اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدHigh-precision CTE measurement of hybrid C/SiC composite for cryogenic space telescopes
101
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
This paper presents highly precise measurements of thermal expansion of a hybrid carbon-fiber reinforced silicon carbide composite, HB-Cesictextregistered - a trademark of ECM, in the temperature region of sim310-10K. Whilst C/SiC composites have been considered to be promising for the mirrors and other structures of space-borne cryogenic telescopes, the anisotropic thermal expansion has been a potential disadvantage of this material. HB-Cesictextregistered is a newly developed composite using a mixture of different types of chopped, short carbon-fiber, in which one of the important aims of the development was to reduce the anisotropy. The measurements indicate that the anisotropy was much reduced down to 4% as a result of hybridization. The thermal expansion data obtained are presented as functions of temperature using eighth-order polynomials separately for the horizontal (XY-) and vertical (Z-) directions of the fabrication process. The average CTEs and their dispersion (1{sigma}) in the range 293-10K derived from the data for the XY- and Z-directions were 0.805$pm$0.003times10$^{-6}$ K$^{-1}$ and 0.837pm0.001times10$^{-6}$ K$^{-1}$, respectively. The absolute accuracy and the reproducibility of the present measurements are suggested to be better than 0.01times10$^{-6}$ K$^{-1}$ and 0.001times(10)^{-6} K^{-1}, respectively. The residual anisotropy of the thermal expansion was consistent with our previous speculation regarding carbon-fiber, in which the residual anisotropy tended to lie mainly in the horizontal plane.
قيم البحث
اقرأ أيضاً
Space missions designed for high precision photometric monitoring of stars often under-sample the point-spread function, with much of the light landing within a single pixel. Missions like MOST, Kepler, BRITE, and TESS, do this to avoid uncertainties
due to pixel-to-pixel response nonuniformity. This approach has worked remarkably well. However, individual pixels also exhibit response nonuniformity. Typically, pixels are most sensitive near their centers and less sensitive near the edges, with a difference in response of as much as 50%. The exact shape of this fall-off, and its dependence on the wavelength of light, is the intra-pixel response function (IPRF). A direct measurement of the IPRF can be used to improve the photometric uncertainties, leading to improved photometry and astrometry of under-sampled systems. Using the spot-scan technique, we measured the IPRF of a flight spare e2v CCD90 imaging sensor, which is used in the Kepler focal plane. Our spot scanner generates spots with a full-width at half-maximum of $lesssim$5 microns across the range of 400 nm - 900 nm. We find that Keplers CCD shows similar IPRF behavior to other back-illuminated devices, with a decrease in responsivity near the edges of a pixel by $sim$50%. The IPRF also depends on wavelength, exhibiting a large amount of diffusion at shorter wavelengths and becoming much more defined by the gate structure in the near-IR. This method can also be used to measure the IPRF of the CCDs used for TESS, which borrows much from the Kepler mission.
Many aspects of the design trade-off of a space-based instrument and its performance can best be tackled through simulations of the expected observations. The complex interplay of various noise sources in the course of the observations make such simu
lations an indispensable part of the assessment and design study of any space-based mission. We present a formalism to model and simulate photometric time series of CCD images by including models of the CCD and its electronics, the telescope optics, the stellar field, the jitter movements of the spacecraft, and all important natural noise sources. This formalism has been implemented in a versatile end-to-end simulation software tool, called PLATO Simulator, specifically designed for the PLATO space mission to be operated from L2, but easily adaptable to similar types of missions. We provide a detailed description of several noise sources and discuss their properties, in connection with the optical design, the allowable level of jitter, the quantum efficiency of the detectors, etc. The expected overall noise budget of generated light curves is computed as a function of the stellar magnitude, for different sets of input parameters describing the instrument properties. The simulator is offered to the scientific community for future use.
Faraday rotation measurements using the current and next generation of low-frequency radio telescopes will provide a powerful probe of astronomical magnetic fields. However, achieving the full potential of these measurements requires accurate removal
of the time-variable ionospheric Faraday rotation contribution. We present ionFR, a code that calculates the amount of ionospheric Faraday rotation for a specific epoch, geographic location, and line-of-sight. ionFR uses a number of publicly available, GPS-derived total electron content maps and the most recent release of the International Geomagnetic Reference Field. We describe applications of this code for the calibration of radio polarimetric observations, and demonstrate the high accuracy of its modeled ionospheric Faraday rotations using LOFAR pulsar observations. These show that we can accurately determine some of the highest-precision pulsar rotation measures ever achieved. Precision rotation measures can be used to monitor rotation measure variations - either intrinsic or due to the changing line-of-sight through the interstellar medium. This calibration is particularly important for nearby sources, where the ionosphere can contribute a significant fraction of the observed rotation measure. We also discuss planned improvements to ionFR, as well as the importance of ionospheric Faraday rotation calibration for the emerging generation of low-frequency radio telescopes, such as the SKA and its pathfinders.
High precision astrometric Space Very Long Baseline Interferometry (S-VLBI) at the low end of the conventional frequency range, i.e. 20cm, is a requirement for a number of high priority science goals. These are headlined by obtaining trigonometric pa
rallax distances to pulsars in Pulsar--Black Hole pairs and OH masers anywhere in the Milky Way Galaxy and the Magellanic Clouds. We propose a solution for the most difficult technical problems in S-VLBI by the MultiView approach where multiple sources, separated by several degrees on the sky, are observed simultaneously. We simulated a number of challenging S-VLBI configurations, with orbit errors up to 8m in size and with ionospheric atmospheres consistant with poor conditions. In these simulations we performed MultiView analysis to achieve the required science goals. This approach removes the need for beam switching requiring a Control Moment Gyro, and the space and ground infrastructure required for high quality orbit reconstruction of a space-based radio telescope. This will dramatically reduce the complexity of S-VLBI missions which implement the phase-referencing technique.
We present a method for the manufacturing of thin shells of glass, which appears promising for the development of active optics for future space telescopes. The method exploits the synergy of different mature technologies, while leveraging the commer
cial availability of large, high-quality sheets of glass, with thickness up to few millimeters. The first step of the method foresees the pre-shaping of flat substrates of glass by replicating the accurate shape of a mold via hot slumping technology. The replication concept is advantageous for making large optics composed of many identical or similar segments. After the hot slumping, the shape error residual on the optical surface is addressed by applying a deterministic sub-aperture technology as computer-controlled bonnet polishing and/or ion beam figuring. Here we focus on the bonnet polishing case, during which the thin, deformable substrate of glass is temporary stiffened by a removable holder. In this paper, we report on the results so far achieved on a 130 mm glass shell case study.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
