Proposed method of determining ISO speed for opto-electronic cameras used in astronomy

Abstract

Many of the more specialised digital cameras produced for use with telescopes have no information provided to describe their sensitivity in terms of a recognised system, such as an ISO speed rating, whereas digital cameras supplied for pictorial imaging usually do have an ISO speed quoted by the manufacturer. The science of photographic sensitometry has become established over the past 110 years and, in many respects, the knowledge gained can be applied satisfactorily to non-photographic camera systems used for astronomy and also for many other scientific and technical applications. A method is described by which an ISO speed rating can be assigned to a digital camera system used for imaging in astronomy.     

A generation of astronomical telescopes,their users,and publications

Research astronomers and the telescopes they use each have typical life spans of about 40 years. Most of their journals live a good deal longer, though the second most important one today is only 40 years old. This paper looks at numbers for productivity and impact of specific astronomical facilities, changes in equality of opportunities and achievements in observational astronomy, and some aspects of national contributions. The focus is on optical astronomy, though something is also said about radio telescopes and astronomy from space. In summary, nothing stays “best of class” for very long; the fraction of the community with access to the most valuable facilities has increased with time (more equality of opportunity); but the fraction of citations earned by the few super-star papers has also increased (less equality of achievement); and the USA remains the host of the most-cited journals and the most productive telescopes, though Europe (meaning in this context the member nations of the European Southern Observatory, the European Space Agency, and the supporters of the journal Astronomy & Astrophysics) are fast closing the gap, with the UK retaining its own journal and some observing facilities not shared with either the USA or other European countries. Detailed examination of specific facilities indicates that size (of telescope, community, and budget) are all of great importance, but that the most significant “focal plane instrument” is still the astronomer at the virtual eyepiece. The changes have happened against a background of enormous increases in numbers of astronomers, sizes of available facilities (but not total number), numbers of papers (but not of journals), and numbers of citations per paper. A significant subset of the conclusions on turnover of people and facilities accompanying major growth: opportunity versus achievement; Europe versus the USA; and the trade-off between community size and the influence of individual scientists undoubtedly apply in many other fields.     

Spatial Chaotic Power Cross-talk in Nonlinear Multibeam Co-propagation in Optical Fibres

Abstract

We investigate spatial power coupling and chaotic cross-talk when beams co-propagate in multimode optical fibres, specifically among four beams that belong to two weakly degenerate mode families. The nonlinear mechanism responsible for the power and phase coupling is the optical Kerr effect in fibres. The power of each of the modes is theoretically demonstrated to be spatially unstable and chaotically dependent on launch conditions. It is shown that the spatial instabilities and irregular energy exchange occur over broad operating conditions as long as the system deviates from its spatial steady states.     

Active Optics

Abstract

A system of ‘active optics’ control for the optical imagery of astronomical telescopes has been under development in the European Southern Observatory for about ten years. Its first application will be in the 3·5 m New Technology Telescope (NTT) scheduled for operation in 1988. A model test with a thin 1 m mirror (aspect ratio 56) has given remarkably successful results which will be reported in Part II of this paper. Part I gives a complete presentation of the theoretical principles of this technique of active optics and its scope of application. The subject is treated from the viewpoint of the temporal band-pass of error sources, ‘active optics’ being concerned with the low-frequency band-pass. The high-frequency band-pass (‘adaptive optics’) is principally concerned with atmospheric correction and is only briefly referred to for comparison. ‘Active optics’ correction of the low-band-pass system errors should bring major improvements in image quality together with a large cost reduction. While its impact on ground-based telescopes seems beyond doubt, the most important application should be in space projects where permanent diffraction-limited performance should be possible with much relaxed tolerances.     

A transmission length limit for space division multiplexing in step-index silica optical fibres

ABSTRACT

By solving the power flow equation, we investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index silica optical fibres with a different strengths of mode coupling. Results show that mode coupling significantly limits the length of these fibres at which the space division multiplexing can be realized with a minimal crosstalk between the neighbour optical channels. This is most pronounced in silica optical fibres with the strongest mode coupling. The two and three spatially multiplexed channels in the investigated step-index silica optical fibres can be employed with a minimal crosstalk up to the fibre lengths of few hundred of meters and few tens of meters, respectively. These lengths are much shorter than kilometer lengths at which these fibres are usually employed without space division multiplexing. Such characterization of optical fibres should be considered in designing an optical fibre transmission system for space division multiplexing.     

Video Transmission for Third Generation Wireless Communication Systems

This paper presents a twin-class unequal protected video transmission system over wireless channels. Video partitioning based on a separation of the Variable Length Coded (VLC) Discrete Cosine Transform (DCT) coefficients within each block is considered for constant bitrate transmission (CBR). In the splitting process the fraction of bits assigned to each of the two partitions is adjusted according to the requirements of the unequal error protection scheme employed. Subsequently, partitioning is applied to the ITU-T H.263 coding standard. As a transport vehicle, we have considered one of the leading third generation cellular radio standards known as WCDMA. A dual-priority transmission system is then invoked on the WCDMA system where the video data, after being broken into two streams, is unequally protected. We use a very simple error correction coding scheme for illustration and then propose more sophisticated forms of unequal protection of the digitized video signals. We show that this strategy results in a significantly higher quality of the reconstructed video data when it is transmitted over time-varying multipath fading channels.     

Near to Mid Infrared Astronomical Optics with Self-achromatic Lenses

Astronomical observatories all over the world are updating their existing and future telescopes by implementing instruments with infrared array detectors. This work is concerned with the design of optics for near and mid (1 to 5 wm) infrared astronomy, and in particular with its achromatization, which is often neglected. Several achromatized optical systems suitable for astronomical instrumentation are described.     

Feed-forward linearisation technique for impulse radar ultra-wideband over fibre

The authors present the linearisation of impulse radar ultra-wideband (IR-UWB) transmission system over fibre. The radio frequency signal of IR-UWB is transmitted over 50 km of the single mode fibre (SMF) using external Mach Zehnder modulator (MZM), amplified, linearised and detected by photodetector receiver. For improving the linearity and suppressing the four-wave mixing (FWM) caused by semiconductor optical amplifier (SOA) and 50 km SMF, the system uses feed-forward technique. Link performance was evaluated for two IR-UWB signals to suppress the FWM. The theoretical simulation results demonstrate a distortion cancellation that is produced by SOA of better than 25 dB over 1550 nm single mode optical fibre cable. In addition, the characteristic transmission of UWB pulse radio through 50 km single mode radio over fibre (RoF) system, such as BER measurements to evaluate the performance of the UWB with respect to different laser power output level, is investigated. This technique has been used to successfully transmit indirectly modulated data using laser diode at 10 GB/s light-wave transmission system.     

The application of interferometry to optical astronomical imaging

In the first part of this review we survey the role optical/infrared interferometry now plays in ground-based astronomy. We discuss in turn the origins of astronomical interferometry, the motivation for its development, the techniques of its implementation, examples of its astronomical significance, and the limitations of the current generation of interferometric arrays. The second part focuses on the prospects for ground-based astronomical imaging interferometry over the near to mid-term (i.e. 10 years) at optical and near-infrared wavelengths. An assessment is made of the astronomical and technical factors which determine the optimal designs for imaging arrays. An analysis based on scientific capability, technical feasibility and cost argues for an array of large numbers of moderate-sized (2 m class) telescopes rather than one comprising a small number of much larger collectors.     

Active optics and modified-Rumsey wide-field telescopes: MINITRUST demonstrators with vase- and tulip-form mirrors

Wide-field astronomy requires the development of larger aperture telescopes. The optical properties of a three-mirror modified-Rumsey design provide significant advantages when compared to other telescope designs: (i) at any wavelength, the design has a flat field and is anastigmatic; (ii) the system is extremely compact, i.e., it is almost four times shorter than a Schmidt. Compared to the equally compact flat-field Ritchey-Chrétien with a doublet-lens corrector, as developed for the Sloan digital sky survey-and which requires the polishing of six optical surfaces-the proposed modified-Rumsey design requires only a two-surface polishing and provides a better imaging quality. All the mirrors are spheroids of the hyperboloid type. Starting from the classical Rumsey design, it is shown that the use of all eight available free parameters allows the simultaneous aspherization of the primary and tertiary mirrors by active optics methods from a single deformable substrate. The continuity conditions between the primary and the tertiary hyperbolizations are achieved by an intermediate narrow ring of constant thickness that is not optically used. After the polishing of a double vase form in a spherical shape, the primary-tertiary hyperbolizations are achieved by in situ stressing. The tulip-form secondary is hyperbolized by stress polishing. Other active optics alternatives are possible for a space telescope. The modified-Rumsey design is of interest for developing large space- and ground-based survey telescopes in UV, visible, or IR ranges, such as currently demonstrated with the construction of identical telescopes MINITRUST-1 and -2, f/5-2 degrees field of view. Double-pass optical tests show diffraction-limited images.     

Quartz-plastic Fibre Technology and Applications

A technique for drawing ‘silica plastic’ fibres is described, along with the transmission and mechanical characteristics of the fabricated fibres. A number of applications of these fibres is also reported, including computer interconnection systems, r.f. reference signal transmission and the propagation of laser radiation through optical fibres for use in surgery.     

Metal matrix composite reinforced with co-continuous Al–Al2O3 fibres produced in situ by reaction of an aluminium matrix with fossil SiO2 spicules

Abstract

Aluminium metal matrix composite (MMC) reinforced with fossil silica fibres was produced by a powder metallurgy extrusion route. The fibres, silica rich spicules known as spongilites, come from abundant natural geological deposits in Brazil. Further processing the MMC by heat treating at high temperatures converts the silica fibres into an interlaced (Al–Si)/Al₂O₃ microcomposite structure, retaining the original fibre morphology. The new co-continuous microstructure of the fibres is a result of a reduction or displacement reaction, where the silicon released from the silica reduction forms at first a liquid Al–Si phase around the fibres and later on diffuses into the solid matrix. The fine internal microstructure of the fibres was studied by FEG-SEM and optical microscopy both on polished and fractured surfaces. Tensile properties of the MMC before and after heat treatment were measured, showing a considerable increase in UTS. Analysis of the fracture surface of the heat treated ruptured specimens showed necking (bridge formation) in the metallic phase of the fibre and no visible pullout.     

Optimal control of laser beams for propagation through a turbulent medium

Concerning the problem of transmitting a laser beam from one telescope to another telescope through a turbulent medium, it is established that using an adaptive optical system on both telescopes to precompensate an outgoing laser beam based on the aberrations measured on the received laser beam leads to an iteration that maximizes the transmission (neglecting attenuation losses) of laser power between the telescopes. Simulation results are presented demonstrating the effectiveness of this technique when the telescopes are equipped with either phase-only or full-wave compensation systems. Simulation results are shown that indicate that for a uniform distribution of the strength of turbulence, 95% transmission of laser power is attained when both telescopes can achieve full-wave compensation provided that the aperture diameter D of the two telescopes is greater than twice the Fresnel length square root of lambdaL, where A is the wavelength of propagation and L is the distance between the two telescopes.     

Analysis of Modulational Instability in Monomode Optical Fibres with High-order Nonlinearity

Abstract

The modulational instability (MI) in monomode optical fibres with fifth-order nonlinearity, fibre loss, higher-order dispersion, and the temporal variation of third-order nonlinearity is studied theoretically. The conditions for the existence of the MI and the maximal modulational growth are given and discussed in detail. The results obtained show that the key factor dominating the producing condition of the MI is the power P of the continuous wave initially launched into the optical fibres. If P falls into 3/10<P/P ₀ <1/2 where P ₀ is defined as characteristic power, the MI can be produced in the range of not only anomalous group velocity dispersion but also the normal in which the final evolution state of the modulated wave is dark soliton.     

A wide dynamic range square-law diode detector [for radioastronomy]

Semiconductor square-law diode detectors are frequently used in radio astronomy to recover signals immersed in the system noise. Their use is commonly restricted to narrow dynamic ranges of very low signal levels where the square-law is valid. A circuit based on operational amplifiers is proposed that would minimize temperature-drift effects within a dynamic range greater than 30 dB, with an efficiency 600 timer greater than the simple high-impedance unbiased detector. Using square-law detector theory, optimum performance is determined for a detector driving source impedance of about 14% of the dynamic resistance     

A three phase method for detecting optical pulsars

A variant route to lock-in amplifier detection of optical emission from radio pulsars is considered. The method uses the radio pulsar frequency signals as a timing clock but splits the optical signals into equal portions of one third the pulsar period. Differences in their intensities offer evidence for an optical signal at the same frequency as the pulsar without the need to know the phase differences between optical and radio signals. The route benefits from sensitivity to pulsed optical signals seen against a steady background and also allows detection of periodic reductions in intensity. The method is insensitive to changes in observational conditions. When used to acquire star images with the viewing field phase split onto three detector areas, the phase block method should provide intensity differences which identify the location of an optical signal with the frequency of the radio pulsar. Supportive data for the method from a simple laboratory based proof of concept experiment are cited. The three phase block route can equally be applied to searches for polarized emission.     

Mechanical properties of natural fibre reinforced polymer composites

During the last few years, natural fibres have received much more attention than ever before from the research community all over the world. These natural fibres offer a number of advantages over traditional synthetic fibres. In the present communication, a study on the synthesis and mechanical properties of new series of green composites involving Hibiscus sabdariffa fibre as a reinforcing material in urea-formaldehyde (UF) resin based polymer matrix has been reported. Static mechanical properties of randomly oriented intimately mixed Hibiscus sabdariffa fibre reinforced polymer composites such as tensile, compressive and wear properties were investigated as a function of fibre loading. Initially urea-formaldehyde resin prepared was subjected to evaluation of its optimum mechanical properties. Then reinforcing of the resin with Hibiscus sabdariffa fibre was accomplished in three different forms: particle size, short fibre and long fibre by employing optimized resin. Present work reveals that mechanical properties such as tensile strength, compressive strength and wear resistance etc of the urea-formaldehyde resin increases to considerable extent when reinforced with the fibre. Thermal (TGA/DTA/DTG) and morphological studies (SEM) of the resin and biocomposites have also been carried out.     

Acoustooptical processor for radio signals with discrete frequency modulation

A study is made of a processor for pulsed radio signals with discrete frequency modulation, where the frequency varies discontinuously within a pulse and takes on N different values. The spectral components are separated with an acoustooptic spectrum analyzer whose spectral plane is imaged onto a linear charged-coupled photodetector by means of a planar (one-dimensional) spatial converter made of optical fibers. In the imaging, the spectral components of the signal injected into the fiber are transposed by a special arrangement (permutation) of the fibers. The charge-coupled detector, which operates in the time-delay and accumulation mode, detects the spectral components and sums them with the required delay, and thereby compresses the signal and separates it from the noise. The compression factor is limited by the value of N and can be several hundred. Results of experimental investigations of a prototype of the processor are presented. Pis’ma Zh. Tekh. Fiz. 23, 12–19 (January 26, 1997)     

A procedure to obtain the complete electromagnetic scattering properties at discontinuities between integrated optical waveguides

Abstract

A method to obtain the complete electromagnetic scattering properties of discontinuities between arbitrary integrated optical waveguides is presented. The method involves a new generalized scattering matrix concept, together with the generalized telegraphist equations formulism and the modal matching technique. Radiation losses, as well as reflection and transmission coefficients between proper modes, can be obtained. Single and multiple discontinuities in planar and channel optical waveguides have been analysed. Numerical results of complex scattering coefficients are given. The possibilities of the method for analysing waveguide photonic crystals, as well as optical devices in waveguide periodic waveguide structures, are demonstrated.