Polishing and testing methods used in the manufacture of the 3.4 m primary mirror of the Iranian National Observatory (INO) telescope are described and the test results of the finished mirror are presented. Mirror lapping and polishing was performed using several rectangular non-rotating tools arranged in a linear array across the mirror radius. Each tool is equipped with two computer controlled force actuators for regulating the surface pressure and removal efficiency during the lapping and polishing operations. The same tool system was used from the lapping phase to the end of the final polishing. The principal optical test method was the interferometric Hartmann test with the aid of a two component null lens in the mirror center of curvature. Mirror measurements were made also with pentaprism test to verify its correct conic constant. The mirror was finished to extremely good surface accuracy and smoothness.
We report the results of the first laser beacon experiment at the astronomical site of La Palma (Canary Islands). A continuous wave low power laser (a few hundreds of mW) system has been set up. The laser, tuned on the sodium D2 line at 589 nm, is launched close to zenith angle. The emission of the mesospheric sodium layer is observed from a telescope located 160 meters away from the laser. The layer is therefore resolved in altitude and the different features of its dynamics are investigated.
The interface between optical design and fine-mechanical design is a creatively fertile stage in the design of an optical system. The optical designer learns `What the system is all about' and the fine-mechanical engineer is liberated creatively by finding out what the priorities are. On the other hand, optical tolerancing as a discipline, which is an attractive mix of optical physics at one end and hard-nosed mechanical pragmatism at the other, has a relatively fragmented literature. It is the purpose of this paper to (1) Point out some cultural contrasts between optical engineers, mechanical engineers and physicists in this context, (2) To present a new method of displaying the optical surface sensitivities so that one may identify datum surfaces in an intuitive way and (3) To summarize some useful formulae which provide a key to the mounting of optical components. Together these three aspects illustrate the scenery of this fertile terrain.
The two-group zoom lens is the simplest arrangement which is capable of achieving a modest useful zoom range and adequate aberration correction for practical implementation. The first order layout of these systems is straight-forward and the real battle that has to be fought is to balance the effects of vignetting and aberration stability. This paper reviews the `+-' type, the `-+' type and shows how the quality of the overall performance evolves with the redistribution of aberration correction between the two groups of the zoom system, finishing with the analysis of a practical system which uses a relatively complex construction to achieve high performance.
In optical systems with modest fields of view, pupil spherical aberration and pupil coma are the principal pupil aberrations of significance, but at extreme fields of view, astigmatic effects and field curvature and distortion effects of the pupil imagery play a part in understanding the subtleties of oblique object-image imagery. Biogon and fisheye wide angle lens systems have been investigated for their pupil aberration properties, along with the investigation of similar effects in a panoramic optical system.
Reflective optical surfaces offer lower aberrations than the equivalent refractive optical components and they are also free of chromatic aberrations; so the use of reflective surfaces in zoom systems should lead to better performance than the equivalent refractive systems. This is clearly a naive view because it ignores the problem of central obstruction and also the very low level of complexity of a reflective zooming group (at least notionally a single surface). On this bases we have exploited the additional complexity that Mangin or back-reflective mirrors offer, showing how a high degree of correction over a modest zoom range may be achieved with remarkably compact constructions.
This paper presents a method of automatically adjusting aberrational weighting factors in a damped least squares optimization program, based on tolerance data provided for each aberration site. The method which is based on previous work by the authors and that of Yao has been developed into a tolerance-based method for aberrational weight adjustment. This enhanced method allows the user to set and adjust weights by a new tolerance-based method. The use of a power index ((rho) ) in the tolerance-based method provides continuously variable control over the amount by which the aberrational weights are adjusted. The practical use of the tolerance-based method is demonstrated using simple and well understood test lenses to reveal the modifications that take place within the iterative design progress, that result from aberrational weight adjustment. Suggestions for suitable tolerance schemes have also been investigated as an integral part of this work, based on the work of Hopkins.
This paper describes the determination and selection of tolerance values used by a tolerance based weighting program written by the authors. Equations for the amounts of low and medium order wavefront aberration permissible in an optical system are plotted as a series of wavefront and transverse ray aberration curves. This data provides the basis for a determination of individual tolerancing schemes for particular optical designs, based on their performance requirements. The maximum amount of wavefront aberration allowable in an optical system, where the required performance is limited by the maximum spatial frequency required to be passed, is also considered. These permissible wavefront aberrations in terms of reduced spatial frequency are presented as a series of curves for a range of reduced spatial frequencies s. For a tolerance-based weighting scheme it is necessary to assign a set of tolerances to each of the aberration sample sites involved in the merit function in such a way that they can be interpreted by the optimization program. The method presented in this paper is to translate the permissible wavefront aberration curves into transverse ray aberration data. In this form the data becomes accessible to transverse ray aberration based optimization algorithms.
Many methods exist for the optimization of lens systems. A great number of these are based on the successful damped least squares (DLS) algorithms. The success of the DLS optimization algorithm applied to optical design has tended to conceal fundamental limitations in the method. The DLS optimization method relies upon the selection of weights for each of, typically fifty aberrations and an optimum choice of these aberration weights is essential in order to achieve both the best possible performance and to guide the design process during convergence. Selection of suitable damping factors is also important for successful convergence towards an optimum solution and is considered here as a dynamic weight adjustment routine based on feedback techniques, to both increase the rate of convergence and improve the quality of the final lens design.
The need to achieve control of specific aberrations and to investigate the aberrational properties of lens systems has led to the development of an interactive design tool based on multi-dimensional quadratic extrapolation. A quadratic approximation to the non-linearly varying aberrations allows the step length of this differential correction technique to be extended and the underlying relationships between goups of constructional parameters and aberrations are revealed. Examples are given.
Due to an apparent conflict between transparency and dispersion the near ultra violet transmission of achromatic lenses is strongly dependant on glass choice. The design of achromats for this waveband is described and the choice of optical glass is reviewed in detail. Designs are given. 1.