection, high transmission efficiency, and can meet the environmental requirements for JWST.

Microshutters arrays for the JWST near-infrared spectrometer

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.

DCATT dispersed fringe sensor: modeling and experimenting with the transmissive phase plates

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 Cyro-Testing Of Infrared Filters And Beamsplitters For The Cosmic Background Explorer's Instruments

The cryo-optical methods used to measure the spectral transmittances of filters and beamsplitters for the Cosmic Background Explorers instruments are described. Measured results demonstrate the temperature sensitivity, or insensitivity, of various infrared filter designs within the wavelength range from 1μm to 1000μm.

Characterization of infrared filters for the wide-field camera 3 of Hubble Space Telescope

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 HSTs 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 filters performance. Details of the characterization apparatus, that include an optical cryostat, and a grating spectrometer are discussed.

Chemical vapor deposited silicon carbide mirrors for extreme ultraviolet applications

Advances in optical coating and materials technology have made possible the development of instruments with substantially im- proved efficiency in the extreme ultraviolet (EUV). For example, the de- velopment 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-scattering 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, is discussed.

Optical design of the developmental cryogenic active telescope testbed

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.

Calibration of the New Horizons Long-Range Reconnaissance Imager

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 NASAs 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 LORRIs 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.

CVD silicon carbide mirrors for EUV applications

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.

Grating efficiencies comparison study: calculations versus metrology for various types of high-groove density gratings at VUV-UV wavelengths

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.