Optical alignment of segmented mirrors to the fluorescence detectors of the Pierre Auger observatory

Segmented mirrors will be used in the telescopes of the Pierre Auger Fluorescence detector. To align the segments, we have developed four methods in which (a) the image of the stop border, (b) the image of a screen with concentric circles, and (c) the Ronchi pattern are used. In addition to these, we have developed a new method, (d), in which instead of the Ronchi ruling, we have used a circular grid. In this case we obtain a moiré pattern for each segment by means of which the experimental setup is simplified, and the sensitivity of the alignment is improved.     

Active control systems for large segmented optical mirrors

A new generation of optical telescopes is on the drawing board. These will be true giants with primary mirrors having a diameter of up to 100 metres. The technology that will enable this revolution to take place was developed at the W. M. Keck Observatory in Hawaii, where the world's largest segmented mirrors are in daily use. This article looks at how the W. M. Keck Observatory proved the mirror technology that will be behind this new generation of telescopes     

Fabrication and alignment issues for segmented mirror telescopes

There is a great demand for new telescopes that use larger primary mirrors to collect more light. Because of the difficulty in the fabrication of mirrors larger than 8 m as a single piece, they must be made with numerous smaller segments. The segments must fit together to create the effect of a single mirror, which presents unique challenges for fabrication and testing that are absent for monolithic optics. This is especially true for the case of a highly aspheric mirror required to make a short two-mirror telescope. We develop the relationship between optical performance of the telescope and errors in the manufacture and operation of the individual segments.     

Design considerations for a highly segmented mirror

Design issues for a 30-m highly segmented mirror are explored, with emphasis on parametric models of simple, inexpensive segments. A mirror with many small segments offers cost savings through quantity production and permits high-order active and adaptive wave-front corrections. For a 30-m f/1.5 paraboloidal mirror made of spherical, hexagonal glass segments, with simple warping harnesses and three-point supports, the maximum segment diameter is approximately 100 mm, and the minimum segment thickness is approximately 5 mm. Large-amplitude, low-order gravitational deformations in the mirror cell can be compensated if the segments are mounted on a plate floating on astatic supports. Because gravitational deformations in the plate are small, the segment actuators require a stroke of only a few tens of micrometers, and the segment positions can be measured by a wave-front sensor.     

Phase discontinuity sensing: a method for phasing segmented mirrors in the infrared

We describe a novel method for phasing segmented optics in which the signal is the difference between inside-of-focus and outside-of-focus long-exposure infrared images. A detailed algorithm based on a correlation of this difference image with theoretical images or templates is presented. In a series of tests of this phase discontinuity sensing (PDS) algorithm at the Keck 1 telescope, at a wavelength of 3.3 mum, the rms piston error (averaged over the 36 primary mirror segments) was repeatedly reduced from approximately 240 to 40 nm or less. Furthermore, the PDS phasing solution was consistent with our previous phasing camera results (to within 66-nm rms), providing strong independent confirmation of this earlier approach.     

Design of an interferometric system for the measurement of phasing errors in segmented mirrors

One of the new problems that has to be solved for segmented mirrors is related to periodic phasing, because for such mirrors to exhibit diffraction-limited performance the segments have to be positioned with an accuracy of a fraction of a wavelength. We describe the optical design of an instrument that measures the phasing errors (i.e., tip, tilt, and piston) between two segments under daylight conditions. Its design is based on a high-aperture white-light Michelson interferometer. It was developed at the Center for Sensors, Instruments and Systems Development (CD6) of the Technical University of Catalunya, Spain, and its final testing was carried out on the Gran Telescopio Canarias test workbench.     

Effect of alignment on the penetration of segmented rods

The purpose of this investigation is to study the effect of alignment on the performance of segmented penetrators. The Eulerian wave propagation code CTH is used for this purpose. A series of calculations using four L/D = 1 tungsten alloy segment trains at varying degrees of misalignment is performed, impacting a single finite-thickness oblique armor steel plate. Obliquity angles of 30° and 60° were considered. This study was performed primarily to investigate the effects of obliquity and is a continuation of a previous study [1] where semi-infinite armor steel plates were examined. It is shown that the obliquity of the plate can have a significant influence on the performance of the segment train. When misalignment is minimal, the performance of the segment train is not adversely affected, particularly if the misalignment positions the train in an orientation aligned with the plate normal. However, for large misalignments, degradation to the performance of the segment train is significant at all orientations.     

Analytical search models for optimum seeking in simulations

Summary Indepently developed analytical and simulation models of the same real world phenomenon can effectively complement each other. A simple analytical model can learn from a more realistic simulation model and can in turn provide evolutionary guidance in optimum seeking to the simulation model.

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Zusammenfassung Unabhängig entwickelte analytische und Simulationsmodelle desselben Realphenomens können sich wirkungsvoll ergänzen. Ein einfaches analytisches Modell kann von einem realistischeren Simulationsmodell lernen und ermöglicht umgekehrt evolutionäre Steuerung der Optimumsuche im Simulationsmodell.

     

Crosstalk corrections for improved energy resolution with highly segmented HPGe-detectors

Crosstalk effects of 36-fold segmented, large volume AGATA HPGe detectors cause shifts in the γ-ray energy measured by the inner core and outer segments as function of segment multiplicity. The positions of the segment sum energy peaks vary approximately linearly with increasing segment multiplicity. The resolution of these peaks deteriorates also linearly as a function of segment multiplicity. Based on single event treatment, two methods were developed in the AGATA Collaboration to correct for the crosstalk induced effects by employing a linear transformation. The matrix elements are deduced from coincidence measurements of γ-rays of various energies as recorded with digital electronics. A very efficient way to determine the matrix elements is obtained by measuring the base line shifts of untriggered segments using γ-ray detection events in which energy is deposited in a single segment. A second approach is based on measuring segment energy values for γ-ray interaction events in which energy is deposited in only two segments. After performing crosstalk corrections, the investigated detector shows a good fit between the core energy and the segment sum energy at all multiplicities and an improved energy resolution of the segment sum energy peaks. The corrected core energy resolution equals the segment sum energy resolution which is superior at all folds compared to the individual uncorrected energy resolutions. This is achieved by combining the two independent energy measurements with the core contact on the one hand and the segment contacts on the other hand.     

Medium-precision null-screen testing of off-axis parabolic mirrors for segmented primary telescope optics: the large millimeter telescope

The feasibility of using null screens for testing the segments of a parabolic segmented telescope mirror for the Large Millimeter Telescope (LMT) is analyzed. An algorithm for designing the null screen for testing the off-axis segments of conic surfaces is described. Actual screen designs for the different classes of segments of the LMT are presented. The sensitivity of the test and the required accuracies for the fabrication and positioning of the screen are analyzed. A measuring accuracy of approximately 12 microm in surface sagitta is within the reach of the technique.     

Analytical study of diffraction effects in extremely large segmented telescopes

We present an analysis of the diffraction effects from a segmented aperture with a very large number of segments-prototype of the next generation of extremely large telescopes. This analysis is based on the point-spread-function analytical calculation for Keck-type hexagonal segmentation geometry. We concentrate on the effects that lead to the appearance of speckles and/or a regular pattern of diffraction peaks. These effects are related to random piston and tip-tilt errors on each segment, gaps between segments, and segment edge distortion. We deliver formulas and the typical numerical values for the Strehl ratio, the relative intensity of higher-order diffraction peaks, and the averaged intensity of speckles associated with each particular case of segmentation error.     

Quarter-wave discrete wavelet design of dichroic highly reflecting-transmitting mirrors for ultrafast solid-state lasers

Thin-film wavelets are further analyzed for the design of dichroic mirrors for ultrafast solid-state lasers that provide both high reflectance on the lasing wavelength range and high transmittance of the pump light. Discrete quarter-wave-thick dielectric thin-film structures of homogeneous refractive indices following a quintic-wavelet envelope are considered. Relations for the reflectance on the lasing wavelength range are given. Adding several index-matching quarter-wave layers to both sides of the discrete wavelet optimizes the transmittance of the pump light. The design is further optimized to get minimum phase distortion on the lasing wavelength range.     

基于重力量测量的振动基座初始对准

针对传统解析法粗对准不能适用于动基座的问题,提出了一种基于重力量测的解析法动基座初始对准新方法。在导航时间中点处的惯性系中建立了中间坐标系;将载体系和导航系中的重力矢量投影到中间坐标系中,并对中间坐标系两侧的投影分别求和以及叉乘运算,得到的两个三维空间分别对应初始导航坐标系和初始载体坐标系,并利用所得三维空间完了成动基座条件下的初始对准;对几种不同工况下的对准过程进行了仿真对比。结果说明,该方法能够适用于动基座对准,在工程上易于实现,并对车载惯导系统实现行进间对准有一定指导意义。     

Electron-pion separation in a highly segmented electromagnetic calorimeter designed for the ALEPH experiment

In this paper, we will emphasize the advantage we can take of highly segmented electromagnetic calorimeters in both transverse and longitudinal directions. Such calorimeters lead particularly to powerful methods of hadron rejection and electron identification.     

Tomographic alignment algorithm for an extremely large three-mirror telescope: invisible modes

We analyze the optical effects due to distortions of a three-mirror telescope that is sufficiently large that all three mirrors must be actively controlled. Numerical experiments on telescopes with both monolithic and segmented primary mirrors reveal the existence of telescope misalignment configurations (modes) that are invisible to a fixed focal station wavefront sensor, even for highly redundant multidirectional tomographic measurement schemes. We describe these modes and give a theoretical explanation for them.     

Analytical expressions for the wave-front aberration coefficients of a tilted plane-parallel plate

An expression is given for the aberration imparted by a tilted plane-parallel plate to a converging or diverging pencil of rays. Analytical expressions for the wave-front aberration coefficients up to the sixth order are derived. These expressions, among others, are of importance when reading an optical disk through its substrate or when using a plane-parallel plate as a beam splitter. Differences with previous expressions from the literature are noted.     

Accurate expressions for the LP modes in a uniform circular-core curved fiber

Accurate forms for the LP(nm) modes (n = 0 and n ≧ 2) in a uniform circular-core curved fiber are given. We show that the LP(nm) modes (n ≧ 2) are composed of two spatially orthogonal components and that, to the zeroth order, there is no special polarization axis for the LP modes in a uniform circular-core curved fiber.     

Construction of superconducting bulk magnet magnetron sputtering apparatus for fabrication of highly reflective optical mirrors

We have developed the two-cathode magnetron sputtering apparatus equipped with superconducting permanent magnet to produce Mo/Si multi-layer films, which would potentially serve as a high-quality optical mirror at extreme ultraviolet (EUV) wavelength of 13.5 nm. The best deposition condition was searched by analyzing the structure of the inter-diffusion layer formed in the Mo/Si bi-layer film prepared under different deposition conditions. It was found that (1) Xe gas should be used as inert gas species, (2) its pressure is lower than 4 × 10⁻² Pa, (3) a throw distance is longer than 250 mm and (4) discharge voltage around 2 kV. By making full use of these data, we synthesized Mo/Si multi-layer films and analyzed the structure and its effect on the reflectivity. The highest EUV-reflectivity so far obtained is 67% in the normal incident condition.