KEYWORDS: Surface finishing, Matrices, Mirrors, Polishing, Off axis mirrors, Data modeling, Space telescopes, Optical components, Manufacturing, 3D modeling
For the first time in Russia, a technology has been developed, using computer-controlled automatic system and a set of special tools, that allows testing and aspherization of off-axis large-sized optical elements of astronomical and space telescopes with a deviation from the nearest sphere up to 20 mm. Computer-controlled process of optical surface figuring is carried out at all stages – milling, grinding and polishing.
JSC LZOS has been using diffraction optical elements (DOE) or Computer Generated Holograms (CGH) for many years to test the surfaces of large-sized optical mirrors for astronomical and space purposes. They are used for aspherical surface figure testing; testing of aspherical surface vertex position relative to the mirror geometric center, calculation of distortion in interferogram image, mutual adjustment of mirrors in testing scheme, etc. Thus, CGH have become an integral part of the up-to-date testing of large-sized optical mirrors and optical systems aspherical surfaces.
Interferometer for figure of large convex hyperboloid mirrors contains concentric meniscus lens and wavefront analyzer for study the wavefront reflected from measured hyperbolic mirror. Concentric lens rotates around the imaginary focus of the tested mirror to measure full surface of the hyperbolic mirror. Main advantage of proposed interferometer is the possibility of figuring the shape of extremely large convex hyperbolic mirrors with a diameter more than 2 meter. In addition, the interferometer may be applied for measure the hyperbolic mirrors with a very large aperture angles in the imaginary geometric focus, for example with the aperture angle of 180° or even more.
JSC LZOS applies various mechanical and optical methods to test optics at all stages of figuring, from milling and grinding to polishing stage. Computer Generated Holograms (CGH) are successfully applied at the polishing stage for the following purposes: testing of large-size optical components, on- and off-axis surfaces; alignment with respect to interferometer; defining of an aspherical surface optical vertex position with respect to the geometrical center for optical systems alignment. All this allows manufacturing of large-size aspherical on- and off-axis astronomical and space mirrors with high accuracy.
Substrate material is important while manufacturing of large-dimensioned optical mirrors for ground and space applications. The properties of Astrositall, its dimensional stability, design features are considered. Various types of supporting cells and optical components bases are used in the process of surfaces figuring. Applying of membranepneumatic support to base the mirror in the course of working surface testing is considered. However, in some cases it is necessary to support the mirror basing it on glued interface elements. In such case, surface figure difference between standard and manufacturing support cell, which is taken into account during figuring, is estimated via modeling.
JSC LZOS successfully applies Computer Generated Holograms (CGH) for testing of on- and off-axis surfaces of largedimensioned optical components, for their alignment with respect to interferometer, for defining of aspherical surface optical vertex position with respect to the geometrical center, for optical systems alignment. All this allows producing of large-dimensioned aspherical on- and off-axis astronomical and space mirrors with high accuracy.
This paper describes the technology of production and testing of mirrors for DAG Telescope (Doğu Anadolu Gözlemevi) produced by Belgium Company AMOS. JSC LZOS fulfils works on production and testing of three mirrors: primary concave hyperbolic mirror of diameter 4 m, secondary convex hyperbolic mirror – 764 mm and tertiary elliptical mirror with flat working surface with dimensions 890х650 mm. The primary mirror is produced from Zerodur, while the secondary and the tertiary ones – from Astrositall. Some auxiliary elements, containers and handling tools, etc. are also produced at JSC LZOS. Special aspects of mirrors machining and testing are also reflected in this article.
Technology with the use of programmable computer-controlled system and a set of special instruments, which makes possible aspherization of optical elements with deviation from the nearest sphere of more than 1 mm at the stage of milling, grinding and polishing, was developed. The equipment and software for testing the surface shape at all stages of processing, was described.
KEYWORDS: Optics manufacturing, Astronomy, Astronomical telescopes, Space telescopes, Telescopes, Mirrors, Off axis mirrors, Surface finishing, Wavefronts, James Webb Space Telescope
The article describes the technology of production of astronomical and space mirrors from Astrositall CO-115M including its properties and stability of these properties over time and presents the results of material long-term testing. The article also describes computer-controlled methods of large-scaled optics production and testing, including high aspherical, off-axis and thin mirrors, using the examples of production of mirrors at JSC LZOS.
Interference method of testing the long-focus focusing lenses allows measuring such optical parameters as focal distances and distortions of wavefront passing through the lens. High measurement accuracy is achieved by means of a special laser measurement system (laser tracker) in autocollimation schematic based on dynamic interferometer. Main sources of instrument error were found out and their relation to the measurement schematic parameters was defined. The article shows that the method allows achieving total instrument measurement error of the 0.003% of the value being measured.
KEYWORDS: Astronomy, Space mirrors, Mirrors, Space telescopes, Telescopes, Temperature metrology, Prototyping, Optical components, Off axis mirrors, Solids
The technology of producing the astronomical and space mirrors from Astrositall material, including its properties and stability of these properties in the course of time, is described. The results of long-term material tests are presented. In particular, the method of grinding and polishing the off-axis segment of the mirror of very large telescope in the stress-strain state is considered.
The article describes the method of testing the absolute profile of large-sized astronomical mirrors grinded aspherical surface and the method of test the aspherical surface decentering relative to the astronomical mirror geometrical center by means of a linear three-point spherometer, which is subsequently moved perpendicular to the direction from the optical surface center to the edge, as well as the method of positioning the interface elements being glued.
Technology with the use of programmable computer-controlled system and a set of special instruments, which makes possible aspherization of off-axis large-size optical elements of astronomical and space telescopes with deviation from the nearest sphere of more than 1 mm, was developed.
There is a described computer - controlled methods of treatment of large optics with testing of ground surfaces by a set
of spherometers and IR-interferometers, testing of lens correctors and CGH correctors, measuring of Radius by a laser
tracker on examples of astronomical mirrors production under projects VST, TNT, VISTA, etc. There is a graph
describing a degree of complexity of fabricated optics for various telescopes all over the world and there is data about
the astronomical mirrors produced in LZOS for different projects.
JSC LZOS under the contract with firm AMOS is carrying out the manufacturing works of Primary and Secondary
Mirrors of Devasthal Optical Telescope (DOT) for Aryabhatta Research Institute of Observational Sciences (ARIES).
Primary mirror specifications: diameter is 3700 mm, vertex radius is 14639 mm (F/1.96), conical constant is -1.03296,
asphericity is 111 microns. Secondary mirror specifications: diameter is 980 mm, vertex radius is 4675 mm (F/1.78),
conical constant is -2.79561, asphericity is 47 microns. The current progress status under this project is presented here in
the manuscript.
Since last years and at present days LZOS, JSC has been producing a range of primary mirrors of astronomical
telescopes with diameter more than 1m under contracts with foreign companies. Simultaneous testing of an aspherical
surface figure by means of a lens corrector and CGH (computer generated hologram) corrector, testing of the corrector
using the CGH allow challenging the task of definite testing of the mirrors surfaces figure. The results of successful
figuring of the mirrors with diameter up to 4m like VISTA Project (Southern European Observatory), TNT (Thai
National telescope, Australia - Thailand), LCO telescopes (Las Cumbres Observatory, USA) and Russian national
projects and meeting these mirrors specifications' requirements are all considered as the sufficient evidence.
"LZOS", JSC has completed the fabrication of M2 Primary mirror for VISTA Project (Visible and Infrared Survey
Telescope for Astronomy). M2 Mirror has diameter 1241mm and hyperbolic figure surface. The Vertex Radius of the
Mirror is 4018.81mm (F/0.96), the Conical constant is -5.548792, aspherical value is 306 μm. The Project works results are presented.
JSC LZOS is carrying out work on the manufacturing of the M1 primary hyperbolic Mirror with the 4100 mm diameter
for the VISTA project (Visible and Infrared Survey Telescope for Astronomy). Vertex radius of Mirror is 8094 mm
(F/0.98), conic constant -1.129792 and asphericity about 880 microns. The current situation of the work carried out is
presented in the manuscript.
LZOS is manufacturing the M3 Mirror flat surface for the GRANTECAN Project. The Mirror has an elliptical configuration and axial dimensions of 1521mm x 1073mm. The results of the M3 Mirror figuring and final polishing, testing in the Ritchey-Common layout at two 30° and 60° angles of incidence of a principal ray are presented in the manuscript.
Nowadays LZOS is carrying out work on the manufacturing of the M1 Mirror and M2 Mirror for the VISTA project (Visible and Infrared Survey Telescope for Astronomy) with the 4100mm diameter primary hyperbolic mirror with asphericity about 800 microns and the 1241mm diameter secondary hyperbolic mirror with asphericity about 350 microns. The current status of the work carried out is presented in the manuscript.
The technical capabilities of LZOS allow optics manufacturing from the phase of melting and annealing of Astrositall® blanks to their final figuring. Nowadays LZOS is working on the several large contracts, among them are manufacturing of 96 hexagonal segment blanks of 1019 mm x 55 mm of Sitall CO-115M for the SALT (Southern African Large Telescope) primary segmented mirror, and 40 sub-mirrors of Zerodur® for the 6.7 m x 6 m segmented Mb mirror for the LAMOST (Large Sky Area Multi-Object Spectroscopic Telescope) project.
LZOS manufactured the flat surface of the M3 mirror for the GRANTECAN Project. The mirror has an elliptical configuration and axial dimensions of 1521 mm x 1073 mm. The results of the M3 mirror figuring and final polishing, testing in the Ritchey-Common layout at two 30° and 60° angles of incidence of a main beam are presented in the manuscript.
Nowadays LZOS is carrying out work on the manufacturing of the M1 Mirror and M2 Mirror for the VISTA project (Visible and Infrared Survey Telescope for Astronomy) with the 4100 mm diameter primary hyperbolic mirror with asphericity about 800 μmi and the 1241 mm diameter secondary hyperbolic mirror with asphericity about 350 μm. The current status of the work carried out is presented in the manuscript.
The technical capabilities of LZOS allow optics manufacturing from the phase of melting and annealing of Astrositall blanks to their final figuring. Now LZOS is working on the several large contracts, among them are manufacturing of 96 hexagonal segment blanks of 1019 mm x 55 mm of Sitall CO-115M for the SALT (Southern African Large Telescope) primary segmented mirror, and 40 sub-mirrors of Zerodur for the 6.7 m x 6 m segmented MB mirror for the LAMOST (Large Sky Area Multi-Object Spectroscopic Telescope) project.
JSC "LZOS" production facilities allow manufacturing optics from casting and annealing of blanks of Sitall CO-115M to final figuring and polishing. LZOS's Sitall capacities allow to produce over 100 tons a year. At the present time Sitall is widely used for manufacturing high-precision astronomical mirrors here at LZOS as well as at other companies. During 1997 - 2002 JSC "LZOS" has fabricated a number of astronomical mirrors including four primary mirrors with hyperbolic figure of 2050 mm (F/3) in diameter and two seconday mirrors of 645 mm (F/2.5) in diameter for Telescope Technologies Ltd, UK, primary mirror of 2280 mm (F/2.3) and secondary mirror of 753 mm (F/2) for the NOA telescope (Astronomical Institute -- National Observatory of Athens, Greece), primary mirror of 2650 mm (F/1.8) and secondary mirror of 938 mm (F/2.3) with an asphericity of 100 μm for the VST telescope (VLT Survey Telescope). We have also completed a number of astronomical mirrors with diameters up to 1300 mm for some European countries and the USA. The rms surface quality of all of the mirrors was in the range from 9 to 12 nm. We used comptuer controlled figuring, polishing and testing. Some mirrors were made of Sitall, producing by LZOS and some of Zerodur, Schott. Our largest current projects include 96 hexagonal segment blanks of 1019 mm x 55 mm for the SALT primary segmented mirror (Southern African Large Telescope), the M1 and M2 mirrors for the VISTA project (Visible and Infrared Survey Telescope for Astronomy) where primary mirror has 4 m diameter and secondary 1.2 m as well as 40 sub-mirrors of the LAMOST MB mirror of about 6.7 m x 6 m for Large Sky Area Multi-Object Spectroscopic Telescope (LAMOST).
JSC "LZOS" manufactures astronomical mirrors from the stage of blanks to finished astronomical mirrors. During 1997-2002 JSC "LZOS" has fabricated a number of astronomical mirrors under the contracts with Carl Zeiss Jena, Germany, up to 2.6m in diameter and up to 100 μm asphericity. Concave surfaces are tested with lens and lens-mirror
wavefront correctors of special design. The Hindle test set-up is used for convex hyperbolic surfaces. For high-aperture hyperbolic surface, we use two Hindle spheres to test one mirror. At the present time, we have developed the test procedures for high-aperture surfaces of primary (diameter 4100 mm, asphericity about 881 μm) and secondary (diameter 1240 mm, asphericity about 364 μm) mirrors for the VISTA Project as well as the Fizeau test set-up for the
LAMOST MB sub-mirrors.
JSC LZOS under the contract with Carl Zeiss Jena, Germany produced three 2050 mm (F/3) primary hyperbolic mirrors for TTL project (Telescope Technologies Limited, Great Britain) during 1997-2001. The asphericity is approximately 20 micrometers from the nearest sphere. The telescope field of view is approximately 40 arcmin. 80% encircle energy within less than 0.2 arcsec was achieved from all mirrors. The surface error RMS is less than 9 nm. 2280 mm (F/2.3) primary mirror for NOA project (Astronomical Institute - National Observatory of Athens, Greece) was produced. The asphericity is approximately 40 micrometers . The telescope field of view with corrector is approximately 1.04 degrees. The primary mirror is classical one with 300 mm thickness and mirror diameter to mirror thickness ratio (aspect ratio) of 7.6:1. The primary mirror has 80% encircle energy within less than 0.2 arcsec and surface error RMS less than 9 nm. 2650 mm (F/1.8) primary mirror for VST project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. The asphericity is approximately 100 micrometers . 1.5 degrees telescope field of view with corrector will be achieved. VST primary adaptive mirror is 140 mm meniscus. The aspect ratio is 19:1.
JSC LZOS under the contract with Carl Zeiss Jena, Germany produced 645 mm (F/2.5) secondary mirror for TTL project (Telescope Technologies Limited, Great Britain) during 1999- 2001. The asphericity is approximately 12 micrometers from the nearest sphere. The system of primary and secondary mirrors has 80% encircle energy within less than 0.2 arcsec. The surface error RMS is about 9 nm. 753 mm (F/2) secondary mirror for NOA project (Astronomical Institute - National Observatory of Athens, Greece) was produced. The asphericity is approximately 26 micrometers . The surface area RMS is about 12 nm. The telescope field of view is approximately 1.04 degrees. The system of primary and secondary mirrors has 80% encircle energy within less that 0.3 arcsec. 938 mm (F/2.3) secondary mirror for VST project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. The asphericity is approximately 100 micrometers . The telescope field of view with corrector will be 1.5 degrees. Three Hindle sphere 1610 mm, 1640 mm and 1985 mm in diameter with radii of 6300 mm, 3995 mm and 2708 mm were used to test three secondary mirrors. Each convex hyperbolic mirror was tested by using two Hindle spheres. The wavefront of tested mirror was determined by wavefront superposition method.
The large technical-scientific achievements made to date in the field of astronomy, space and laser technology have been feasible to a great extent due to forthcoming of the new high-precision otpical ground and space-based system. In this connection, the requirements to the quality of optical surface have enhanced; a range of their overall dimensions has increased; components with off-axial aspherical surfaces, and with an arbitrary shape of outer perimeter of the components and their holes have been used often. Alongside the traditional materials used in the optical manufacture, non-traditional materials find ever-growing use.
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