China
Africa
Explore chapters and articles related to this topic
Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
The one remaining class of optical telescopes is the catadioptric group, of which the Schmidt camera is probably the best known. A catadioptric system uses both lenses and mirrors in its primary light gathering section. Very high degrees of correction of the aberrations can be achieved because of the wide range of variable parameters that become available to the designer in such systems. The Schmidt camera uses a spherical primary mirror so that coma is eliminated by not introducing it into the system in the first place! The resulting spherical aberration is eliminated by a thin correcting lens at the mirror’s radius of curvature (Figure 1.57). The only major remaining aberration is field curvature, and the effect of this is eliminated through the use of a detector shaped to match the focal surface or through the use of additional correcting lenses (field flatteners). The correcting lens can introduce small amounts of coma and chromatic aberration but is usually so thin that these aberrations are negligible. Diffraction-limited performance over fields of view of several degrees with focal ratios as fast as f1.5 or f2 is possible.
Telescopes
Published in Daniel Malacara-Hernández, Brian J. Thompson, Fundamentals and Basic Optical Instruments, 2017
Marija Strojnik, Maureen S. Kirk
The elementary implementation of the Schmidt telescope incorporates a spherical primary with a stop and a corrector plate at the center of the curvature of the primary mirror. The freedom from coma and astigmatism results from monocentric construction with the stop at the mirror's center of curvature. For the object at infinity, the image is free of spherical aberration, coma, astigmatism, and distortion. The focal surface has a radius equal to its focal length. Rosete-Aguilar and Maxwell [22] thoroughly discussed the design of the corrector plates.
High-frame-rate liquid crystal phase modulator for augmented reality displays
Published in Liquid Crystals, 2019
Ran Chen, Yuge Huang, Jian Li, Minggang Hu, Juanli Li, Xinbing Chen, Pei Chen, Shin-Tson Wu, Zhongwei An
Liquid-crystal-on-silicon (LCoS) panel has been widely used for intensity modulation in augmented reality (AR) displays, such as Google Glass and Microsoft HoloLens, because of its high resolution, low driving voltage and low power consumption [1–3]. To overcome the focus-cue mismatch issue, phase-only LCoS has found useful applications in focal surface displays [4–6]. For these applications, a large phase modulation depth (≥2π) is required. Meanwhile, the liquid crystal (LC) response time should be less than 4 ms in order to achieve the desired 240 Hz frame rate. From [4,5], both Oculus and Microsoft devices were operated at 60 Hz, which is 4 times too slow to meet the required 240 Hz. Therefore, there is urgent need to develop a phase-only LCoS device with response time less than 4 ms, while keeping a 2π phase change and low operation voltage (<6 V) without the overdrive and undershoot driving circuitry [7].
Curvatures of smectic liquid crystals and their applications
Published in Journal of Information Display, 2018
Among the defect structures in smectics, the most common is FCDs, modeled as families of the so-called Dupin cyclides that appear like asymmetric donut shapes. Borrowing the expression of Lavrentovich and Kleman, in order to understand the morphogenesis of layers in FCDs, it is necessary to consider a simple saddle-like smectic layer and parallel additions of the same featured layers, Si, where their normals surround two focal surface F1 and F2, having centers of principal curvature C1 and C2 and normals indicating the molecular director, n (Figure 3(a)) [27]. These focal surfaces show singularities of the director field, where the order parameter of the smectic layers is broken, and the energy associated with the singularities scales with the area of the focal surfaces. Because the elastic energy is concentrated around the focal surfaces, the smectic layers want to have smaller areas. Therefore, focal surfaces are degenerated into two lines which are a paired hyperbola and ellipse (the ellipse is perpendicular to the plane including hyperbola), or even smaller, a point (Figure 3(b)) [71]. The shrinkage of the focal surfaces into a point results in a concentric packing of layers with spherical shapes, which is rarely found during the formation of FCDs with free surfaces but appears in the sublimable smectics that will be discussed in Section 3 [68].
Effect of new punched vortex generators in a rectangular channel on heat transfer using Taguchi method
Published in Experimental Heat Transfer, 2022
S. Caliskan, A. Dogan, U. R. Sahin
For this reason, heat transfer calculations were performed by equalizing the upper surface and lower surface temperatures of the stainless-steel foil. Thermal images are obtained with a thermal camera positioned below the heater perpendicular to the channel. FLIR brand A640 thermal imager was used to determine the temperature distributions on the heating surface. Thanks to the AGEMA Researcher software and computerized thermography system, the camera can measure temperatures from −20°C to 1200°C with an uncertainty of ± 2%. The thermal imager system consists of 320 to 240 pixels of uncooled focal surface determinator between 7.5 and 13 micrometers. The field of view is 25°x18.8°/0.4 m, and the current field of view is 1.3 m-rad, and the thermal sensitivity is between 0.07°C and 30°C. Images obtained with the thermal imager were visualized and recorded using the FLIR-Quick Report program with the help of a computer for later analysis. A three-phase fan provides the forced air movement to the test section with a capacity of 2950 m3/h. VACON brand frequency controller is used to adjust the fan flow rate. The frequency controller operates in a range of 0–50 Hz and has a sensitivity of 0.01 Hz. A flexible circular pipe is mounted to minimize the vibrations between the fan outlet and the rectangular channel. The flow rate is measured with a hotwire-anemometer (The KIMO brand LV-107 model) with an accuracy of ± 3%. Pressure measurements were made by using FISCHER (DE39) brand pressure gauge. The probes for the pressure gauge are correctly mounted in the inlet and outlet sections of the test section of the rectangular channel. Temperatures at the inlet and outlet of the test zone were measured using a digital thermometer using a calibrated T-type thermocouple.