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Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
A widely used means of mounting the telescope tube so that it may be pointed at an object and then moved to follow the object’s motion across the sky is the equatorial mounting. This is a two-axis mounting, with one axis, the polar axis, aligned parallel with the Earth’s rotational axis and the other, the declination axis, perpendicular to the polar axis. The design has the enormous advantage that only a single constant velocity motor is required to rotate the mounting around the polar axis to track an object. It is also convenient in that angular read-outs on the two axes give the hour angle or right ascension and declination directly.
Mechanical systems and design
Published in D.A. Bradley, N.C. Burd, D. Dawson, A.J. Loader, Mechatronics, 2018
D.A. Bradley, N.C. Burd, D. Dawson, A.J. Loader
In order to track a particular object in the sky, as the earth rotated, the equatorial mounting was developed to synchronize with the earth’s axis of rotation. On large telescopes, such as the 200 inch Mount Palomar unit, this resulted in large structures driven by constant speed motors. Using a mechatronics approach, a compact balanced support structure can be designed with just two degrees of freedom which will track any object in the heavens under computer control.
Monteiro da Rocha and the international debate in the 1760s on astronomical methods to find the longitude at sea: his proposals and criticisms to Lacaille’s lunar-distance method
Published in Annals of Science, 2022
Fernando B. Figueiredo, Guy Boistel
Monteiro da Rocha also provides this kind of ‘immutable tables’, eleven tables on the whole to correct the dip and refraction effects, the moon’s and sun’s parallax and semi-diameter. For refraction Monteiro da Rocha uses both values provided by Lacaille's and Bradley's tables, calculating the arithmetic mean of the values. He also computed some other tables to help with some astronomical calculations required by his methods. For instance, there is a table (table III) used to help latitude calculations that allow for reduction all observations before and after the meridian passage of the Sun. Another (table VII) helps convert equatorial into ecliptic coordinates (mainly right ascensions into longitudes). Other (table VIII) gives the angular distances of the ecliptic points to the place's zenith. There is another one (table IX) that provides the angle between the ecliptic and the meridian planes. Tables X and XI are used for converting units (arc degrees in time and vice versa). Although presented at the end of the manuscript (MS BNP 511, fls.96–105), their explanations are made before the methods to determine longitude. To a better and easier understanding of their use, Monteiro always gives elucidative and practical examples.
Molecular-weight dependence of phase structure and viscosity in a liquid crystalline polyester with strong π–π interaction
Published in Liquid Crystals, 2019
Rong Yang, Lv Ding, Weilong Chen, Xin Zhang, Jinchun Li
Because XRD analysis of high-molecular-weight PBDPS while heating was conducted in our previous work [24,27], the 2D WAXS pattern at room temperature and WAXD patterns during heating of PBDPS5.2k were obtained to verify the phase structure of PBDPS. Figure 4 shows that before the 2D WAXS test, the fibre was spun at 100°C and then annealed at 60°C. With the fibre in the equatorial direction, the layer scattering up to the third order appears on the equator. In the high-angle region, two scattering arcs are on the meridian, which should mainly originate from the interference between side chains parallel to each other. These results proved that PBDPS showed a SmB phase with a chain-folding packing and side benzene rings stacked like a zipper perpendicular to the mesogen unit. A few changes were noticed in the variable-temperature WAXD patterns when the temperature increased from 60°C to 80°C. First, in the low-angle range, the peaks at 2.9° remained, whereas the other peaks at 5.7° and 7.2° disappeared, which suggested that PBDPS5.2k showed a smectic phase at 80°C but that the smectic layer was not perfect. In the high-angle range, the peak at 20.0° became broad, and the peak at 22.4° disappeared, which indicated that there is no order in the smectic layer (SmA phase). When the temperature was increased to 100°C, the peak at 2.9° disappeared, and the peak at 20.0° became amorphous, indicating an isotropic phase, which is in accordance with the DSC results. However, the peak at 27.6° remained even when the temperature increased up to 140°C. These results demonstrated that the ordered benzene rings from different domains were retained in the isotropic phase of PBDPS.
Investigating the impact of the latitudinal velocity profile on nonlinear gradient drift instability development in the subauroral ionosphere
Published in Radiation Effects and Defects in Solids, 2022
Lujain Almarhabi, Chirag R. Skolar, Wayne Scales, Bhuvana Srinivasan
Subauroral polarization streams (SAPS) are regions in the mid-latitude ionosphere with large westward flow driven by a poleward electric field. Ionospheric irregularities have been observed in SAPS from space weather radar, GPS, and satellite data. Ionospheric irregularities are small-scale structures in the plasma density created by various plasma instabilities, which are driven by combinations of plasma drifts, density gradients, and electric fields [1]. Ionospheric processes producing the mid-latitude GPS scintillations (i.e. amplitude and phase fluctuations of the signal that can be deleterious to the performance of the communication link) are less understood due to a lack of models and observations that can explain their characteristics and distributions [2]. The mid-latitude ionosphere is an integral part of radio communications, in that the presence of such irregularities impacts communication systems by producing mid-latitude GPS scintillation. The mid-latitude ionosphere is known as a buffer zone that lies between the high-latitude ionosphere (above ) and the equatorial ionosphere (below to the equator) [3]. In this work, the gradient drift instability (GDI) and the Kelvin-Helmholtz instability (KHI) are explored in order to understand how they may potentially cause turbulence in the ionosphere. We focus in particular on the gradient drift instability (GDI), since it is a major mechanism of plasma structuring in the F ionospheric regions [4]. A better understanding of the formation and development of ionospheric turbulence can provide insight on improving models of space weather, hence improving the accuracy of communication signals.