A corner cube (CC) articulation model has been developed to evaluate the SIM internal metrology (IntMet) optical delay bias (with the accuracy of picometer) due to the component imperfections, such as vertex offset, reflection coating index error, dihedral error, and surface figure error at each facet. This physics-based and MATLAB-implemented geometric optics model provides useful guidance on the flight system design, integration, and characterization. The first portion of this paper covers the CC model details. Then several feature of the model, such as metrology beam footprint visualization, roofline straddling/crossing analysis, and application to drive the sub-system design and the error budget flow-down, are demonstrated in the second part.
The Space Interferometer Mission (SIM) flight instrument will not undergo a full performance, end-to-end system test on the ground due to a number of constraints. Thus, analysis and physics-based models will play a significant role in providing confidence that SIM will meet its science goals on orbit. The various models themselves are validated against the experimental results of severl "picometer" testbeds. In this paper we describe a set of models that are used to predict the magnitude and functional form of a class of field-dependent systematic errors for the science and guide interferometers. This set of models is validated by comparing predictions with the experimental results obtained from the MicroArcsecond Metrology (MAM) testbed and the Diffraction testbed (DTB). The metric for validation is provided by the SIM astrometric error budget.
The current design of the Space Interferometry Mission (SIM) employs a 19 laser-metrology-beam system (also called L19 external metrology truss) to monitor changes of distances between the fiducials of the flight system's multiple baselines. The function of the external metrology truss is to aid in the determination of the time-variations of the interferometer baseline. The largest contributor to truss error occurs in SIM wide-angle observations when the articulation of the siderostat mirrors (in order to gather starlight from different sky coordinates) brings to light systematic errors due to offsets at levels of instrument components (which include corner cube retro-reflectors, etc.). This is the external metrology wide-angle field-dependent error. Physics-based model of field-dependent error at single metrology gauge level is developed and linearly propagated to errors in interferometer delay. General formulation of delay error sensitivity to various error parameters is developed. The essence of the linear error model is contained in an errormapping matrix. A corresponding Zernike component matrix approach is developed in parallel with its advantages discussed. As a first example, dihedral error model is developed for the corner cubes (CC) attached to the siderostat mirrors. Average and worst case residual errors are computed when various orders of field-dependent terms are removed from the delay error. These serve as guidelines for arriving at system requirements given the error budget allocation. Highlights of the non-common vertex error (NCVE) model are shown as a second example followed by discussions.
Global astrometry is the measurement of stellar positions and motions. These are typically characterized by five parameters, including two position parameters, two proper motion parameters, and parallax. The Space Interferometry Mission (SIM) will derive these parameters for a grid of approximately 1300 stars covering the celestial sphere to an accuracy of approximately 4uas, representing a two orders of magnitude improvemnt over the most precise current star catalogues. Narrow angle astrometry will be performed to a 1uas accuracy. A wealth of scientific information will be obtained from these accurate measurements encompassing many aspects of both galactic and extragalactic science. SIM will be subject to a number of instrument errors that can potentially degrade performance. Many of these errors are systematic in that they are relatively static and repeatable with respect to the time frame and direction of the observation. This paper and its companion define the modeling of the contributing factors to these errors and the analysis of how they impact SIM's ability to perform astrometric science.
This paper summarizes two different strategies envisioned for calibrating the systematic field dependent biases present in the Space Interferometry Mission (SIM) instrument. The Internal Calibration strategy is based on pre-launch measurements combined with a set of on-orbit measurements generated by a source internal to the instrument. The External Calibration strategy uses stars as an external source for generating the calibration function. Both approaches demand a significant amount of innovation given that SIM's calibration strategy requires a post-calibration error of 100 picometers over a 15 degree field of regard while the uncalibrated instrument introduces tens to hundreds of nanometers of error. The calibration strategies are discussed in the context of the wide angle astrometric mode of the instrument, although variations on both strategies have been proposed for doing narrow angle astrometry.
An experimental beam combiner (BC) is being developed to support the space interferometry program at the JPL. The beam combine forms the part of an interferometer where star light collected by the sidestats or telescopes is brought together to produce white light fringes, and to provide wavefront tilt information via guiding spots and beam walk information via shear spots. The assembly and alignment of the BC has been completed. The characterization test were performed under laboratory conditions with an artificial star and optical delay line. Part of each input beam was used to perform star tracking. The white light interference fringes were obtained over the selected wavelength range from 450 nm to 850 nm. A least-square fit process was used to analyze the fringe initial phase, fringe visibilities and shift errors of the optical path difference in the delay line using the dispersed white-light fringes at different OPD positions.
This paper describes the Micro-Precision Interferometer (MPI) testbed and its major achievements to date related to mitigating risk for future spaceborne optical interferometer missions. The MPI testbed is ground-based hardware model of a future spaceborne interferometer. The three primary objectives of the testbed are to: (1) demonstrate the 10 nm positional stability requirement in the ambient lab disturbance environment, (2) predict whether the 10 nm positional stability requirement can be achieved in the anticipated on-orbit disturbance environment, and (3) validate integrated modeling tools that will ultimately tools that will ultimately to be used to design the actual space missions. This paper describes the hardware testbed in its present configuration. The testbed simulation model, as it stands today, will be described elsewhere. The paper presents results concerning closed loop positional stabilities at or below the 10 nm requirement for both the ambient and on-orbit disturbance environments. These encouraging results confirm that the MPI testbed provides an essential link between the extensive ongoing ground-based interferometer technology development activities and the technology needs of future spaceborne optical interferometers.
This paper describes the design and performance of a brassboard astrometric beam combiner. The beam combiner was developed as part of the JPL Interferometry Technology Program . The purpose of this program is to test out design concepts in hardware that will eventually be used for the Space Interferometry Mission. The label brassboard implies that the beam combiner is flight-like in terms of fit and function. The beam combiner met its design performance except for fringe visibility. Although it has not been environmentally tested as an assembly, the beam combiner was designed to survive the appropriate thermal and vibration tests.
The multi-angle imaging spectroradiometer (MISR) will provide global data sets from Earth orbit using nine pushbroom cameras, each viewing in a fixed, unique direction. Data will be acquired for day-lit portions of the orbit at an average rate of 3.3 Mbits s-1 for the entire six year mission. Automated ground processing will make use of the instrument radiometric, spectral, and geometric calibrations, to produce registered images at the nine view angles. This, the Level 1 product, provides top- of-atmosphere scene radiances, weighted by the spectral band profile for the instrument. Initially, processing will proceed with pre-flight determined radiometric response coefficients. In-flight radiometric calibration of the sensor will then provide monthly updates to these coefficients, to account for degradation which may occur during the mission. THe spectral response profiles are invariant in time, and are provided only by the pre-flight measurements. These include an out-of-band spectral calibration of each channel. These spectral data are used as input to the radiometric calibration of the instrument, and also to produce certain Level 2 products for which an out- of-band correction is made. This paper describes the calibration program, with emphasis on results from the recently completed pre-flight calibration.
The multi-angle imaging spectro-radiometer (MISR) instrument, which is scheduled to fly on the EOS AM1 platform, contains nine refractive cameras (four different lens designs) at preselected view angles which image in the push broom mode. Each focal plane contains four charge coupled device (CCD) line arrays consisting of 1504 active pixels; each array is preceded by one of the MISR spectral filters. In order to facilitate registration of the data generated by the 36 arrays during the initial phase of the mission, the crosstrack pointing angle of each pixel in each array was measured in the laboratory at the camera subsystem level. These measurements were particularly challenging because the pixels had to be calibrated under flight conditions (in a vacuum over the temperature range 0 to 10 degrees Celsius) to an accuracy of 1/8 pixel or 2.6 micrometer. Given the first order properties of the various lenses, this requirement implies that the distortion had to be calibrated to better than 10 arcsec. This paper will discusses the hardware and software techniques utilized to accomplish this stringent calibration.
A unique, highly automated thermal-vacuum facility for optical testing of lenses and cameras is described. In particular, measurements of MTF, boresight, and geometric image distortion over a large parameter space including wavelength, field of view and temperature will be discussed. Unique aspects of the facility include a 'virtual nodal bench' opto-mechanical metrology system and fiber-optic illumination of mechanical reference features.
MISR will provide global data sets from Earth orbit using nine discrete cameras, each viewing at unique view directions. The design of this instrument is complete and has been refined following assembly and testing of an engineering model. The engineering model has been invaluable in identifying correctable design flaws, in resolving subsystem interface issues early in the program, and in providing the science team with as-built performance data to be used in the algorithm development. MISR will fly with an on-board calibrator consisting of Spectralon diffuse panels and photodiode detector standards. Both the use of Spectralon and flight detector standards have been developed by the MISR team. Currently the engineering team is assembling and testing the flight cameras, and the data teams are preparing for the post-launch geometric and radiometric calibration of the instrument, as well as developing algorithms to provide the science products. With a 3.3 Mb orbital average data rate, and global coverage each nine days, processing will be automated and standardized. Deliverables include calibrated, registered data sets, as well as aerosol/land surface, and cloud parameters.
The EOS/MISR instrument contains nine cameras; each camera focal plane consists of four closely spaced, linear CCD arrays. Each of the four arrays is preceded by a narrow bandpass filter covering a distinct spectral region. Testing of two engineering model cameras revealed spatial and spectral optical crosstalk levels not predicted by stray light analysis. A detailed investigation of filter coating anomalies and the filter assembly/CCD geometry elucidated several mechanisms responsible for optical crosstalk. These mechanisms, as well as recommendations concerning the design of future focal planes, are presented in this paper.
The second generation Wide-Field/Planetary Camera (WF/PC-II) for the Hubble Space Telescope (HST) was modeled to access the impact of manufacturing, alignment, and environmental tolerances on performance. This analysis showed that the lateral registration of the image of the Optical Telescope Assembly (OTA) pupil to the surface providing the spherical correction must be aligned and maintained through launch to 50 microns; WF/PC-I was an order of magnitude less sensitive. Inherited WF/PC-I hardware was subjected to new WF/PC-II environmental tests. As a result WF/PC-II was reconfigured to ensure on-orbit performance: the focus mechanism was removed to increase stability through launch and on- orbit, and four formerly fixed mirrors were actuated, to provide capability for on-orbit pupil alignment. This paper traces the evolution of the WF/PC-II error budget from its WF/PC-I beginnings to the current configuration.
The second generation Wide-Field/Planetary Camera (WF/PC-II) for the Hubble Space Telescope (HST) was modeled to access the impact of manufacturing, alignment, and environmental tolerances on performance. This analysis showed that the lateral registration of the image of the optical telescope assembly (OTA) pupil to the surface providing the spherical correction must be aligned and maintained through launch to 50 microns; WF/PC-I was an order of magnitude less sensitive. Inherited WF/PC-I hardware was subjected to new WF/PC- II environmental tests. As a result WF/PC-II was reconfigured to ensure on-orbit performance: the focus mechanism was removed to increase stability through launch and on- orbit, and four formerly fixed mirrors were actuated, to provide capability for on-orbit pupil alignment. This paper traces the evolution of the WF/PC-II error budget from its WF/PC-I beginnings to the current configuration. This information should be of general interest to designers of future HST instruments.
This paper describes the use of a Hartmann-type pupil mask and CCD camera to perform wavefront and focus validation of the Hubble Space Telescope simulator before and during environmental testing of the second-generation Wide Field and Planetary Camera. The method yields a focus accuracy at F/24 of about +/- 100 micrometers even in the presence of 3.5 waves of surface spherical aberration. The method avoids the introduction of potentially imperfect auxiliary optical tooling (e.g., null correctors).