Magomed A. Abdulkadyrov*, Sergey P. Belousov, Alexandr N. Ignatov, Vladimir E. Patrikeev, Vitaliy V. Pridnya, Andrey V. Polyanchikov, Victor V. Rumyantsev, Anatoly V. Samuylov, Alexandr P. Semenov, Yury A. Sharov

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 m from the nearest sphere. The telescope field of view is approximately 40 arcmin. 80% encircle energy within less than 0.2 arcsec was achieved for 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 m. 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. 2650mm (F/1.8) primary mirror for VST project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. The aspericity is approximately 100 m. 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.

Keywords: telescopes, optical fabrication, optical testing, aspheres

* Correspondence: e-mail: lastro@comail.ru; Telefone: 007-095-552-15-72; Fax 007-095-552-15-86

Series of optical instruments of 2-, 3-, 4- meter class with excess requirements for image and execution equipment quality, considerable increasing of the field of view along with the essential decreasing of dimension due to increasing of the primary and secondary mirrors aperture up to 1:3 is under development nowadays. JSC LZOS in cooperation with Carl Zeiss (Jena) company has filled several orders on fabrication of optics for telescopes of 2 - 3 meter class with modern requirements to the surface quality and optical components complexity.

To create telescopic mirror systems to ground and space-based the JSC LZOS widely applies fabrication of mirror made of glass ceramic Sitall CO-115M, an analog for Zerodur of Schott company (Germany). Long experience of optical components shaping has shown its reliability and effectiveness for manufacturing of astronomic and space devices and instruments with monolithic, lightweighted and thin large-size optical elements.

According to the contract with Carl Zeiss (Germany) during 1997 - 2000 the JSC LZOS produces three primary mirrors with hyperbolic surface of 2050 mm in diameter and the secondary mirror of 645 mm in diameter made of Sitall CO-115M for TTL project (Telescope Technologies Limited, Great Britain). Primary mirrors have relative aperture D/f = 1:2.9 being peculiar for modern telescopes. The secondary mirror has D/f = 1:2.5.

The optical complex for NOA telescope (Astronomical Institute - National Observatory of Athens, Greece) with the primary mirror of 2280 mm in diameter, D/f = 1:2.3, the secondary mirror of 753 mm in diameter and three lens field corrector was manufactured in parallel. The telescope field of view is about 1.04^o. The telescope will be mounted in Balkans. The main mirror is classical with thickness of 300 mm and the ratio of mirror thickness to the diameter 1:7.6.

In 1999 - 2001 the optical complex for survey telescope VST¹ (VLT Survey telescope Osservatorio Astronomico di Capodimonte Napoli) was manufactured with the primary mirror of 2650 mm in diameter, D/f = 1:1.8 and the secondary mirror of 938 mm in diameter. With the field corrector the telescope will have the filed of view 1.5^o. The VST telescope mirror is adaptive and it is shaped as meniscus of 140 mm in thickness form. So the ratio of mirror thickness to its diameter is 1:19. The telescope will be mounted in Paranal observatory, Chile near the system of 4-, 8-meter VLT telescopes.

Three main mirrors of TTL project, NOA mirror and the main VST mirror have the following parameters:

specifications	project
	TTL	NOA	VST
material	Sitall CO-115M
shape	concave, hyperbolic
outer diameter	2050 mm	2280 mm	2650 mm
inner diameter	450 mm	460 mm	600 mm
thickness	200 mm	300 mm	140 mm (meniscus)
clear aperture	2000 mm	2200 mm	2600 mm
curvature	12000 ± 50 mm	10560 + 50 mm	9509 ± 10 mm
conic constant	-1.0703	-1.07943	-1.139899
maximum asphericity	19 m	40 m	100 m

According to the specification the following output parameters of telescope optics are required.

TTL project:
light concentration (80%) 0.2 arcsec diameter.
NOA project:
image quality (80% encircled energy)
on axis uncorrected: 0.35 arcsec
on axis corrected: 0.35 arcsec
off axis to 5 arcmin radius, uncorrected: 0.5 arcsec
off axis to 20 arcmin radius, corrected: 0.5 arcsec
VST project:
Geometrical spot concentration (80%) on axis, two mirror system, within 0.30 arcsec diameter, after removal of the coefficients constant, focus, decentering coma.
Geometrical spot concentration (80%) on axis, two mirror system, within 0.15 arcsec diameter, after additional removal of the coefficients 3^rd order spherical aberration, 3^rd order astigmatism, triangular coma, quadratic astigmatism.

figure 2: Handling toll for transportation of detail with 2650 mm in diameter

Production and technical complex JSC LZOS has thermostatically controlled optical chambers, vacuum control test setup, testing equipment, shaping equipment and automated software controlled machines for final finishing. JSC LZOS has large experience for creation of new light mirror constructions and computer controlled polishing of optical aspheric surfaces of light and thin details with free configuration of outer perimeter and apertures. Existing technology essentially accelerates the process of high precision optic fabrication and provides exact prognosis for surface shape and optical mirrors manufacturing time.

3.1. Primary mirrors blank manufacturing Technological equipment of JSC LZOS and Sitall CO-115M blanks manufactured by JSC LZOS were used for primary mirrors manufacturing2. The production potential of JSC LZOS provides optical mirrors manufacturing of Sitall blank from melting and annealing up to the final shaping. The turner for optical details up to 4 meters in diameter (Fig. 1) was manufactured for manipulations with mirror blank. The handling tool (Fig. 2) that was used for mirror transportation during preprocessing and monitoring was made for VST mirror and it will be used for mirror transportation during its mounting into working telescope cell. Special container (Fig. 4) was developed and produced for mirror transportation for aluminizing and its shipping to the observatory.

figure 1: Turner for details up to 4 m in diameter

The special areas for 84 legs were made on the back convex mirror surface for adaptive mirror monitoring system. Specialists of Carl Zeiss glued adaptive mirror elements to those areas and to mirror generatrix. Fig. 3 shows the mirror with elements of adaptive unloading system glued-on.

figure 3: VST mirror with glued-on elements

figure 4: Transportation container

3.2. Supporting and testing of primary mirrors On the stage of primary mirrors aspherization the surface profile was measured with the spherometer complex according to special procedure and mathematical algorithm. This procedure provides successful surface aspherization with declination from the target profile less than 1-2 m. After surface preprocessing the mirror is mounted into membrane-pneumatic technological cell on the automated tool being a part of vertical test set-up. Vertical test set-up for optical mirrors (fig. 6) has a control area that is set at different height depending on the controlled mirror radius. The test set-up provides control of details with radius of curvature up to 12 meters. In order to protect from air wind and for more precious determination of controlled mirror surface form the test set-up includes a jacket for protection against (fig. 5). Optimal location of membrane-pneumatic support system was calculated for each mirror.

figure 5: Jacket for protection against airflows

figure 6: Vertical test set-up

Membrane-pneumatic cell provides stable position of optical mirror during testing for successful computer controlled polishing. Supporting cell is equipped with automated system for stabilization of mirror position on the cell under changing of outer conditions (atmospheric pressure, humidity) during surface shape testing that provides stable surface shape with necessary precision. Due to more stable mirror position during testing it was possible to get high quality of processed optical surfaces. During surface aspherization on the primary VST mirror the surface shape was tested with IR interferometer with CO2 laser and wavefront mirror corrector. Wavefront corrector transforms the spherical wavefront into aspheric one that corresponds to the shape of the primary VST mirror shape. Control during final finishing was provided in the vertical set-up. Interferometer with wavefront corrector and automated processing of wavefront interferograms with CCD chamfer was used to testing the surface shape. Two-lens wavefront corrector has the input aperture about 60 mm and it transfers the plane wavefront into aspheric one. Calculation and production of wavefront correctors are performed at our plant (Fig. 8).

3.3 Primary mirrors shaping

Shaping of optical mirrors includes the stage of shaping and computer controlled surface finishing. Pre-shaping of primary mirrors for TTL, NOA and VST project was made according to the classical methods. Firstly the nearest sphere was fabricated and then the surface aspherization was performed by grinding and further polishing (Fig. 9).

At primary VST mirror surface aspherization process the computer controlled shaping by means of grinding was applied. After fabrication of aspheric surface on the mirror by grinding the surface shape was determined with the help of IR interferometer with CO₂-laser and wavefront mirror corrector (Fig. 7). According to the constructed topography of the mirror surface the processing sessions on the automated tool were calculated. After elimination of local defects and astigmatism on the detail surface by computer controlled methods the surface aspherization was continued according to classical technology. The mirror was located on the technological cell of membrane-pneumatic type in the vertical test set-up both at the stage of processing and at the stage of surface testing.

figure 9: VST mirror during pre-shaping

For finishing of the mirror surfaces the computer controlled complex for test results processing and shaping of optical mirror surfaces up to 4 m in diameter was used3. This technology is based on the program control of the fabrication complex with automated surface shape data processing, calculation, adjusting and prediction of technological modes, control over small oscillating instruments movement. Technological program complex is applied for wavefront interferogram of the testing detail processing in real time, for calculation of technological parameters of automated shaping, for automated correction of technological session according to the automated processing session, for prediction of the target surface.

Before computer controlled finishing of optical mirrors the surface shape had range of defects about 10. The initial VST mirror shape had declination after preprocessing about 25. Computer controlled finishing (fig. 10) took about 1.5 - 2 months for TTL and NOA mirrors and about 3 months for VST mirror because the last one had a large initial defect and had the maximum asphericity.

figure 10: Optical mirror surface finishing

The wavefront error on the optical surfaces was:
TTL 1,2,3: 17-21 nm (Wavefront RMS)
NOA: 19 nm (RMS)
VST: < 20 nm (RMS)
On fig. 11 the final interferograms for three TTL project mirrors are shown. At all primary mirrors encircle energy was 80% within the spot less than 0.2 arcsec in diameter.
On fig.12 the initial and the final interferograms for the main NOA mirror are shown. At the primary mirror encircle energy was 80% within the spot less than 0.18arcsec in diameter.
On fig.13 the initial and the final interferograms for the VST primary mirror are shown. At the primary mirror encircle energy was 80% within the spot less than 0.12 arcsec in diameter.

wavefront error 17nm (RMS) 80% energy concentration in a diameter 0.16 arcsec

wavefront error 21nm (RMS) 80% energy concentration in a diameter 0.20 arcsec

wavefront error 19nm (RMS) 80% energy concentration in a diameter 0.19 arcsec

figure 11: Interferograms for three TTL project mirrors

It must be noted that during TTL project mirrors fabrication two wavefront correctors with different input aperture were applied. So more reliable data of the wavefront shape for the primary mirrors and of the mirror parameters, vertex radius and surface eccentricity that correspond the specification were achieved.

wavefront error 7.5 (P-V), 1.5 (RMS)

wavefront error 19nm (RMS) 80% energy concentration in a diameter 0.18 arcsec

figure 12: Initial and final interferograms of NOA mirror of 2280 mm in diameter at the stage of computer controlled finishing

wavefront error 20.5 (P-V), 2.5 (RMS)

80% energy concentration in a diameter 0.12 arcsec after removal of the coefficients 3rd order spherical aberration, 3rd order astigmatism, triangular coma, quadratic astigmatism

figure 13: Initial and final interferograms of VST mirror of 2650 mm in diameter at the stage of automated finishing

The second wavefront corrector for VST mirror was manufactured having in mind a preliminary measured actual radius of curvature in order to get more precise values for vertex radius and surface eccentricity meeting the specification.

JSC LZOS produced three 2050 mm (F/3) primary hyperbolic mirrors for TTL project (Telescope Technologies Limited, Great Britain) during 1997-2000. 80% encircle energy within less than 0.2 arcsec was achieved for all mirrors. The surface error RMS is less than 10 nm. The first light from the first telescope on the plant in Liverpool is got now.

2280 mm (F/2.3) primary mirror for NOA project (Astronomical Institute – National Observatory of Athens, Greece) was produced. The primary mirror has 80% encircle energy within less than 0.18 arcsec and surface error RMS less than 10 nm.

2650mm (F/1.8) primary mirror for VST ESO project (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli) was produced. Geometrical spot concentration (80%) on axis within 0.20 arcsec diameter, after removal of the coefficients constant, focus, decentering coma. Geometrical spot concentration (80%) on axis, two mirror system, within 0.12 arcsec diameter, after additional removal of the coefficients 3^rd order spherical aberration, 3^rd order astigmatism, triangular coma, quadratic astigmatism.

REFERENSES

1. D. Mancini, G. Sedmak, M. Brescia, F. Cortecchia, D. Fierro, V. Fiume Garelli, G. Marra, F. Perrotta, F. Rovedi, P. Schipani, VST project: technical overview. Proceedings of SPIE, 4004, pp. 79-90, 2000.
2. M. A. Abdulkadyrov, S. P. Belousov, A. N. Ignatov, V. V. Rumyantsev, Non-traditional technologies to fabricate lightweighted astronomical mirrors with high stability of surface shape. Proceedings of SPIE, 3786, pp. 468-473, 1999.
3. A. P. Semenov, V. E. Patrikeev, A. V. Samuylov, Y. A. Sharov, Computer-controlled fabrication of large-size ground and space-based optics from glass ceramic Sitall CO-115M. Proceedings of SPIE, 3786, pp. 474-479, 1999.