Flat Focal Surface Research Articles

Design and implementation of a Si3N4 three-stigmatic-point arrayed waveguide grating with a resolving power over 17,000.

To provide a solution to the issue of the non-flat focal surface in traditional Rowland AWGs, we have designed and implemented a Si3N4 three-stigmatic-point arrayed waveguide grating (TSP AWG) with three inputs, and a spectral resolving power over 17,000 has been achieved experimentally. The flat focal surface of this AWG can accommodate a butt-coupled detector array positioned at the output facet without any reduction of the resolving power of the edge channels. Therefore, it is particularly advantageous to some astronomical applications which require an AWG as a light-dispersing component to obtain a complete 2D spectrum. As a proof-of-concept for next generation devices, the multi-input aspect of the design accommodates multiple single-mode fibers coming into the AWG. In addition, because the device is implemented on a high-index-contrast platform (Si3N4/SiO2), a compact size of ∼9.3 × 9.3 mm2 is achieved.

Two-mirror aplanatic telescopes with a flat field

A complete description is given of two-mirror telescopes with a flat medial focal surface, on which the images of stars are circles of least confusion. Particular attention is paid to aplanats, since their field of view is noticeably larger than that of classical systems. Two sets of appropriate solutions correspond to Schwarzschild and Gregorian telescopes. As a result, it becomes possible to use flat light detectors with wide-field two-mirror telescopes. New designs are of particular interest when as few reflective surfaces as possible are required, which is typical for space exploration and non-optical observations.

Variable Curvature Displays: Optical Designs and Applications for VR/AR/MR Headsets

In the present paper, we discuss the design of a projection system with curved display and its enhancement by variably adjusting the curvature. We demonstrate that the focal surface curvature varies significantly with a change of the object position and that it can easily be computed with the Seidel aberration theory. Using this analytically derived curvature value as the starting point, we optimise a refocusable projection system with 90 ° field of view and F / # = 6.2 . It is demonstrated that such a system can provide stable image quality and illumination when refocusing from infinity to 1.5 m. The gain in spatial resolution is as high as 1.54 times with respect to a flat focal surface. Furthermore, we prove that a silicon die can be curved to the required shape with a safety factor of 4.3 in terms of the mechanical stress. Finally, it is shown that the developed system can be used in a virtual reality headset providing high resolution, low distortion and a flexible focusing mode.

Compact Survey Telescope with a Diameter of 3.6 m

Abstract An optical layout of a corrected Cassegrain telescope with an aperture of 3.6 m (f/3.0) and a 2.°5 field of view is proposed. The primary and secondary mirrors are standard hyperboloids; all five corrector lenses are made of fused silica and have spherical surfaces. The total length of the optical system is 4.36 m. The rms spot diameter in the “g,” “r,” “i” integral waveband of 0.40–0.82 μm is about 0.″3 across the flat focal surface; the D 80 star diameter is about 0.″4. Due to the low light vignetting, the effective aperture diameter reaches 3.3 m. The comparative simplicity and compactness of the optical layout allow us to consider this system promising for conducting deep sky surveys.

ON THE CAPABILITIES OF SURVEY TELESCOPES OF MODERATE SIZE

ABSTRACT To explore the capabilities of moderate-size optical telescopes used in surveys, a set of nine new wide-field designs, having apertures of up to 1 m, was created. The designs were optimized to ensure the widest possible field of view with an image quality better than 3″ across the field. All but one of the systems have an angular field in a range of 3.°5–10° and a flat focal surface; the field of the last system is 45° in diameter with an aperture 0.5 m and a spherical focal surface. Relations between the expected limiting magnitude, survey speed, and exposure time allow one to choose the system that is best suited for the objectives of specific observations. In particular, a single wide-field telescope with an aperture of approximately 1 m can detect objects brighter than 22.5 m over the entire hemisphere within one night. Since the optical layouts are of practical interest, their complete descriptions are given.

Luneburg lens with extended flat focal surface for electronic scan applications.

Luneburg lens with flat focal surface has been developed to work together with planar antenna feeds for beam steering applications. According to our analysis of the conventional flattened Luneburg lens, it cannot accommodate enough feeding elements which can cover its whole scan range with half power beamwidths (HPBWs). In this paper, a novel Luneburg lens with extended flat focal surface is proposed based on the theory of Quasi-Conformal Transformation Optics (QCTO), with its beam steering features reserved. To demonstrate this design, a three-dimensional (3D) prototype of this novel extend-flattened Luneburg lens working at Ku band is fabricated based on 3D printing techniques, whose flat focal surface is attached to a 9-element microstrip antenna array to achieve different scan angles. Our measured results show that, with different antenna elements being fed, the HPBWs can cover the whole scan range.

Planar, flattened Luneburg lens at infrared wavelengths

Employing artificially structured metamaterials provides a means of circumventing the limits of conventional optical materials. Here, we use transformation optics (TO) combined with nanolithography to produce a planar Luneburg lens with a flat focal surface that operates at telecommunication wavelengths. Whereas previous infrared TO devices have been transformations of free-space, here we implement a transformation of an existing optical element to create a new device with the same optical characteristics but a user-defined geometry.

Exact optics - III. Schwarzschild's spectrograph camera revised

Karl Schwarzschild identified a system of two mirrors, each defined by conic sections, free of third-order spherical aberration, coma and astigmatism, and with a flat focal surface. He considered it impractical, because the field was too restricted. This system was rediscovered as a quadratic approximation to one of Lynden-Bell's ‘exact optics’ designs which have wider fields. Thus the ‘exact optics’ version has a moderate but useful field, with excellent definition, suitable for a spectrograph camera. The mirrors are strongly aspheric in both the Schwarzschild design and the exact optics version.

High-order laser harmonics detection in the EUV and soft x-ray spectral regions

The design and performance of a spectroscopic instrument able to simultaneously measure spectral features, angular divergence, and conversion efficiency of high-order harmonics generated by an ultrashort pulsed laser focused onto a gas jet are presented. It combines an advanced optical setup with a EUV-sensitive bidimensional detector. The grazing-incidence spectrometer consists of two optical elements: a toroidal mirror and a spherical variable-line-spaced grating with flat-field properties. The mirror focuses the radiation tangentially on the grating entrance plane, in order to increase the flux collected by the grating. The grating has a variable groove spacing with parameters optimized to produce an almost flat focal surface in the 5–40 nm region. A 40 mm diameter microchannel plate intensifier is used to acquire the spectrum. The mirror does not compensate for the grating astigmatism, so the length of the spectral lines on the spectrum is proportional to the sagittal angular divergence of the high-order harmonic emission. The absolute response of the system was measured in order to obtain the conversion efficiency of the harmonic generation process. Some experimental examples reported here show very high sensitivity, which makes the system able to work in the single-shot operation, thus allowing us to follow the dynamic evolution of the harmonic generation process.

Extremely achromatic f/10 all-spherical camera constructed for the high-resolution echelle spectrometer of the Keck telescope

We present a very large f/1.O prime focus, all-spherical, all-fused-silica catadioptric camera. It contains a two-element airspaced corrector, an f/0.76 primary mirror, and a singlet final element. It accommodates a chromatic range from 0.3 to 1.1µm or more without refocus. It is optimized with an external entrance pupil but can be reoptimized for other pupil distances. In spite of its 30-in. (76.2-cm) focal length, it delivers 12.6-µm (rms) average image diameters to a 3.6-in.- (9.1-cm-) diameter flat focal surface. It is thus well matched to the (7-15µm) pixels and to the size of a (2 × 2) mosaic of today's largest available CCD's.

Fundamental and practical limitations of FUV/EUV photon-counting image detectors

In EUV and FUV space-astronomy applications, the best contemporary detector designs are based on the use of microchannel plates due to their ability to deliver photon-counting performance with high efficiency. The major unresolved issue is the choice of position-readout system for the individual photoevents. Electrical event-readout systems are divided into two classes: the discrete wire anodes that perform coordinate digitization by wire-group selection, and the continuous centroid-position encoders for which coordinates are digitized in the associated electronics. The centroid-position encoder techniques are discussed in terms of how they overcome the four chief limitations of the discrete-wire readouts - their limited format size, their flat focal surface, their fundamental hex-channel vs squared-pixel moire pattern, and their image undersampling. With these limitations overcome, microchannel based image systems can deliver the performance demanded by the forthcoming generation of applications in space astronomy.