Large Telescope Mirrors Research Articles

Generalized figure-control algorithm for large segmented telescope mirrors

A fundamental discovery in segmented-mirror active optics is described: symmetrizing the geometry of the sensor–actuator array provides a computationally effective symmetrization of the mathematical description for the figure control of a Keck-type telescope mirror. The author establishes a universal mathematical control model and provides an efficient algorithm to solve this model equation set. This model can be applied to multifarious Keck-type mirror configurations with a similar sensor–actuator geometry design, no matter what kind of outline shapes and how many segments they have. With the underlying symmetry, a further extension of this algorithm is possible without increasing the number of parameters to be estimated for the recently proposed extremely large telescopes, such as the 30-m California Extremely Large Telescope (CELT) and the 100-m OverWhelmingly Large (OWL) telescope. Moreover, careful choice of boundary conditions in conjunction with the proper choice of minimization algorithm yields results that exceed the performance of the current existing techniques given by Nelson and Mast [Appl. Opt.21, 2631 (1982)]. This method allows noise performance analysis. Several computer simulation models for application of this algorithm are given for the Keck 10-m Telescope and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope’s (LAMOST’s) MA (Reflecting Schmidt plate) and MB (spherical primary mirror).

Direct ray aberration estimation in Hartmanngrams by use of a regularized phase-tracking system

The Hartmann test is a well-known technique for testing large telescope mirrors. The Hartmann technique samples the wave front under analysis by use of a screen of uniformly spaced array of holes located at the pupil plane. The traditional technique used to gather quantitative data requires the measurement of the centroid of these holes as imaged near the paraxial focus. The deviation from its unaberrated uniform position is proportional to the slope of the wave-front asphericity. The centroid estimation is normally done manually with the aid of a microscope or a densitometer; however, newer automatic fringe-processing techniques that use the synchronous detection technique or the Fourier phase-estimation method may also be used. Here we propose a new technique based on a regularized phase-tracking (RPT) system to detect the transverse aberration in Hartmanngrams in a direct way. That is, it takes the dotted pattern of the Hartmanngram as input, and as output the RPT system gives the unwrapped transverse ray aberration in just one step. Our RPT is compared with the synchronous and the Fourier methods, which may be regarded as its closest competitors.

The eye of mercury

The mirrors of large telescopes require adaptive optical systems to correct the distortion stemming from atmospheric fluctuations and the flexing of the mirrors under their own weight. Daedalus is now working on a really adaptive, liquid mirror using mercury. His proposal is to employ an array of electrodes set in a large dish of mercury, and a set of coils by which a complex magnetic field can be imposed on the mercury. The result — forces that will produce hydrostatic pressure in the liquid and allow its surface to be shaped to order.

Cleaning of optical surfaces by excimer laser radiation

The effect of particle removal from Al mirror surfaces by the influence of pulsed UV laser radiation has been studied. The investigations are closely related to the demands of astronomers, who are looking for a more effective way to clean future very large telescope (VLT) mirrors [1]. A systematic parameter study has been performed in order to determine the irradiation conditions which yield the highest dust removal efficiency (i.e. reflectivity increase) on contaminated samples. The particle removal rate increases with increasing laser fluence, being limited however by the damage threshold of the coating. Data indicate that on Al coated BK7 and Zerodur samples KrF laser radiation yields the optimum result, with cleaning efficiencies comparable to polymer film stripping. The initial reflectivity of the clean coating can nearly be restored, in particular when an additional solvent film on the sample surface is applied.

Nondistorting lateral edge support of large telescope mirrors

Simply by push-pull shear action at their outer edge, large circular telescope mirrors can be laterally supported free from bending distortion, if the upright mirror is kept in balance by a cosine distribution of weak axial forces applied at its outer rim and possibly also at the central hole. The flexure-induced comalike wave-front aberration of a thin 8-m meniscus mirror was reduced to an rms value of 0.5 nm over the full aperture; the largest path difference was reduced to 2.4 nm. A comparable result has also been calculated for a meniscus mirror of different geometry and a material with different elastic constants. Realizing such possibilities in practice demands accurate engineering. A helpful artifice is investigated for the correct application of the tangential supporting forces.

A Lateral Restraining Device for Large Telescope Mirrors

Abstract Telescope mirrors—like every rigid body—have six degrees of freedom (DOFs) to move: translation along three axes of space and rotation around these three axes. Hydraulic support systems, which are designed to hold the mirror in the required best optical shape, have to couple all six DOFs between the mirror and its supporting cell structure. While three DOFs are covered by the axial support system and two DOFs by the lateral support system, the last degree of freedom often remains uncovered. A good example is the lateral support system for the 8 m primary mirror of ESO's Very Large Telescope, which did not couple movements along the horizontal axis between the mirror and its cell and therefore required a lateral restraining device. Based on this example, a general method is shown how to control all degrees of freedom at a given supporting force distribution without any unwanted additional forces. According to the theory of modal control, a hydraulic network is used to decouple the oil flow to the hydraulic actuators and thereby to control the components of the supporting forces independently. The theory may be applied for other hydraulic support systems, too.

Lateral Support of Very Large Telescope Mirrors by Edge Forces Only

Abstract Until now a diameter of about 4 m seemed to be the upper-size limit of telescope mirrors that still permitted cost-saving designs of lateral supports by edge forces alone. In some designs, the supporting edge forces comprised not only basic push-pull action normal to the edge but also a specific, moderate amount of tangential shear. However, this was a by-product of design economy rather than the result of understanding the potential of tangential support forces as a means of reducing mirror flexure systematically, down to residuals in the 1% region. The surprising possibility of extending the usefulness of pure edge supports is demonstrated by the example of the 8-m mirror of the European Southern Observatory's Very Large Telescope. Fitted with a lateral support at the outer edge alone, this thin mirror will exhibit a wavefront aberration with a calculated rms value of only 18 nm, without taking into account possible active control.

Reflection anisotropy in evaporated aluminum: Consequences for telescope mirror coatings

Aluminum coatings deposited in the presence of small quantities of oxygen onto substrates at oblique incidence are optically anisotropic, even at normal incidence. Values of the normal incidence reflection anisotropy are given for films deposited under controlled pressures and rates and for the range of angles 0°–45° which is appropriate to a circular array of sources for coating a large telescope mirror. Experimental arrangements for measuring anisotropy are described. One of these leads to a map showing the angular displacement of an incident linearly polarized beam which is reflected by a coated concave mirror.

Thermal expansion uniformity of materials for large telescope mirrors

Uniformity of thermal expansion has been measured for fused quartz (Heraeus-Amersil TO8E) and borosilicate glass (Schott Duran and Ohara E6). The variation of expansion coefficient for three melts of TO8E was 5 x 10(-9)/K over a temperature range of 300 to 100 K and was found to vary linearly with position in the melt. This spatial gradient averaged 3.5 x 10(-11)/K cm. The room-temperature thermal expansivity variation of Duran (Tempax) glass was approximately 27 x 10(-9)/K, while that of E6 glass was approximately 52 x 10(-9)/K.

Mirror Compensator for Testing Convex Secondary

The testing of convex secondary mirror of large telescope is known as a difficault thing, because it needs for a large mirror or lens. In USA, they used to use Hindle sphere for testing convex hyperboloids, and in West Germany as I know, they often use concave matrix probe for testing convex mirrors. In the case of Hindle method, the diameter of testing sphere always is about 2 or 3 times as the convex mirror under testing, in the later case, the matrix probe must has the same size as the convex mirror, further more, the probe itself needs another testing facilities. Some other methods had been suggested by R. Wilson, E, Gaviola and others but they have rarely been used in practice.

Thin mirrors for large optical telescopes

The use of thin mirrors in large aperture, optical telescopes offers substantial benefits without necessarily causing any loss in optical performance. This paper discusses the design requirements for supporting thin mirrors in a telescope, the consequences which result, and the limitations of using thin mirrors.

Testing large telescope mirrors in the optical shop by an autocollimation method with multiple pendulum flat mirrors

Design and a compensation technique are presented to enable slope error measurements to be made via use of a pendulum having a small flat mirror mounted on the lower end. A multiplicity of such pendulum mirrors can act as a substitute for a full diameter flat for testing large astronomical primary and secondary mirrors.

Well baffled two-mirror corrector for a parabola

The two-mirror Baker corrector for a parabolic telescope mirror is a well-known, very elegant design. In some variations, both corrector mirrors are spherical. The only thing marring the beauty of this solution to providing anastigmatic performance for large parabolic telescope mirrors is that the system is difficult to baffle. An alternate two-mirror corrector is presented here-based on a different design principle-that is easy to baffle. One of the two corrector mirrors is spherical, and there is a field stop to further aid in baffling. The penalty paid is that the signs.

Portable visible-infrared reflectometer

A simple reflectometer has been developed for measuring the conditions of aluminum coatings on large astronomical telescope mirrors.

Field Correctors for Large Telescopes

The aberrations of large telescope mirrors can be corrected by lens systems located near to the focal plane. The three-element systems, designed by Ross for the 200-in. (5-m) Mount Palomar telescope, are shown to be amenable to a higher degree of aberration correction, and designs of this form are given, adapted for use at the focus of the primary mirror of a Ritchey–Chrétien telescope. A different form of three-component system gives still better aberration correction, and examples of this are given, suitable for use on the Kitt Peak projected 150-in. (3.8-m) f/2.8 telescope. Simple two-element systems to correct the field aberrations at the focus of the two-mirror Ritchey–Chrétien telescope are also described.

New Method of Testing Large Telescope Mirrors

New Method of Testing Large Telescope Mirrors

The Evaporation Process and its Application to the Aluminizing of Large Telescope Mirrors

The Evaporation Process and its Application to the Aluminizing of Large Telescope Mirrors