Pre-Phase A Report - Lisa - Nasa

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118 Chapter 4 Measurement Sensitivity thickness shells ranging all the way out to cosmological distances, and thus will be nearly isotropic. The small anisotropies in the background due to nearby concentrations of stars such as the LMC, M31, and the Virgo cluster, as discussed by Lipunov et al. [117], will be difficult to detect. The only handle for separating the possible cosmic backgrounds from the dominant isotropic part of the extragalactic binary background is the spectrum. There is enough uncertainty in the ratios of the numbers of binaries of various kinds in other galaxies to the numbers in our galaxy so that the strength of the extragalactic binary background cannot be predicted reliably. However, since the types of binaries contributing most strongly at the frequencies of interest probably will be evolving mostly by emitting gravitational radiation, the spectrum may be known quite well. If the spectrum of a cosmic background were significantly different and the amplitude were large enough, such a background could be separated and quantified. Perhaps the most significant question is how well all of the backgrounds can be separated from instrumental noise. To discuss this question, it is useful to divide the instrumental noise above roughly 10−4 Hz into three different types. One is stationary noise with steady amplitude at all frequencies of interest. The second is noise which varies at one and two cycles/year, in such a way that it mimics the interaction of the galactic background with the rotating antenna pattern. The third is noise with all other types of time variations. The first and second types cannot be separated from isotropic and galactic backgrounds, respectively, except if they are substantially higher than experimental limits which can be put on instrumental noise. The third type of noise would not be confused with real background signals. For measuring the difference in distances between proof masses, the main noise sources are photon shot noise and phase shifts from fluctuations in laser beam pointing. The shot noise contribution can be calculated from the received light level and at least partly subtracted out. For beam pointing fluctuations, it is difficult to say how much of the noise may be of the first two types, but an estimate of a third or less of the level given in the error budget seems reasonable. Specific experiments during the mission to characterize the noise by changing the gain of the beam-pointing servo loops and temporarily defocusing the beams somewhat should be considered. For spurious accelerations of the proof masses, there are a number of items of comparable size in the error budget. A few, like random residual gas impacts on the proof masses, may be quite stationary, although they also may have variations at annual and six month periods from spacecraft temperature variations. However, it seems likely that most of the spurious acceleration sources will not be predominantly stationary. For example, this would apply to sources such as the interaction of the average charge on the proof mass with the fluctuations in the solar wind magnetic field. Consideration will be given to including diagnostic experiments, such as changing the average proof-mass charge or changing how tightly the spacecraft follow the proof masses, in order to characterize the spurious acceleration noise sources as well as possible. Overall, it seems reasonable to estimate that perhaps a third of the total instrumental noise in the distance measurement and spurious acceleration error budgets would be difficult to separate from real background signals. The extent to which data from the third arm of LISA would aid in searching for background signals has not yet been investigated. 3-3-1999 9:33 Corrected version 2.08
Chapter 5 Payload Design 5.1 Payload structure design concept The design objectives for the payload structure are: 1. To ensure that the structure exhibits no modes of vibration below 60 Hz during launch. 2. To ensure that the structure exhibits no modes of vibration between 0.1 Hz and 10−4 Hz during operation. 3. To allow independent pointing adjustment for the two laser systems. 4. To minimise gravitational and optical disturbances due to thermally induced distortions. An iterative design study, involving 3D-CAD modelling and Finite Element Analysis, has produced a conceptual design that meets the first three requirements. The fourth requirement is met to a greater extent, but further work is required to quantify acceptable levels of distortion and to model the behaviour of the structure in more detail. The design has a number of features that arise from the requirement specifications: • All of the optical components are mounted together within rigid subassemblies – the optical assemblies – to minimise changes to critical alignment dimensions. • These two subassemblies are enclosed within the arms of the rigid Y-shaped payload thermal shield. They are attached to the inside by flex-pivot assemblies and pointing actuators to allow the alignment of the optical assemblies to be adjusted. These adjustments are made by moving each entire optical assembly within the payload thermal shield; the shield itself does not move with respect to the spacecraft during alignment. • The payload thermal shield acts as the main structural member. In addition to the optical assemblies, the radiator plate is supported from the Y-tube’s underside. It carries the lasers and their drive electronics, along with the UV discharge unit. Mounting the radiator plate in this manner minimises the number of interfaces with the spacecraft. Corrected version 2.08 119 3-3-1999 9:33
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LISA Laser Interferometer Space Ant
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LISA Laser Interferometer Space Ant
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Foreword The first mission concept
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The result of the Team-X study was
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Contents Foreword iii Executive Sum
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Contents ix 4.2.3 Acceleration nois
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Contents xi 9 Science and Mission O
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Executive Summary 1 Executive Summa
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Executive Summary 3 Complementarity
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Mission Summary 5 LISA Mission Summ
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Chapter 1 Scientific Objectives By
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1.1 Theory of gravitational radiati
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Chapter 2 Different Ways of Detecti
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2.2 Ground-based detectors 39 2.2 G
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2.2 Ground-based detectors 41 Count
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2.4 Spacecraft tracking 43 2.4 Spac
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2.7 Heliocentric versus geocentric
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2.8 The LISA concept 47 telescopes
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2.8 The LISA concept 49 Figure 2.6
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2.8 The LISA concept 51 seem rather
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Chapter 3 Experiment Description 3.
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3.1 The interferometer 55 Fiber to
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3.1 The interferometer 57 3.1.4 Sys
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3.1 The interferometer 59 Figure 3.
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3.1 The interferometer 61 3.1.6 Las
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3.1 The interferometer 63 signal sh
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3.1 The interferometer 65 for a sin
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Chapter 9 Science and Mission Opera
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9.2 Mission operations 171 9.2 Miss
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9.3 Operating modes 173 Power savin
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Chapter 10 International Collaborat
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10.3 Schedule 177 The ESA Project M
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References [1] C.W. Misner, K.S. Th
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References 181 [35] C. Cutler, T.A.
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References 183 [77] J.E. Faller and
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References 185 [104] E. Grun, H.A.
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List of Acronyms - and other abbrev
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List of Acronyms 189 LHC Large Hadr
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Rear cover figure : This Report has
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