KEYWORDS: Stars, Interferometers, Charge-coupled devices, Visualization, Superposition, Target detection, Photons, Signal detection, Error analysis, Signal to noise ratio
SIM PlanetQuest will measure star positions to an accuracy of a few microarcseconds using precise white light
fringe measurements. One challenge for the SIM observation scenario is "star confusion," where multiple stars
are present in the instrument field of view. This is especially relevant for observing dim science targets because
the density of number of stars increases rapidly with star magnitude. We study the effect of star confusion on
the SIM astrometric performance due to systematic fringe errors caused by the extra photons from the confusion
star(s). Since star confusion from multiple stars may be analyzed as a linear superposition of the effect from single
star confusion, we quantify the astrometric errors due to single star confusion surveying over many spectral types,
including A0V, F0V, K5III, and M0V, and for various visual magnitude differences. To the leading order, the
star confusion effect is characterized by the magnitude difference, spectral difference, and the angular separation
between the target and confusion stars.
Strategies for dealing with star confusion are presented. For example, since the presence of additional sources
in the field of view leads to inconsistent delay estimates from different channels, with sufficient signal to noise
ratio, the star confusion can be detected using chi-square statistics of fringe measurements from multiple spectral
channels. An interesting result is that the star confusion can be detected even though the interferometer cannot
resolve the separation between the target and confusion stars when their spectra are sufficiently different. Other
strategies for mitigating the star confusion effect are also discussed.
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.
Like all astrometric instruments, the Space Interferometry Mission (SIM) suffers from field-dependent errors requiring calibration. Diffraction effects in the delay line, polarization rotations on corner cubes, and beam walk across imperfect optics, all contribute to field-distortion that is significantly larger than is acceptable. The bulk of the systematic error is linear across the field - that is, it results in magnification and rotation errors. We show that the linear terms are inconsequential to the performance of SIM because they are inseparable from baseline length and orientation errors. One approach to calibrating the higher-order terms is to perform 'external' calibration; that is, SIM periodically makes differential measurements of a field of bright stars whose positions are not precisely known. We describe the requirements and constraints on the external calibration process and lay the groundwork for a specific procedure detailed in accompanying papers.
Optical systems, which operate over a wide range of Fresnel numbers, are often times performance-limited by diffraction effects. In order to characterize such effects at the 40-100 picometer level, a diffraction testbed has been built which has the capability of measuring diffraction effects at this level. Concurrently, mathematical diffraction modeling tools have been developed that propagate an input wavefront through an optical train, while retaining amplitude and phase information at a grid resolution sufficient for yielding picometer-resolution diffraction test data. This paper contains a description of this diffraction hardware testbed, the diffraction modeling approach, and a comparison of the modeled and hardware test results, which then serves as validation of the diffraction modeling methodology.
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.
KEYWORDS: Data modeling, Digital filtering, Optical filters, Space operations, Interferometers, Data acquisition, Finite element methods, Fourier transforms, Interfaces, Matrices
This paper presents a modeling methodology used to predict the performance of a flexible structure, such as a space telescope, in the presence of an on-board vibrational disturbance source, such as a reaction wheel assembly (RWA). Both decoupled and coupled analysis methods are presented. The decoupled method relies on blocked RWA disturbances, measured with the RWA hardmounted to a rigid surface. The coupled method corrects the blocked RWA disturbance boundary conditions using 'force filters' which depend on estimates of the interface accelerances of the RWA and spacecraft. Both methods were validated on the Micro-Precision Interferometer testbed at the Jet Propulsion Laboratory. Experimental results are encouraging, indicating that both methods provide sufficient accuracy compared to measured values; however, the coupled method provides the best results when the gyroscopic nature of the spinning RWA is captured in the RWA accelerance model. Additionally, the RWA disturbance cross spectral density terms are found to be influential.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.