Productivity and impact of large optical telescopes

An attempt is made to provide quantitative measures of the amount of data gathered at large optical telescopes throughout the world and the impact these data have on astronomical research. The data base comprises 1163 papers reporting data from 39 telescopes, published between January 1990 and June 1991, and 4052 citations to them in 1993. Productivity measured in papers per square meter of telescope mirror varies by a factor of six, and impact measured in citations per paper varies by a factor of more than 10. Predictably, high productivity and high impact are associated with telescopes located at good sites and fully supported for many years by organizations with large budgets. Low productivity and low impact are associated with less favorable locations, short periods of operation, and financial stringency. In addition, the most productive telescopes seem to be ones whose users include astronomers from a wide range of geographical locations.January to June.July to December     

Holographic correction and phasing of large sparse-array telescopes

I have constructed a 1-m-diameter telescope using separate, low-quality spherical primary mirror segments. A single hologram of the mirrors is used to correct the random surface distortions as well as spherical aberration, while simultaneously phasing the individual apertures together. I present experimental results of the removal of an error of thousands of waves to produce a diffraction-limited instrument operating over a narrow bandwidth. This technique promises to have many benefits in future space-based telescopes for imaging, lidar, and optical communications.     

Effects of misalignment errors on the optical transfer functions of synthetic aperture telescopes

A segmented or diluted aperture optical system will undergo phase errors as a result of errors in the positioning of the segments. The errors associated with a segmented primary mirror limit the image quality obtainable with synthetic aperture telescopes. Here we study the effects of segmentation errors on image quality, considering both the phase angle and the amplitude of the optical transfer function (OTF). We show that, in these kinds of telescope, phasing and alignment errors among segments reduce the amplitude and distort the phase angle of the OTF.     

Single-element diffractive optical system for real-time processing of synthetic aperture radar data

I present an optical system for the polar formatting of data in a spotlight-mode synthetic aperture radar. This system is implemented with only one diffractive optical element (DOE). Previously such a DOE could not be produced because the phase of the required transmission function of the DOE does not obey the continuity condition, which is a prerequisite for the conventional implementation of such optical transforms. Here I show how a DOE can be produced to perform the complete polar-formatting transform by incorporating branch-point phase singularities in the transmission function of the DOE. The computation of the transmission function is shown, and numerically computed diffraction patterns obtained from this DOE are also shown.     

Adaptive optics sky coverage modeling for extremely large telescopes

A Monte Carlo sky coverage model for laser guide star adaptive optics systems was proposed by Clare and Ellerbroek [J. Opt. Soc. Am. A 23, 418 (2006)]. We refine the model to include (i) natural guide star (NGS) statistics using published star count models, (ii) noise on the NGS measurements, (iii) the effect of telescope wind shake, (iv) a model for how the Strehl and hence NGS wavefront sensor measurement noise varies across the field, (v) the focus error due to imperfectly tracking the range to the sodium layer, (vi) the mechanical bandwidths of the tip-tilt (TT) stage and deformable mirror actuators, and (vii) temporal filtering of the NGS measurements to balance errors due to noise and servo lag. From this model, we are able to generate a TT error budget for the Thirty Meter Telescope facility narrow-field infrared adaptive optics system (NFIRAOS) and perform several design trade studies. With the current NFIRAOS design, the median TT error at the galactic pole with median seeing is calculated to be 65 nm or 1.8 mas rms.     

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.     

Calculation of the point source effective collimator aperture for collimated x-ray and γ-ray telescopes

Techniques for calculating the energy dependent near and distant point source effective collimator aperture of a collimated detector have been developed. The results are found to be in good agreement with the experimentally determined values from a shielded γ-ray telescope.     

Very large scale simulations of materials failure

With present-day supercomputers, simulation is becoming a powerful tool for providing immediate insights into the nature of fracture dynamics. Atomistic simulations yield ab initio information about crack-tip formation and deformation at length-scales unattainable by experimental measurement and unpredictable by continuum elasticity theory. We will describe several atomistic dynamics studies concerning brittle fracture and ductile deformation.     

The diffraction pattern of large aperture bowl transducers

A new examination has been made of the diffraction pattern in the vicinity of the focus, for large aperture concave bowl ultrasonic transducers. The study gives results which show a departure from those derived from optical theory, hitherto accepted. This departure increases progressively with increase in bowl aperture and leads to the expectation that large aperture ultrasonic microscopes will provide a resolution somewhat greater than indicated by classical theory.The study has also shown that the major part of the wave energy radiated by the transducer is confined near the focus within a cylindrical region, in which it is propagated as a plane wave. By a suitable choice of bowl dimensions and ultrasonic frequency the radius of this region may be made a fraction of a milimetre, while its length is several milimetres.From the realisation of the plane wave nature of propagation near the focus a technique has been developed for the determination of the ultrasonic velocity in samples of solids of very small volume of the order of 50 mm³.     

大口径多孔透镜离轴成像研究

设计并采用微细加工技术制作了直径D=50mm,特征尺寸为10μm的大口径多孔透镜(photon sieve).在一定的视场下采用数值计算和实验方法分析了其聚焦特性和MTF曲线,为克服多孔透镜一级衍射效率较低,成像质量受背景光干扰的影响,采用了同轴和离轴成像实验.实验结果与聚焦特性一致,两种实验方法得出的不同实验结果表明:离轴成像系统效果克服了同轴成像受0级背景光的影响,改善了像质,为很多一级衍射效率较低的衍射透镜成像提供一种新的成像方式.     

大口径光学元件瞬态波前检测

为瞬态测量大口径光学元件波前,提出一种基于斜入射结构的近红外反射式错位点衍射干涉原理的Φ400 mm瞬态波前检测方法。该方案将待测光分成两束互相错位的参考光与测试光,从而在干涉图中引入高线性载频,采集到对比度良好的干涉图后,利用傅里叶变换相位解调法从单幅干涉图中提取待测波前相位,实现瞬态波前动态测量。实验光路总长近20 m,极易受气流的影响,且由于气流干扰随时间变化,该系统本身可以看作是大口径光学元件瞬态波前发生与检测装置。测试结果与SID4波前传感器比较,波前均方根（RMS）小于1/50 λ,可知所提方法可以实现大口径瞬态波前的高分辨率与高精度检测。

     

Projection approach to complexity reduction in tomographic alignment of extremely large telescopes

We describe a complexity reduction approach intended to solve the tomographic alignment problem for the Thirty Meter Telescope by means of its alignment and phasing system (APS) with little loss of information. This approach is computationally efficient enough to perform detailed Monte-Carlo simulations of the APS on a standard PC. We present sample simulations to model error propagation through the system and to build a preliminary APS alignment error budget.     

Efficient method for the reduction of large piston errors in segmented-mirror telescopes

Phase discontinuity sensing (PDS) is one of two successful approaches to segment phasing that are currently in use at the Keck telescopes, but it has only a limited capture range. We describe and present numerical simulations of a broadband version of the current (narrowband) PDS algorithm that can extend the capture range from 0.4 to 40 microns. Like the original algorithm, the new broadband PDS algorithm requires no special-purpose hardware but only a high-resolution area detector operating in the 2-3-microns range. The potential application of this algorithm to extremely large telescopes is also discussed.     

Comparison of designs of off-axis Gregorian telescopes for millimeter-wave large focal-plane arrays

We compare the diffraction-limited field of view (FOV) provided by four types of off-axis Gregorian telescopes: the classical Gregorian, the aplanatic Gregorian, and the designs that cancel astigmatism and both astigmatism and coma. The analysis is carried out with telescope parameters that are appropriate for satellite and balloonborne millimeter- and submillimeter-wave astrophysics. We find that the design that cancels both coma and astigmatism provides the largest flat FOV, approximately 21 square deg. We also find that the FOV can be increased by approximately 15% by means of optimizing the shape and location of the focal surface.     

Coupling of large telescopes and single-mode waveguides: application to stellar interferometry

The coupling between a turbulence-distorted optical beam and a single-mode waveguide is addressed. The coupling efficiency and the coupled phase are derived, both without aberrations and with small aberrations. These analytical expressions are validated by numerical simulations. Correction with adaptive optics is investigated. In the general case, the Strehl ratio is a pessimistic estimator, and the coupled phase is different and has a smaller variance than the classical phase averaged over the pupil. Application fields are heterodyne detection and stellar interferometry, for which spatial and modal filtering are distinguished.     

Multiconjugate adaptive optics for large telescopes: analytical control of the mirror shapes

We present an analytical algorithm for deriving the shapes of the deformable mirrors to be used for multiconjugate adaptive correction on a large telescope. The algorithm is optimal in the limit where the overlap of the wave-front contributions from relevant atmospheric layers probed by the guide stars is close to the size of the pupil. The fundamental principle for correction is based on a minimization of the sum of the residual power spectra of the phase fluctuations seen by the guide stars after correction. On the basis of the expressions for the mirror shapes, so-called layer transfer functions describing the distribution of the correction of a single atmospheric layer among the deformable mirrors and the resulting correction of that layer have been derived. It is shown that for five guide stars distributed in a regular cross, two- and three-mirror correction will be possible only up to a maximum frequency defined by the largest separation of the conjugate altitudes of the mirrors and by the angular separation of the guide stars. The performance of the algorithm is investigated in the K band by using a standard seven-layer atmosphere. We present results obtained for two guide-star configurations: a continuous distribution within a given angular radius and a five-star cross pattern with a given angular arm length. The wave-front fluctuations are subjected to correction using one, two, and three deformable mirrors. The needed mirror dynamic range is derived as required root-mean-square stroke and actuator pitch. Finally the performance is estimated in terms of the Strehl ratio obtained by the correction as a function of field angle. No noise has been included in the present analysis, and the guide stars are assumed to be at infinity.     

Laser guide star wavefront sensing for ground-layer adaptive optics on extremely large telescopes

We propose ground-layer adaptive optics (GLAO) to improve the seeing on the 42?m European Extremely Large Telescope. Shack-Hartmann wavefront sensors (WFSs) with laser guide stars (LGSs) will experience significant spot elongation due to off-axis observation. This spot elongation influences the design of the laser launch location, laser power, WFS detector, and centroiding algorithm for LGS GLAO on an extremely large telescope. We show, using end-to-end numerical simulations, that with a noise-weighted matrix-vector-multiply reconstructor, the performance in terms of 50% ensquared energy (EE) of the side and central launch of the lasers is equivalent, the matched filter and weighted center of gravity centroiding algorithms are the most promising, and approximately 10×10 undersampled pixels are optimal. Significant improvement in the 50% EE can be observed with a few tens of photons/subaperture/frame, and no significant gain is seen by adding more than 200 photons/subaperture/frame. The LGS GLAO is not particularly sensitive to the sodium profile present in the mesosphere nor to a short-timescale (less than 100?s) evolution of the sodium profile. The performance of LGS GLAO is, however, sensitive to the atmospheric turbulence profile.     

Electron-beam writing system and its application to large and high-density diffractive optic elements

An electron-beam writing system that is chartacterized by a stationary electron beam and a continuously spinning table is proposed and developed for diffractive optic elements with axisymmetric patterns. This system allows us to fabricate continuously concentric or radialized high-density microgratings with effective areas over the electron-beam deflection limits. For a fine electron-dose distribution, we adopted multiple-revolution writing and electron-beam modulation. Several test gratings, including a rotary encoder disk with a 1.57-μm grating pitch and a 15-mm diameter and a blazed micro-Fresnel lens with a 4.5-mm diameter and a N.A. of 0.45, are successfully demonstrated with real-time data processing.     

Directional narrowing of the diffractive pattern through a combination of spherical and spiral optical elements within a single aperture

The intensity pattern produced by the zero-order Bessel beam can be squeezed along certain directions if it interferes with the Bessel beam of a higher order. The concept of directional narrowing can be extended onto the zone plates by the division of the aperture into a set of concentric annuli; within some of the apertures the phase function of the spherical optical element is substituted or supplemented by the spiral optical element. The proposed approach is verified by the numerical simulation of the interference of Bessel beams, linear axicons, and spherical zone plates of zero order and second order.