The Laser Interferometer Space Antenna (LISA) mission is a space-based gravitational wave detector consisting of three spacecraft with two transceiver telescopes per spacecraft. In addition to tight wavefront error control as expected for an interferometric system, there are tight pupil imaging and optical path length specifications. We use concepts gleaned from pupil aberration theory to understand these latter two constraints and show how these concepts led to a successful design for the LISA transceiver.
The Ocean Radiometer for Carbon Assessment (ORCA) is a new design for the next generation remote sensing
of ocean biology and biogeochemistry. ORCA is configured to meet all the measurement requirements of the
Decadal Survey Aerosol, Cloud, and Ecology (ACE ), the Ocean Ecosystem (OES) radiometer and the Pre-ACE
climate data continuity mission (PACE). Under the auspices of a 2007 grant from NASA Research Opportunity
in Space and Earth Science (ROSES) and the Instrument Incubator Program (IIP) , a team at the Goddard
Space Flight Center (GSFC) has been working on a functional prototype with flightlike fore and aft optics and
scan mechanisms. As part of the development efforts to bring ORCA closer to a flight configuration, we have
conducted component-level optical testing using standard spectrophometers and system-level characterizations
using nonflight commercial off-the-shelf (COTS) focal plane array detectors. Although these arrays would not be
able to handle flight data rates, they are adequate for optical alignment and performance testing. The purpose
of this presentation is to describe the results of this testing performed at GSFC and the National Institute of
Standards and Technology (NIST) at the component and system level. Specifically, we show results for ORCA's
spectral calibration ranging from the near UV, visible, and near-infrared spectral regions.
The Ocean Radiometer for Carbon Assessment (ORCA) is a new design for the next generation remote sensing of
oceans biology and biogeochemistry satellite. ORCA is configured to meet the requirements of the Decadal Survey
recommended Aerosol, Cloud, and Ecology (ACE ), the Ocean Ecosystem (OES) radiometer and the Pre-ACE climate
data continuity mission (PACE). Under the auspices of a 2007 grant from NASA's Research Opportunity in Space and
Earth Science (ROSES) and the Instrument Incubator Program (IIP) , a team at the Goddard Space Flight Center
(GSFC) has been working on a functional prototype of a hyperspectral imager with flightlike optics and scan
mechanisms. This paper discusses the requirements and optomechanical design of this prototype.
The Ocean Radiometer for Carbon Assessment (ORCA), currently being developed at Goddard, is a hyperspectral
instrument with a spectral range extending from 350nm to 880nm in the UV and visible wavelength. Its radiometric
measurement accuracy will depend, in part, on the extent to which it is insensitive to linearly polarized light. A wedge
type depolarizer is used to reduce ORCA's polarization sensitivity over its entire spectral range. The choice for this
approach is driven by the large spectral range and to a certain extent is also influenced by the currently orbiting SeaWifs
instrument's use of a wedge depolarizer and its low polarization sensitivity. The wedge depolarizer's design, its modeled
and measured depolarization characteristics are presented.
The Ocean Radiometer for Carbon Assessment (ORCA) is a new design concept for the next generation ocean
biology and biogeochemistry satellite sensor. The wavelength range will be from the near UV, through the visible
and to the Short Wave infrared. The challenge in this design is to remove the polarization effects from the optical
performance of this hyper spectral observing instrument. In order to remove any polarization sensitivity during
observation, the design calls for a front-end depolarizer that consists of two wedged birefringent magnesium
fluoride crystals. Here we discuss the polarimetry measurements performed on this polarization scrambler, the
depolarizer design and compare these results with model calculations.
The Space Infrared Interferometric Telescope (SPIRIT) was designed to accomplish three scientific objectives: (1) learn
how planetary systems form from protostellar disks and how they acquire their inhomogeneous chemical composition;
(2) characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how
and where planets of different types form; and (3) learn how high-redshift galaxies formed and merged to form the
present-day population of galaxies. SPIRIT will accomplish these objectives through infrared observations with a two
aperture interferometric instrument. This paper gives an overview into the optical system design, including the design
form, the metrology systems used for control, stray light, and optical testing.
The Solar TErrestrial RElations Observatory (STEREO), the third mission in NASA's Solar Terrestrial Probes program,
was launched in 2006 on a two year mission to study solar phenomena. STEREO consists of two nearly identical
satellites, each carrying an Extreme Ultraviolet Imager (EUVI) telescope as part of the Sun Earth Connection Coronal
and Heliospheric Investigation instrument suite. EUVI is a normal incidence, 98mm diameter, Ritchey-Chrétien
telescope designed to obtain wide field of view images of the Sun at short wavelengths (17.1-30.4nm) using a CCD
detector. The telescope entrance aperture is divided into four quadrants by a mask near the secondary mirror spider
veins. A mechanism that rotates another mask allows only one of these sub-apertures to accept light over an exposure.
The EUVI contains no focus mechanism. Mechanical models predict a difference in telescope focus between ambient
integration conditions and on-orbit operation. We describe an independent check of the ambient, ultraviolet, absolute
focus setting of the EUVI telescopes after they were integrated with their respective spacecraft. A scanning Hartmann-like
test design resulted from constraints imposed by the EUVI aperture select mechanism. This inexpensive test was
simultaneously coordinated with other integration and test activities in a high-vibration, clean room environment. The
total focus test error was required to be better than ±0.05mm. We cover the alignment and test procedure, sources of
statistical and systematic error, data reduction and analysis, and results using various algorithms for determining focus.
The results are consistent with other tests of instrument focus alignment and indicate that the EUVI telescopes meet the
ambient focus offset requirements. STEREO and the EUVI telescopes are functioning well on-orbit.
The James Webb Space Telescope's (JWST) Integrated Science Instrument Module (ISIM) contains the observatory's four science instruments and their support subsystems. During alignment and test of the integrated ISIM at NASA's Goddard Space Flight Center (GSFC), the Optical telescope element SIMulator (OSIM) will be used to optically stimulate the science instruments to verify their operation and performance. In this paper we present the design of two cryogenic alignment fixtures that will be used to align the OSIM to the ISIM during testing at GSFC. These fixtures, the Master Alignment Target Fixture (MATF) and the ISIM Alignment Target Fixture (IATF), will provide continuous, six degree of freedom feedback to OSIM during initial ambient alignment as well as during cryogenic vacuum testing. These fixtures will allow us to position the OSIM and detect OSIM-ISIM absolute alignment to better than 180 microns in translation and 540 micro-radians in rotation. We will provide a brief overview of the OSIM system and we will also discuss the relevance of these fixtures in the context of the overall ISIM alignment and test plan.
We report results of a recently-completed pre-Formulation Phase study of SPIRIT, a candidate NASA Origins Probe mission. SPIRIT is a spatial and spectral interferometer with an operating wavelength range 25 - 400 μm. SPIRIT will provide sub-arcsecond resolution images and spectra with resolution R = 3000 in a 1 arcmin field of view to accomplish three primary scientific objectives: (1) Learn how planetary systems form from protostellar disks, and how they acquire their chemical organization; (2) Characterize the family of extrasolar planetary systems by imaging the structure in debris disks to understand how and where planets form, and why some planets are ice giants and others are rocky; and (3) Learn how high-redshift galaxies formed and merged to form the present-day population of galaxies. Observations with SPIRIT will be complementary to those of the James Webb Space Telescope and the ground-based Atacama Large Millimeter Array. All three observatories could be operational contemporaneously.
The James Webb Space Telescope (JWST) is a segmented deployable telescope that will require on-orbit alignment
using the Near Infrared Camera as a wavefront sensor. The telescope will be aligned by adjusting seven degrees of
freedom on each of 18 primary mirror segments and five degrees of freedom on the secondary mirror to optimize the
performance of the telescope and camera at a wavelength of 2 microns. With the completion of these adjustments, the
telescope focus is set and the optical performance of each of the other science instruments should then be optimal
without making further telescope focus adjustments for each individual instrument. This alignment approach requires
confocality of the instruments after integration and alignment to the composite metering structure, which will be verified
during instrument level testing at Goddard Space Flight Center with a telescope optical simulator. In this paper, we
present the results from a study of several analytical approaches to determine the focus for each instrument. The goal of
the study is to compare the accuracies obtained for each method, and to select the most feasible for use during optical
The NGST Wavefront Control Testbed (WCT) is a joint technology program managed by the Goddard Space Flight Center (GSFC) and the Jet Propulsion Laboratory (JPL) for the purpose of developing technologies relevant to the NGST optical system. The WCT provides a flexible testing environment that supports the development of wavefront sensing and control algorithms that may be used to align and control a segmented optical system. WCT is a modular system consisting of a Source Module (SM), Telescope Simulator Module (TSM) and an Aft-Optics (AO) bench. The SM incorporates multiple sources, neutral density filters and bandpass filters to provide a customized point source for the TSM. The telescope simulator module contains a flip-in mirror that selects between a small deformable mirror and three actively controlled spherical mirror segments. The TSM is capable of delivering a wide range of aberrated, unaberrated, continuous and segmented wavefronts to the AO optical bench for analysis. The AO bench consists of a series of reflective and transmissive optics that images the exit pupil of the TSM onto a 349 actuator deformable mirror that is used for wavefront correction. A Fast Steering Mirror (FSM) may be inserted into the system (AO bench) to investigate image stability and to compensate for systematic jitter when operated in a closed loop mode. We will describe the optical design and performance of the WCT hardware and discuss the impact of environmental factors on system performance.
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.
By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.
This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.
The Next Generation Space Telescope will depart from the traditional means of providing high optical quality and stability, namely use of massive structures. Instead, a benign orbital environment will provide stability for a large, flexible, lightweight deployed structure, and active wavefront controls will compensate misalignments and figure errors induced during launch and cool-down on orbit. This paper presents a baseline architecture for NGST wavefront controls, including initial capture and alignment, segment phasing, wavefront sensing and deformable mirror control. Simulations and analyses illustrate expected scientific performance with respect to figure error, misalignments, and thermal deformation.
Various efforts are underway to demonstrate hardware for the NGST. One such effort is the development of the DCATT testbed. This testbed is an NGST effort geared to demonstrating in hardware the end-to-end system functionality. The system includes a segmented telescope, active optics subsystem with a deformable mirror (DM), and a wavefront sensor. The degree to which the DCATT can demonstrate this functionality depends crucially on its performance. A system performance analysis of this testbed is presented. The analysis is based on the design of the DCATT developed by Goddard and JPL. In the analysis, the performance of the system as a function of key system factors is calculated. These factors include the following: control, environmental, fabrication, alignment, and design. The performance after correction by the DM is required to be diffraction-limited at a wavelength of 2.0 microns. The flowdown of this performance is called the corrected error budget. To fully characterize the testbed performance as a system, one must develop a budget for the performance of the system before action of the DM. The flowdown of this performance is called the uncorrected error budget. The top line of this budget is related to the correction capability of the DM.
Over the past two years, a team of researchers led by the Goddard Space Flight Center has developed a conceptual design for the NGST. The optical design of the optical telescope assembly (OTA) as well as the integrated science instrument module (ISIM) has presented many challenges. As currently envisioned, the NGST is an 8 m class telescope capable of diffraction-limited imaging at a wavelength of 2 microns operating at L2 at a temperature of around 40 K. The baseline design incorporates features such as a segmented primary mirror, deployable optical components, and active optics including a deformable mirror and fast steering mirror. In this paper, we describe the development of the conceptual design, discuss the trade-offs involving performance versus complexity, packaging, and cost, and then highlight some of the more important lessons that have been learned in the process. The interaction between the OTA and the ISIM is also discussed. It is hoped that this paper can provide insight and/or guidance to those who ar or will be working on the continuing refinement of the optical design of the NGST.
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.
As part of the technology validation strategy of the next generation space telescope (NGST), a system testbed is being developed at GSFC, in partnership with JPL and Marshall Space Flight Center, which will include al of the component functions envisioned in an NGST active optical system. The system will include an actively controlled, segmented primary mirror, actively controlled secondary, deformable, and fast steering mirrors, wavefront sensing optics, wavefront control algorithms, a telescope simulator module, and an interferometric wavefront sensor for use in comparing final obtained wavefronts from different tests. The developmental cryogenic active telescope testbed will be implemented in three phase. Phase 1 will focus on operating the testbed at ambient temperature. During Phase 2, a cryocapable segmented telescope will be developed and cooled to cryogenic temperature to investigate the impact on the ability to correct the wavefront and stabilize the image. In Phase 3, it is planned to incorporate industry developed flight-like components, such as figure controlled mirror segments, cryogenic, low hold power actuators, or different wavefront sensing and control hardware or software. A very important element of the program is the development and subsequent validation of the integrated multidisciplinary models. The phase 1 testbed objectives, plans, configuration, and design will be discussed.
There is no doubt that astronomy with the `new, improved' Hubble Space Telescope will significantly advance our knowledge and understanding of the universe for years to come. The Corrective Optics Space Telescope Axial Replacement (COSTAR) was designed to restore the image quality to nearly diffraction limited performance for three of the first generation instruments; the faint object camera, the faint object spectrograph, and the Goddard high resolution spectrograph. Spectacular images have been obtained from the faint object camera after the installation of the corrective optics during the first servicing mission in December of 1993. About 85% of the light in the central core of the corrected image is contained within a circle with a diameter of 0.2 arcsec. This is a vast improvement over the previous 15 to 17% encircled energies obtained before COSTAR. Clearly COSTAR is a success. One reason for the overwhelming success of COSTAR was the ambitious and comprehensive test program conducted by various groups throughout the program. For optical testing of COSTAR on the ground, engineers at Ball Aerospace designed and built the refractive Hubble simulator to produce known amounts of spherical aberration and astigmatism at specific points in the field of view. The design goal for this refractive aberrated simulator (RAS) was to match the aberrations of the Hubble Space Telescope to within (lambda) /20 rms over the field at a wavelength of 632.8 nm. When the COSTAR optics were combined with the RAS optics, the corrected COSTAR output images were produced. These COSTAR images were recorded with a high resolution 1024 by 1024 array CCD camera, the Ball image analyzer (BIA). The image quality criteria used for assessment of COSTAR performance was encircled energy in the COSTAR focal plane. This test with the BIA was very important because it was a direct measurement of the point spread function. But it was difficult with this test to say anything quantitative about the aberration content of the corrected images. Also, from only this test it was difficult to measure important pupil parameters, such as pupil intensity profiles and pupil sizes and location. To measure the COSTAR wavefront accurately and to determine pupil parameters, another very important test was performed on the COSTAR optics. A Hartmann test of the optical system consisting of the RAS and COSTAR was conducted by the Goddard Independent Verification Team (IVT). In this paper, we first describe the unique Hartmann sensor that was developed by the IVT. Then we briefly describe the RAS and COSTAR optical systems and the test setup. Finally, we present the results of the test and compare our results with results obtained from optical analysis and from image tests with the BIA.
Many large optical telescopes have a wide field camera as one of their primary instruments. Camera designs with all reflective surfaces are useful because they give stable image quality over a broad spectral band. Systems with two conic mirrors can correct the residual aberrations, field curvature and astigmatism, of Ritchey-Chretien telescopes. We present first- and third-order optical design relations which can facilitate the formation of design concepts during preliminary trade studies. We apply the relations to the Hubble Space Telescope in the design of a new wide field camera which compensates the existing spherical aberration, as well as the astigmatism and field curvature, of the telescope.
Lyman/FUSE, the Far Ultraviolet Spectroscopic Explorer, is a proposed low earth orbit mission to explore the
1OO-16OGi spectra of diverse astronomical sources. The simultaneous design goals of high spectral resolution, high
sensitivity, high signal to noise ratio, limited slit imaging capability, wide spectral coverage, compactness, and high
stability were a formidable challenge. In the design, a Wolter type II glancing incidence telescope (70 cm aperture,
f/1O) feeds two spectroscopic channels. A boresighted fme error sensor is used to point the instrument. In the
4oo-16oc1i range, a 1.84m Rowland circle spectrograph uses five near normal incidence gratings with a common
MAMA detector. To achieve acceptable resolving power and limited imaging with a fast telescope, the grating
aberrations must be significantly reduced below those obtained with toroidal or ellipsoidal gratings. A modified
ellipsoidal grating is used to achieve resolving powers of 30000 in the three high resolution bands covering 91O-125Q.
A separate EUV channel will explore the 1OO-35O range with lower spectral and spatial resolution. The throughput
in this range is improved by more than an order of magnitude over the Extreme Ultraviolet Explorer (EUVE), when
EUVE is operating in its spectroscopic mode. The optical design and results of ray tracing studies are presented, as
well as the expected effective area in each channel.