Comparison of measurements of the outer scale of turbulence by three different techniques

We have made simultaneous and nearly simultaneous measurements of L0, the outer scale of turbulence, at the Palomar Observatory by using three techniques: angle-of-arrival covariance measurements with the Generalized Seeing Monitor (GSM), differential-image-motion measurements with the adaptive-optics system on the Hale 5-m telescope, and fringe speed measurements with the Palomar Testbed Interferometer (PTI). The three techniques give consistent results, an outer scale of approximately 10-20 m, despite the fact that the spatial scales of the three instruments vary from 1 m for the GSM to 100 m for the PTI.     

Method of estimating time scales of atmospheric piston and its application at Dome C (Antarctica)

Temporal fluctuations of the atmospheric piston are critical for interferometers as they determine their sensitivity. We characterize an instrumental setup, termed the piston scope, that aims at measuring the atmospheric time constant, tau(0), through the image motion in the focal plane of a Fizeau interferometer. High-resolution piston scope measurements have been obtained at the observatory of Paranal, Chile in April 2006. The derived atmospheric parameters are shown to be consistent with data from the astronomical site monitor, provided that the atmospheric turbulence is displaced along a single direction. The piston scope measurements of lower temporal and spatial resolution were recorded for what is believed to be the first time in February 2005 at the Antarctic site of Dome C. Their reanalysis in terms of the new data calibration sharpens the conclusions of a first qualitative examination [Appl. Opt. 45, 5709 (2006)].     

Experimental measurements of estimator bias and the signal-to-noise ratio for deconvolution from wave-front sensing

Deconvolution from wave-front sensing (DWFS) has been proposed as a method for achieving high-resolution images of astronomical objects from ground-based telescopes. The technique consists of the simultaneous measurement of a short-exposure focal-plane speckled image, as well as the wave front, by use of a Shack-Hartmann sensor placed at the pupil plane. In early studies it was suspected that some problems would occur in poor seeing conditions; however, it was usually assumed that the technique would work well as long as the wave-front sensor subaperture spacing was less than r(0) (L/r(0) < 1). Atmosphere-induced phase errors in the pupil of a telescope imaging system produce both phase errors and magnitude errors in the effective short-exposure optical transfer function (OTF) of the system. Recently it has been shown that the commonly used estimator for this technique produces biased estimates of the magnitude errors. The significance of this bias problem is that one cannot properly estimate or correct for the frame-to-frame fluctuations in the magnitude of the OTF but can do so only for fluctuations in the phase. An auxiliary estimate must also be used to correct for the mean value of the magnitude error. The inability to compensate for the magnitude fluctuations results in a signal-to-noise ratio (SNR) that is less favorable for the technique than was previously thought. In some situations simpler techniques, such as the Knox-Thompson and bispectrum methods, which require only speckle gram data from the focal plane of the imaging system, can produce better results. We present experimental measurements based on observations of bright stars and the Jovian moon Ganymede that confirm previous theoretical predictions.     

Theoretical wave structure function when the effect of the outer scale is significant

The wave structure function (WSF) for a plane wave, calculated from the basic Rytov theory, is usually expressed as 6.88(r/r(0))(5/3), but this does not include the effect of a finite outer scale (or of a nonzero inner scale) of turbulence. When separation distance r is only 5% of the outer scale, this expression overpredicts the WSF by a factor of approximately 2. Accurate evaluations of the Rytov formulas are given for the WSFs of plane and spherical waves in Kolmogorov and von Karman turbulence and for the structure function of the atmosphere's index of refraction. Simple formulas make the results easy to use.     

Self-pumped phase-conjugate interferometer with a photorefractive iron-doped lithium-niobate crystal

A single object wave is amplitude divided by a beam splitter into two waves of equal intensity that are made to interfere at the back surface of an iron-doped lithium-niobate crystal so that the normal to the back surface is the angular bisector of the input waves. The interference results in the formation of a phase grating (Bragg grating) in the volume of the crystal. These waves are diffracted at the Bragg grating on both the front focal plane and the back focal plane of the crystal. The wave diffracted in the back focal plane from the Bragg grating and counterpropagating to the incident wave is observed to be the phase conjugate of the input object wave. The wave diffracted in the front focal plane of the Bragg grating is incorporated into the design of an interferometer to measure a specific in-plane displacement of the object wave. It is theoretically evaluated and experimentally demonstrated that interferometers such as those that incorporate conjugate-wave pairs are highly sensitive.     

Use of a surface-mounted chirped fibre Bragg grating sensor to monitor delamination growth in a double-cantilever beam test

A surface-mounted chirped fibre Bragg grating (CFBG) sensor has been used for the first time to monitor delamination growth within a composite material (a transparent, unidirectionally reinforced glass fibre/epoxy resin double-cantilever beam (DCB) specimen). The specimens were tested using a constant displacement rate, with the delamination length being measured using complementary techniques: (i) in situ photography, (ii) surface-mounted strain gauges, and (iii) the surface-mounted CFBG sensors. The unidirectionally reinforced DCB specimens showed characteristics typical of such material which complicate the curvature of the beams, i.e. the development of extensive fibre bridging and pronounced R-curve behaviour. To validate the interpretation of the CFBG reflected spectrum, the experimentally determined strains from the surface-mounted strain gauges have been used, together with in situ photographs of the position of the delamination front. Using the CFBG sensor technique, the delamination length was measured to within about 4 mm over the 60 mm sensor length.     

高精度光栅测长机校准及不确定度分析

本文论述了测长机和测长仪的区别,高精度光栅测长机应归于测长仪类,适用于JJF 1189-2008《测长仪校准规范》.校准高精度光栅测长机,需严格控制环境温度,测量小尺寸量块需考虑两球形测头不同心引入的误差,测量内尺寸以两环规直径差作为标准值,测得值可用平均值代替最大值,分析了二等量块校准光栅测长机外尺寸示值误差的测量不...     

First retrieval of vertical profiles of turbulence characteristics and horizontal wind velocity from solar transmission measurements at 212 and 405 GHz

We report on the investigation and successful application of vertical profiling of the structure parameter C2m and of the outer scale L0 of absorption fluctuations and of the horizontal wind velocity (vector) during daytime by the analysis of solar transmission measurements. The method is relatively simple and straightforward so that the presented (or a similar) technique could be used in the routine remote sensing of daytime C2m, L0, and wind profiles. It requires multiple beams pointing in different directions at the Sun. The retrieved profiles are consistent with the current knowledge of atmospheric physics. Simultaneous in situ wind velocity measurements agree with the retrieved wind velocity in the lowest 100 m above ground within the measurement uncertainties of less than +/-2 m/s. The derived values of C2m at 200 m above ground are in good agreement (within a factor of 1.5) with the findings of an earlier investigation at the same test site. Finally, it is shown that irradiance fluctuations of millimeter and submillimeter waves are dominantly affected by humidity fluctuations, even at a dry and elevated site.     

A versatile, PC-based gamma ray monitor

This paper describes a new computer-based gamma ray monitor for laboratory and field measurements. The monitor consists of a universal peripheral device, and GM counter gamma ray probe, connected via USB to the PC. The peripheral device is generic; in addition to the GM probe, it accepts sensors for spectrometric measurements as well as sensors for the measurement of other environmental parameters. Owing to its low-power consumption, it is used with laptop or palmtop computer, as a portable field measurement instrument. The proposed solution has advantages over conventional instrument: (1) open architecture; (2) user-defined functions and (3) easy network connection.     

Coherence properties of a source array derived from a Gaussian Schell-model beam

The spatial coherence properties of a source array derived from a quasi-monochromatic Gaussian Schell-model (GSM) beam are analyzed. The method for realizing the source array by use of a GSM source and a Gaussian amplitude grating was originally proposed in our preceding paper [J. Opt. Soc. Am. A 11, 2112 (1994)], in which the intensity distribution was analyzed. One result that we obtain is that the degree of spatial coherence of each element in the source array is the same as that at the other elements. In particular, each element becomes equivalent to the GSM source and has a high degree of spatial coherence with the other elements if three conditions are satisfied simultaneously: (1) the spatial-coherence length of the GSM source is larger than the GSM source size, (2) the GSM source size normalized by a period of the grating is 0.3 to 0.5, and (3) each aperture size of the grating normalized by the period is 0.05 or less.     

Interferometric seeing measurements on Mt. Wilson: power spectra and outer scales

We have measured power spectra of atmospheric phase fluctuations with the Mark III stellar interferometer on Mt. Wilson under a wide variety of seeing conditions. On almost all nights, the high-frequency portions of the temporal power spectra closely follow the form predicted by the standard Kolmogorov-Tatarski model. At lower frequencies, a variety of behavior is observed. On a few nights, the spectra clearly exhibit the low-frequency flattening characteristic of turbulence with an outer-scale length of the order of 30 m. On other nights, examination of individual spectra yields no strong evidence of an outer scale less than a few kilometers in size, but comparison of the spectra on different interferometer baselines shows a saturation of the spatial structure function on long baselines. This saturation is consistent with the assumption of an outer-scale length similar to that derived for the nights when low-frequency flattening of the spectra is clearly seen. We discuss possible explanations of this behavior and conclude that power spectra from a single interferometer baseline are a poor diagnostic for the effective outer scale compared with multiple-baseline spectra.     

两种莫尔条纹信号对光栅系统测量精度的影响

对两种莫尔条纹（光闸条纹和横向条纹）信号的谐波含量以及它们对光栅系统测量精度的影响进行了分析，并用实测数据加以比较，指出在光栅系统中，取横向莫尔条纹信号的谐波量小，正弦性好，细分误差小，在其它参数相同的情况下，可提高系统的测量精度2倍左右。     

Tilt angular anisoplanatism and a full-aperture tilt-measurement technique with a laser guide star

A method is presented for sensing atmospheric wave-front tilt from a laser guide star (LGS) by observing a laser beacon with auxiliary telescopes. The analysis is performed with a LGS scatter model and Zernike polynomial expansion of wave-front distortions. It is shown that integration of the LGS image over its angular extent and the position of the auxiliary telescope in an array reduce the tilt sensing error associated with the contribution from the downward path. This allows us to single out only the wave-front tilt of the transmitted beam on the uplink path that corresponds to the tilt for the scientific object. The tilt angular correlation is analyzed in the atmosphere with a finite turbulence outer scale. The tilt correlation angle depends on the angular size of the telescope and the outer scale of turbulence. The tilt sensing error increases with the auxiliary telescope diameter, suggesting that an auxiliary telescope must be small. The Strehl ratio associated with the contribution from the downward path is in the range from 0.1 to 0.9 when the relative telescope diameter D/r(0) varies from 4 to 93 and the turbulence outer scale is in the 10-150-m range. Tilt correction increases the Strehl ratio compared with the uncorrected image for all the system parameters and seeing conditions considered. The method discussed gives a higher performance than the conventional technique, which uses an off-axis natural guide star. A scheme for measuring tilt with a beam projected from a small aperture is described. This scheme allows us to avoid phosphorescence of the main optical train for a sodium LGS.     

用神经网络技术实现的高精度光栅测量装置

描述了一种用神经网络技术实现的高精度光栅测量装置。该装置采用BP神经网络对光栅信号进行细分，在仅用7个训练样本的情况下，细分精度可达0.18μm，使装置的分辨率得到很大提高，同时，简化了硬件设计，提高了系统的可靠性。     

Using ultrasonic wave reflection to monitor false set of cement paste

The standard mechanical penetration approach for monitoring cement paste stiffening (Vicat needle method, ASTM C191) does not distinguish responses associated with false set of cement paste caused by secondary gypsum formation. The objective of this research is to determine whether ultrasonic wave reflection, using a testing set up with high measurement sensitivity, could be used to monitor false set of cement paste. Penetration resistance, P-wave, and S-wave reflection coefficients were measured on cement pastes with water-to-cement ratio 0.5. The S-wave reflection coefficient showed a sharp and abrupt linear drop associated with secondary gypsum formation, thereby indicating that S-wave ultrasonic wave reflection can be used to monitor false set of cement pastes. False set could not be distinguished in penetration resistance or P-wave UWR test data.     

圆光栅检测仪的研制

介绍了一种高精度、精密、自动化、动态测量的光栅检测仪,测量光栅盘的扩展不确定度为０．０８”。该仪器采用了高回转精度的空气静压轴承及多头均化技术,减小了参考盘刻划误差的影响,同时,采用了计算机系统控制测量过程,进行数据处理,实现了自动化测量,检测结果和误差曲线由打印机打印输出。     

Sensing a wave front by use of a diffraction grating

A method is presented to sense the wave front at the exit of an optical surface. This method uses a set of diffracted rays generated when a He-Ne laser impinges on a rectangular diffraction grating. The grating was placed near the curvature center of the surface to be tested. After they are reflected in the test surface, the diffracted rays have the information of the slopes of the wave front, like in the Hartmann test. The Hartmann pattern was registered near the curvature center and captured with a CCD camera. The slopes for each ray are measured from the experimental pattern, and they are compared with the ideal ones simulated in a computer. The evaluation was carried out by use of Seidel polynomials to obtain the information of the aberrations of a mirror 53 cm in diameter.     

Performance optimization and light-beam-deviation analysis of the parallel-slab division-of-amplitude photopolarimeter

A division-of-amplitude photopolarimeter that uses a parallel-slab multiple-reflection beam splitter was described recently [Opt. Lett. 21, 1709 (1996)]. We provide a general analysis and an optimization of a specific design that uses a fused-silica slab that is uniformly coated with a transparent thin film of ZnS on the front surface and with an opaque Ag or Au reflecting layer on the back. Multiple internal reflections within the slab give rise to a set of parallel, equispaced, reflected beams numbered 0, 1, 2, and 3 that are intercepted by photodetectors D(0), D(1), D(2), and D(3), respectively, to produce output electrical signals i(0), i(1), i(2), and i(3), respectively. The instrument matrix A, which relates the output-signal vector I to the input Stokes vector S by I = AS, and its determinant D are analyzed. The instrument matrix A is nonsingular; hence all four Stokes parameters can be measured simultaneously over a broad spectral range (UV-VIS-IR). The optimum film thickness, the optimum angle of incidence, and the effect of light-beam deviation on the measured input Stokes parameters are considered.     

HIGH CONCENTRATION DUST MASS MONITOR

The development and testing of a portable, rugged, self-contained monitoring prototype instrument capable of detecting and measuring airborne, potentially explosive, dust levels at concentrations in the range of 20 to 500 g/m³, has been completed. The output signal provided by this monitor is designed to be essentially independent of particle size and composition. Direct mass concentration readout as well as internal memory capabilities for unattended operation have been incorporated in this device. The design emphasizes rugged-ness, versatility of operation, and adherence to intrinsic safety requirements.

The sensing method incorporated in this monitoring instrument is based on direct air-path beta attenuation detection across a gap of the order of 2 centimeters using a very low energy beta-emitting isotope (Ni-63). The measurement system includes automatic periodic clean-air referencing in order to compensate for any air density fluctuations and/or detector drifts. Measurements, directly indicated in g/m³, are updated automatically every 10 seconds.

Operating at a beta count rate of approximately 6000 to 7000 sec _-1, over the 10-second count period, the observed standard deviation of the measurements was of the order of 2.5 g/m³, around the zero-concentration level.     

Design and testing of a two-monochromator reference spectrofluorimeter for high-accuracy total radiance factor measurements

A two-monochromator reference spectrofluorimeter has been developed at the National Research Council of Canada in accordance with International Commission on Illumination and American Society for Testing and Materials colorimetry standards to permit high-accuracy total spectral radiance factor measurements of fluorescent materials. This fully automated instrument employs a xenon-arc light source, all-reflective optics, two grating monochromators with order-sorting filters, a cooled photomultiplier tube analyzing detector, and a calibrated silicon photodiode monitor detector. The instrument operating range is 250-1050 nm with a selectable bandpass (optimized for a 5-nm resolution), and the measurement geometry is 45 degrees annular illumination and 0 degrees viewing (45/0). We describe the instrument's design, testing, and verification procedures. Systematic errors that have been determined and corrected for include instrument polarization, beam nonuniformity, wavelength shifts, stray light, and system drift. The wavelength accuracy and reproducibility are estimated to better than +/-0.1 and +/-0.03 nm, respectively. The photometric short-term repeatability and long-term reproducibility are estimated to be better than +/-0.15% and +/-0.5%, respectively. The overall photometric accuracy is better than 1% of the value over a wide range of reflectances, and the reproducibility of the color specification of a fluorescent material is better than 0.25 DE(ab) units.