TPF-I SWG Report - Exoplanet Exploration Program - NASA

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C HAPTER 4 0 ±π/2 0 ±π/2 ±π ±3π/2 ±π X-Array ±3π/2 0 ±2π/3 ±4π/3 Linear DCB Single hop ±π Linear 3 0 ±π 0 ±π/2 Z-Array Triangle ±2π/3 Diamond DCB ±3π/2 Multi-hop 0 ±4π/3 Two-hop ±π/2 ±3π/2 Figure 4-8. Nulling configurations considered in the trade study. The target star is in the direction normal to the page. The phasing of each collector is given for each of the two chop states. Pale blue circles represent collector spacecraft; yellow circles are dedicated combiner spacecraft; hatched yellow and blue circles are collector spacecraft with combining optics. Colored lines indicate the beam relay paths for each aperture. A number of other configurations were ruled out, as described in the main text. Originally implemented in MathCad, the model has now been migrated to MatLab. The starting point is the catalog of 1014 <strong>TPF</strong>-I candidate target stars described in Section 2.5 and illustrated by the circles in Figure 4-7. Given a specific nulling configuration and collector diameter, the program cycles through each star, determining first whether it meets the criteria for ecliptic latitude (solar shading constraint) and inner working angle. It then estimates the time needed to achieve an SNR of 5 for a broad-band detection of an Earth-sized planet, using a simplified version of the full performance model. The list of targets is then sorted in ascending order of integration time. With 2 years of mission time available for the initial survey, we assume that 50% of this time will be spent integrating on targets and that each target will be visited three times to ensure a completeness in excess of 90%. The number of targets that can be surveyed is therefore given by the star in the sorted list by which the cumulative integration time has reached 2 x 50% / 3 = 122 days. The colored circles in Figure 4-7 indicate the integration time needed to detect an Earth-sized planet around the star. The mid-IR signal from an Earth-sized planet in the mid-Habitable Zone depends only on the distance to the system. The noise is dominated by contributions from the local zodiacal dust (invariant with distance) and stellar size leakage (which decreases with distance but increases for the earlier spectral types). As a result the easiest targets in red are the nearby K stars. Integration times increase with distance and with the intrinsic size of the star. The solid line shows schematically a contour 68
D ESIGN AND A R C H I T E C T U R E T RADE S TUDIES of constant integration time; targets to the left are limited by local zodiacal emission, while those to the right are limited by stellar leakage. This technique can also be extended to predict the number of targets that can be spectroscopically characterized in a given amount of time. This depends on the prevalence of Earth-like planets in the Habitable Zone. If planets are rare then the average distance to the systems being characterized is high, the integration times are long, and only a few systems can be accommodated in the time available. If planets are very common, however, then we will be able to characterize a much larger number of nearby systems. The simple algorithm to account for this is described in Dubovitsky and Lay (2004). A more sophisticated model to predict and optimize the program completeness is now underway. Based on similar analysis for the <strong>TPF</strong>-C mission, the algorithm assesses the observability for each of 1000 planets around each star. For each week of the mission, only the most productive stars are selected in order to maximize the number of planets found. 4.3 Nulling Configurations We define an architecture by the combination of nulling configuration, collector aperture diameter, beam routing between spacecraft, beam combiner design, number of launches, and type of launch vehicle. The nulling configuration includes the number and relative locations of the collectors, and the amplitudes and phases with which each collector beam is combined. All are significantly constrained in geometry by the need for equal optical path lengths from each collector to the combiner. The six basic architectures compared in this study are listed in Figure 4-8, ranging from three to five spacecraft. The first four are all part of the Dual Chopped Bracewell (DCB) family, in which the four apertures have phases of 0, π/2, π, and 3π/2 radians. The Linear DCB (Beichman et al. 1999) can be phased in two ways, with either separated or interleaved nulling baselines. In the analysis we choose the optimal case for each observing scenario. The X-array (Lay and Dubovitsky 2004) chosen for study has a fixed 2:1 aspect ratio (a tunable aspect ratio is discussed in Section 7). The Diamond DCB and Z-Array were both proposed by Anders Karlsson (ESA) as a means of reducing the number of spacecraft; the hatched circle in the schematic indicates a spacecraft that functions as both a collector and combiner. The Z-Array uses multiple relays between the collectors to balance the path lengths. The four DCB architectures have identical beam combiners. The Triangle and Linear 3 are based on a three-way nulling strategy with phases of 0, 2π/3, and 4π/3 (Karlsson et al. 2004). The other three-spacecraft designs have been proposed by Serabyn and Mennesson (2004). In all cases, the spacecraft are confined to a plane perpendicular to the target star direction for thermal reasons. The beams must be routed such that the path lengths from the star to the combiner are equal; this is achieved with a single hop from the collector to the combiner in the X-array, two hops for the Linear DCB, Diamond DCB, Triangle and Linear 3, and as many as four hops for the Z-Array. Many nulling configurations were not considered in the trade study. The single Bracewell nuller is attractive for its simplicity (only two collectors), but it cannot accommodate a phase chop, making it extremely vulnerable to slow systematic effects (a variation of 1 photon per second of scattered light can look like a planet, for example). There are also a number of configurations with higher-order nulls, in which the central null of the response on the sky has a broader null than the quadratic profiles seen in 69
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JPL Publication 07-1 Terrestrial Pl
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Abstract Over the past two years, t
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Acknowledgements Twenty-two represe
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Table of Contents 1 Introduction...
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4.8.3 Post-Nulling Calibration.....
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6.4.5 Double Fourier Interferometry
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I NTRODUCTION 1 Introduction Over 2
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I NTRODUCTION et al. 2004), and res
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I NTRODUCTION Standard Interferomet
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I NTRODUCTION 1.4 References Angel,
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C HAPTER 2 0.7 to 1.5 AU scaled by
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C HAPTER 2 Table 2-2. Illustrative
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C HAPTER 2 Classical interferometry
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C HAPTER 2 trivial in the absence o
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Page 38 and 39: C HAPTER 2 Lines of constant stella
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Page 48 and 49: C HAPTER 2 Beichman, C. A., Fridlun
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Page 52 and 53: C HAPTER 2 Reach, W. T., Franz, B.
Page 54 and 55: C HAPTER 3 3.1.1 Darwin/TPF-I Prope
Page 56 and 57: C HAPTER 3 clouds or an OB associat
Page 58 and 59: C HAPTER 3 Figure 3-3. Three possib
Page 60 and 61: C HAPTER 3 the circumstellar disk,
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Page 64 and 65: C HAPTER 3 Continuum interferometry
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Page 68 and 69: C HAPTER 3 3.3 Conclusions Darwin/T
Page 70 and 71: C HAPTER 3 Nguyen, H. T., Kallivaya
Page 73 and 74: D ESIGN AND A R C H I T E C T U R E
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C HAPTER 5 a) c) b) d) IWA = 43 mas
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C HAPTER 5 planets found. Figure 5-
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C HAPTER 5 30 25 5 M earth 3 M eart
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C HAPTER 5 5.8.4 Angular Resolution
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C HAPTER 5 Table 5-3. Angular Resol
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C HAPTER 5 The optimization works b
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T E C H N O L O G Y R OADMAP FOR TP
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F UTURE D EVELOPMENTS 7 Preparatory
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F UTURE D EVELOPMENTS • To comple
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F UTURE D EVELOPMENTS Table 7-1. Pr
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D ISCUSSION AND C ONCLUSION 8 Discu
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Appendices 167
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T E C H N O L O G Y A DVISORY C O M
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F L I G H T S YSTEM C ONFIGURATION
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F L I G H T S YSTEM C ONFIGURATION
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A PPENDIX C 2. Identical FACS fligh
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A PPENDIX C 2. Recall that the coef
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A PPENDIX C Once the formation has
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A PPENDIX C Figure C-3. Example of
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A PPENDIX C Figure C-4. FAST Flight
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A PPENDIX C The “true” time of
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A PPENDIX C References Cohen, S., a
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A PPENDIX D DC DCB DM DM DOCS DOD D
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A PPENDIX D NaCl NAR NASA NASTRAN N
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A PPENDIX E Appendix E TPF-I Review
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A PPENDIX F Alice Quillen, Universi
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A PPENDIX F Figure 3-1 - Original f
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