Large Telescope Mirrors Research Articles

Experiment study on large solar telescope mirror seeing

Experiment study on large solar telescope mirror seeing

Adaptive parabolic membrane mirrors for large deployable space telescopes.

A key element for the development of extremely large telescopes in space or balloon-borne observatories will be a reduction in the areal weight of the primary mirror. Large membrane mirrors offer a very low areal weight but are difficult to manufacture with the optical quality needed for astronomical telescopes. This paper demonstrates a practical method to overcome this limitation. In a test chamber we have successfully grown optical quality parabolic membrane mirrors on a rotating liquid in a test chamber. These polymer mirror prototypes of up to 30cm in diameter show a sufficiently low surface roughness and can be coated with reflective layers. By manipulating the parabolic shape locally using radiative adaptive optics methods, it is shown that imperfections or changes in the shape can be corrected. With only tiny local temperature changes induced by the radiation, many micrometers of stroke have been achieved. Scaling the method investigated to produce mirrors with diameters of many meters is possible using available technology. This approach opens the possibility to produce affordable extremely large primary mirrors for space telescopes. With the flexibility of the membrane material, this type of mirror can be compactly rolled up when stored in the launch vehicle, and then be deployed in space.

Design of truss adjusting mechanism for secondary mirror of large aperture telescope

Design of truss adjusting mechanism for secondary mirror of large aperture telescope

New capabilities for laser holographic testing during assembly and collimation of large segmented telescope mirrors

Subject of study. Several novel options for interferometric testing of large aspheric segmented primary telescope mirrors during all phases of construction (assembly, collimation, and performance assessment) are proposed. In this paper, we will describe and validate new engineering design solutions for interferometric surface testing of large concave aspheric segmented telescope mirrors. Methods. All proposed test configurations are based on a reflecting optical compensator in the form of an axial synthetic holographic optical element or a reflective convex aspheric surface of revolution. We propose a quasi-autocollimative light path in which an optical compensator reverses the object wavefront; however, the object wavefront itself does not directly produce the image of the surface being tested in the plane where the interference and shadow patterns are recorded, thereby eliminating significant distortion-like errors. Main results. We provide the calculated parameters for quasi-autocollimative systems used to test the surfaces of the aspheric concave primary mirror segments of four well-known world-class telescopes: the Millimetron telescope, diameter 10 m; the James Webb Space Telescope, diameter 6.5 m; the Extremely Large Telescope (ELT), diameter 39.3 m; and the postponed European Southern Observatory conceptual design known as the Overwhelmingly Large Telescope (OLT), diameter 100 m. We show that simultaneous use of a series of similar coaxial optical compensators on the object leg of the interferometer will almost completely eliminate any restrictions related to the size, asphericity, and curvature of aspheric segments of primary mirrors in telescopes. Using a conical substrate for the working surface of the holographic compensator will substantially reduce the maximum spatial frequency of the compensator. The calculations were performed using Mathcad and Zemax. We propose that traditional non-autocollimative laser holographic test configurations without wavefront reversal be used on the object leg during the initial assembly of the primary mirror segments. Laser holographic topography is also suitable for this purpose. Practical significance. The novel ideas, methods, and systems engineering solutions proposed here are based on wavefront reversal by a reflecting optical compensator or a series (“chain”) of such compensators and will support in-process testing and certification testing of aspheric primary mirror segments, with interferometric accuracy, for any of the optical telescopes currently operating or under development, whether ground based or space based. These capabilities will support full-scale interferometric testing of the 12 m diameter Millimetron aspheric segmented primary mirror initially proposed by Academician N. S. Kardashev, but will also undoubtedly be especially important for space-based telescopes.

Study of linear phase shift algorithms and application to deflectometry

For a number of optical methods, including deflectometry, information is encoded in fringe patterns. To extract the phase data from intensity values, a number of phase shift algorithms have been designed. In deflectometry, the nonlinearity of the fringe display implies that the displayed fringe pattern presents a harmonic content even if the input pattern is perfectly sinusoidal. The propagation of these harmonics through phase shift algorithms creates parasitic fringe patterns, reminiscent of the initial fringe pattern on the estimated phase. This phenomenon, known as print-through, has been identified as a serious performance limitation.In this paper, we revisit Surrel’s work on harmonic insensitive phase shift algorithms and demonstrate that the class of Discrete Fourier Transform (DFT) phase shift algorithms he defines is very appropriate for the field of deflectometry. We show how to choose the most suitable one depending on the application by performing a complete modeling of the harmonic print-through phenomenon for these DFT algorithms and studying the error propagation for shot noise and temporal perturbations. In a deflectometry context, we demonstrate by means of simulations that carefully chosen DFT algorithms can simultaneously be robust to print-through and perform better with respect to noise than the state of the art nonlinear phase shift algorithms.Lastly, by comparing experimental mirror shape measurements of the matrix of the secondary mirror of the European Extremely Large Telescope made on the one hand by DFT deflectometry and on the other hand by phase shift interferometry, we demonstrate that the use of DFT algorithms can substantially improve the high spatial frequency measurement capabilities of a deflectometry setup, without the need for a calibration of the display’s nonlinearities.

Об осесимметричном нагружении пластины на локальной опоре в рамках теории Родионовой – Титаева – Черных

The problem of an axisymmetric bending of a plate with constant thickness under its own weight is considered. The plate has a circular support. The problem is solved in an axisymmetric statement using a nonclassical shell theory of Rodionova-Titaev-Chernykh (RTCh), which takes plate compression in thickness into account. The solution for this theory is obtained using Godunov's orthogonal sweep method. This solution is compared with the solution obtained using the general three-dimensional theory of elasticity, implemented in an open-source package Code_Aster using axisymmetric finite elements. The motivation for this study is the description of a stress-strain state of some variants of primary mirrors of large optical telescopes under the action of gravity. The obtained results characterizing the optical quality of the mirror surface are: the peak value (PV) and the root-mean square (RMS) of its displacement. A parametric study was carried out, i.e., the thickness of the plate and the half-width of the support were varied. The two methods were compared. It is shown that, as the plate thickness increases or the half-width of the support decreases, non-physical behaviour of the mirror surface takes place within the limits of the nonclassical theory of RTCh. A criterion of its applicability is therefore proposed.

南极大型望远镜主镜运输的抗振缓冲系统

南极大型望远镜主镜运输的抗振缓冲系统

南极大型望远镜主镜运输的抗振缓冲系统

南极大型望远镜主镜运输的抗振缓冲系统

Surface Correction Control Based on Plasticized Multilayer P(VDF‐TrFE‐CFE) Actuator—Live Mirror

AbstractThe interdisciplinary approach presented here creates next‐generation large mirrors using electroactive polymer (EAP) actuators without classical glass abrasive polishing (“live mirrors”). The outstanding electromechanical coupling properties of terpolymer are taken advantage of, particularly when doped with plasticizer, e.g., diisononyl phthalate (DINP). This doped terpolymer creates a large strain response as well as excellent mechanical energy density under relatively low electric fields. Classical EAPs (e.g., polyurethane, silicone) require extremely high input voltages to reach sufficient mechanical strain. Using the high‐permittivity doped terpolymer and the concept of stacking multilayers, high displacements and large forces are generated. The actuation performance of multilayered terpolymer filled with DINP has been proven to shape mirror glass with a preliminary prototype of an 8‐layer actuator stack. The experimental results demonstrate surface deformations under load conditions of several microns. This is large enough to usefully control large optical telescope mirrors. This technology may enable much larger high‐quality optical mirror systems for ground‐ and space‐based astronomy and communications telescopes.

Calculation of the Optical Telescope Mirror

This work considers the problem of bending a thick annular plate of variable thickness on point supports under the action of its own weight. The problem describes the stress-strain state of the primary mirrors of large optical telescopes when the mirror axis is directed to the zenith. The main feature of this problem is the transverse displacements of the plate reference surface, which coincides with the rear flat base of the plate where the supports are located, and the front surface from which the incident light is reflected. Errors of the wavefront of the reflected light, including the standard deviation and the magnitude of the wavefront, are associated with the transverse displacement. The problem is solved with the use of two nonclassical theories of plates, the Timoshenko-Reissner theory of plates and the Palii-Spiro theory of mean thickness shells. The case of the optimal location of the supports corresponds to the smallest values of the deviation of the wavefront magnitude and the standard deviation. Calculations according to the Timoshenko-Reissner and the Palii-Spiro theories gave the same optimal location of the supports. The Palii-Spiro theory, which takes into account the variation of the transverse displacement along the thickness of the plate, is preferable for calculating the distortion of the reflecting surface of the primary mirrors of optical telescopes.

Thermal control for light-weighted primary mirrors of large ground-based solar telescopes

A light-weighted primary mirror is the most important optical element for a large ground-based solar telescope. It receives solar radiation of more than 1000 W / m2, of which about 10% is converted into heat energy, bringing in mirror seeing effect and surface shape distortion. Thus, a thermal control system (TCS) is necessary and important. Many studies have discussed the factors that influence the temperature difference between mirror surface and ambient air, but few refer to TCS modeling and optimization for the sake of parameter estimation. A thermal control model for the parameter estimation is proposed. The analytical process and the numerical analysis results of the model on the Chinese large ground-based solar telescope are given. A 60-cm prototype of open solar telescope (POST) is developed, on which practical experiment is taken place. According to the parameters estimated results, we select devices for the TCS of the POST. The experiment results show that the maximum temperature difference was restricted within ±0.5 ° C, despite the temperature variation of the ambient air, whereas without TCS, the maximum temperature difference rises up to 6.5°C. That validates the feasibility and effectiveness of the proposed model, and it can be referred for other large ground-based solar telescopes.

Subaru Portable Spectrophotometer: in-situ reflectivity measurement for large telescope mirror

The reflectivity of a telescope primary mirror is one of the fundamental parameters that determines the telescope performance. Due to a lack of suitable instruments, however, measuring the absolute value of the reflectivity, in particular wide spectral measurements in-situ, has been almost impossible. To solve this problem, we developed a portable spectrophotometer called the Subaru Portable Spectrophotometer (SPS). SPS covers a spectral range between 380 and 1000 nm with a resolution of 2 nm. Its dimension and weight enable in-situ measurement on the primary mirror. A modified V-N method is applied to SPS for obtaining the absolute reflectivity. A sequential measurement makes SPS compensate the instrumental drift. The great advantage of SPS is its capability of getting absolute spectral reflectivity in-situ, even after the primary mirror is mounted on a telescope. In the case of Subaru Telescope, SPS clarified the reflectivity of the primary mirror coated with aluminum 4 years ago. Periodic measurements have been on-going since the primary mirror recoating in 2017. It is now possible to study the telescope reflectivity degradation with SPS.

Control limitations from distributed sensing: Theory and Extremely Large Telescope application

We investigate performance bounds for feedback control of distributed plants where the controller can be centralized (i.e. it has access to measurements from the whole plant), but sensors only measure differences between neighboring subsystem outputs. Such “distributed sensing” can be a technological necessity in applications where system size exceeds accuracy requirements by many orders of magnitude. We formulate how distributed sensing generally limits feedback performance robust to measurement noise and to model uncertainty, without assuming any controller restrictions (among others, no “distributed control” restriction). A major practical consequence is the necessity to cut down integral action on some modes. We particularize the results to spatially invariant systems and finally illustrate implications of our developments for stabilizing the segmented primary mirror of the European Extremely Large Telescope.

High-accuracy numerical model of the Southern African large telescope mirror support truss

In order to calibrate the primary mirror of the Southern African large telescope, a high-accuracy numerical model of the existing mirror support truss and its actual, current behavior is required. In this investigation, structural optimization techniques are used to tune a numerical model so that it accurately matches the measured output of a structure under a given load or combination of loads. A methodology for obtaining an accurate numerical model of such a structure by first investigating the behavior of the different strut types before attempting to model the structure as a whole is presented. This method yielded vast improvements in the numerical accuracy of a smaller, simpler laboratory structure’s numerical model.

Modeling and validation of polishing tool influence functions for manufacturing segments for an extremely large telescope

We present a simulation technique to predict tool influence functions (TIFs) based on the Precessions polishing process, which is driven by addressing mass fabrication of the European Extremely Large Telescope mirror segments. Precessions polishing requires accurate and predictable TIFs to optimize the multiple process parameters, particularly when sequential polishing runs are performed by different polishing tools. In this paper, the static and dynamic TIFs are simulated based on the Preston equation. The velocity distribution is calculated according to the geometry of the precession motion. The pressure distribution at the polishing spot is calculated by means of finite element analysis (FEA). The FEA result is validated by direct force measurement with a simulation error of 4.3%. The simulation results of TIFs are verified by an experiment that shows the residual errors are less than 5% for both static and dynamic TIFs.

Fabricating the optical elements of compound mirrors for large astronomical telescopes

The technical possibilities of the Lytkarino Optical Glass Factory make it possible to manufacture optics from the stage of melting and annealing the blanks to the fabrication of finished astronomical mirrors. This paper describes the factory’s experience in investigating blanks and finished segments made from Astrositall for telescopes with compound segmented mirrors. An arrangement is shown for monitoring the figure of the spherical surface of the segments of a compound mirror, using a Fizeau lens. To implement the arrangement, a large-diameter Fizeau lens was fabricated and a special test stand was developed and fabricated that makes it possible to monitor both the spherical surface of the lens itself and a spherical element of the compound mirror.

Hartmann method for monitoring the shape of convex aspheric mirrors of large optical telescopes

This paper discusses the possibility of using the Hartmann method to monitor the shape of convex aspheric mirrors of telescopes under manufacturing conditions. A Hartmann stop in the form of regular small-diameter apertures in an opaque screen is placed outside the mirror to be monitored and is illuminated by a pencil of parallel rays directed perpendicular to the mirror's symmetry axis. The role of recording device can be played by a linear array of CCDs or a photographic plate on whose sensitive surface a so-called Hartmann pattern is recorded. Subsequent mathematical processing makes it possible to determine the shape errors of the meridional cross section. To provide monitoring of the entire surface, the latter is rotated around the symmetry axis by any given angle any number of times, and this makes it possible to completely automate the monitoring process of both ground and polished surfaces under manufacturing conditions.

An experiment of ultra-precision ELID grinding for large astronomical telescope mirror

An experiment of ultra-precision ELID grinding for large astronomical telescope mirror

Development of Large Optics Fabrication/Finishing (LOF) Process(Nanoprecision Elid-grinding (continued))

The ELID-grinding R&D activities on development of large optics for astronomical observatory and semiconductor fabrication, have been accelerated in these years. The authors developed the large aspheric nanoprecision ELID-grinders such as "L-uaoe-GIC", "N-aou-VEL", etc. for those purposes. In this paper, the authors introduce the examples of ELID-ground large mirrors and the new turning grinding method with the exclusive large and indexable rotary table which was newly developed specifically for efficient fabrication of large telescope mirror segments. The developed rotary table was successfully mounted on a large aspheric ELID-grinder. A new additional finishing process method was also devised on the machine.

New phasing algorithm for large segmented telescope mirrors

We propose a new phase calibration method, called the sensor-by-sensor phase calibration method (SSPC), which is applied for the phasing of a large segmented telescope mirror. Instead of making the entire segmented mirror in phase at one time and obtaining the de- sired sensor readings accordingly, we serially make the local areas in phase at each sensor location and thus obtain the desired sensor read- ings one by one. Then we run the telescope active control system to make the segments cophase. For the case of Keck telescopes, the SSPC method uses all the 168 phase measurements at 168 phase sen- sor locations; then we obtain 168 desired sensor readings for the seg- mented mirror active control system. The SSPC method is completely insensitive to the residual tilt errors from the segment alignment and the residual errors from the active control system. The validity of the SSPC method has been demonstrated in an active optics experimental system, which has three segments and a 500-mm-diam overall aperture. The accuracy of the SSPC method is better than 20-mm rms in this experi- mental system, and it is easy to obtain diffraction-limited images at a visible wavelength l5650 nm over an aperture of 220 mm. © 2002 Society