Revisiting the Effectiveness of Large Optical Telescopes

• Main Author: V. V. Sychev
• Format: Article
• Language: Russian
• Published: MGTU im. N.È. Baumana 2015-01-01
• Series: Nauka i Obrazovanie
• Subjects: large optical telescopes coefficient of invariance; informativeness invariant; the diffraction image of a point
• Online Access: http://technomag.edu.ru/jour/article/view/383

<p>To create large-size optical telescopes, various design concepts have been used. Each concept inevitably faced the challenge to optimize technical characteristics and parameters of the telescope. There was always a question: what concept to choose, how to estimate efficiency of such telescopes and by what criteria and how to estimate expediency of this or that project of the large-size telescope. It is, obviously, insufficient to make a resolution-based estimation. An estimate by the angular field size is inappropriate too. Well, it may be also an estimate by the stellar magnitude. All these criteria are related to each other. Improvement of one of these parameters inevitably leads to deterioration of the others. Obviously, the certain generalized criterion considering all parameters and features of the design concept of the large-size telescope is necessary here. As such can serve the criterion of informational content of the telescope.</p><p>The article offers a complex criterion allowing not only to estimate efficiency of large-size optical telescopes, but also to compare their conceptual and technological level among themselves in terms of obtaining information.</p><p>The article suggests a new term, i.e. the informational content invariant to characterize informative capacities of the chosen concept and of the realizing technology. It will allow us to avoid unjustified complications of technical solutions, wrong accents in designing and excess material inputs when developing the project.</p><p>The informational content criterion-based analysis of the existing projects of large-size telescopes has been convincingly shown that, conceptually, there are three best telescopes, namely: GSMT, CELT, and ACT-25. And, in terms of informational content, the АCТ-25 is 10 times more than GSMT and CELT, and the existing Keck-telescope exceeds by 30 times. Hence, it is hard to escape a conclusion that it is more favourable to implement one ACT-25, than to do 10 GSMT or CELT. Once again the conclusion drawn by the author in March, 2000 [11] is confirmed: there is no common sense to create telescopes of land basing with a diameter of the main mirror more than 25 m to register images of extremely remote astronomical objects. And creation of telescopes with diameters from 30 to 100 m, as it is seen from calculations, does not give any advantages over telescopes of smaller diameter, and only extremely complicates and raises the price of a problem.</p><p>It is shown that introduction of new concept of an invariant of informational content for large-size optical telescopes will allow to have a new look at the development process of complicated optic-electronic complexes. The informational content invariant as a criterion of efficiency enables an assessment and comparison of various technical solutions at the stage of search for optimum ways of increasing informational content of telescopes.</p><p>Besides, and it is quite essential, the invariant of informational content will disable the misapprehension regarding a possibility to increase amount of information by increasing a mirror diameter of the telescope and will prevent the scientific-and technological community from unsuccessful projects and unjustified material inputs.</p><p>In the early 1990’s when design and implementation of the fourth generation of optical telescopes of a 10-meter class were under development scientists and engineers already started being engaged in problems of creating the super telescopes of the 5-th generation (25-meter and more). In recent years of the XX century when implementation of the main projects of telescopes of the fourth generation entered the finishing phase, these researches started extending and going deep. Despite the complicated problems the offers of 25-meter telescopes were followed by the avant-projects of telescopes with an aperture of 50 meters, and even 100 meters:</p><p>- influence of laser radiation on design elements and propagation medium and, as consequence, the back action of design elements and propagation medium on the transmitted radiation wave-front;</p><p>- lack of a basic source of radiation on the laser radiation wavelength, which is necessary for the adaptive correction methods to be applied to distorted wave-front;</p><p>- inherent only in laser systems additional distorting factors available in transmitting systems.</p><p>Such distorting factors are as follows:</p><p>• length of optical path because of spacing necessary for a high-power laser source with a large number of the consistent optical elements;</p><p>• thermal self-influence of powerful laser radiation in the channel of radiation transportation before its input in the forming optical system;</p><p>• instability of spatiotemporal characteristics of laser radiation source itself, which worsens passing radiation conditions both in an optical path and in the free atmosphere;</p><p>• thermal heterogeneity and thermal deformations.</p><p>It is shown that adaptive systems are distinguished from active optics by the fact that correction of the radiation wave-front distortions occurs in real time on the entire set of the distorting factors (not only on influence of the atmosphere) with the speed much exceeding action of distortions. Thus, the quality correction is assessed according to criterion of quality of primary image.</p><p>Thus, correction continuously considers information on the current spatial, temperature, temporary, and justified parameters of the optical system, providing quality maintenance of the image under conditions of distorting factors.</p><p>The main postulates of adaptive correction are formulated and offered.</p><p>Postulates represent the set of statements and provisions allowing us to realize effective remedies of adaptive correction of distortions.</p><p>It is also shown what real opportunities are open by using methods and means of adaptive optics in effective application of laser radiation power and what ways allow us to solve these tasks. First of all, it is:</p><p>- forming a system of assumptions and minimizing the distortions in the optical path, which includes, actually, a laser resonator, a channel for transportation of powerful laser radiation with beam-deflecting mirrors to form the telescope with a compound main mirror;</p><p>- forming the efficiency criteria of adaptive optical systems;</p><p>- multi-loop system for adaptive correction of distortions.</p><p>The paper discusses test results of transporting powerful laser radiation in the horizontal pathway and shows visual appearance of forming optical system of the test complex bench.</p><p>It is convincingly proved that the use of offered postulates in development or modernization of optical complexes ensures the minimum level of residual distortions and the overall performance of adaptive optics.</p><p>The offered postulates of adaptive correction of radiation wave-front and a positive experience of their use in full-scale optical complexes will substantially reduce terms and costs in creating effective aids to watch remote objects, as well as to form and supply the energy to the space objects for its various use such as power supply, communication, fight against space debris, ensuring asteroid safety, etc.</p><p>It is possible to draw a conclusion that the state of domestic optical science, its potential in creation of adaptive means to provide formation and transportation of powerful laser radiation, and results of theoretical and pilot studies, encourage a reasonable hope for future creating a multi-purpose highly effective large-size optic-electronic facility.</p>