To create ground and space-based telescope mirror systems LZOS widely applies mirrors made of glass-ceramic Sitall CO-115M (AstroSitall^®), an analogous material to Zerodur of Schott, Germany. Many years of experience in optical components figuring has shown Sitall's reliability and suitability for the manufacturing of astronomical and space instruments with solid, lightweighted and thin large optical elements¹.

LZOS has accumulated substantial experience in designing and manufacturing of optics for large-size astronomical telescopes, development of new lightweiting mirror structures and high-accuracy computer-controlled figuring of aspheric surfaces of lightweighted, thin and off-axial components of an arbitrary shape².

Our effective and successful cooperation with Carl Zeiss Jena, Germany, in the manufacturing of large size optics for some European countries has allowed us to flow into the manufacturing process of optics for the 2-3 meter class telescopes with modern high quality optical requirements and optical components complexity^3,4.

Within 1997-2001 LZOS manufactured four primary hyperbolic mirrors of 2050 mm in diameter and two secondary mirrors of 645 mm in diameter of Sitall for the TTL (Telescope technologies Limited, UK). Asphericity from the best-fit sphere of the primary mirrors is about 20m and the secondary mirrors about 12m. We achieved an rms of about 0.03 (=0.6328m) for the primary mirrors. The similar quality has been achieved for the secondary mirrors.

LZOS manufactured a set of optics for the NOA Telescope (National Observatory of Athens, Greece) with a 2280 mm primary mirror, 753 mm secondary mirror and three-lens field corrector. Asphericity of the primary mirror is about 40m and the secondary mirror about 26m. An encircled energy of a system of the primary and secondary mirrors is 80% in a spot less than 0.3".

In 2001 we manufactured a unique set of optics for the VST Telescope (VLT Survey Telescope, Osservatorio Astronomico di Capodimonte Napoli, Italy) with a 2650 mm primary mirror and 938 mm secondary mirror both of them having 100m aspherisity. The VST primary mirror is an adaptive meniscus mirror of 140 mm thick, a thickness to diameter ratio is 1:19. At present a replica of the VST primary mirror is being manufactured at LZOS.

Now LZOS is involved in some large projects such as manufacturing of 96 primary mirror blanks for the South African Large Telescope (SALT)⁵, manufacturing and testing of the primary and secondary mirrors for the VISTA Telescope (Visible and Infrared Survey Telescope for Astronomy, UK)⁶ and shaping and final figuring of 37 sub-mirrors of the LAMOST primary mirror (Large Sky Area Multi-Object Spectroscopic Telescope, China)^7,8.

Since 1959 LZOS has mastered production of a glass-ceramic material possessing an extremely low coefficient of thermal expansion (CTE) Sitall CO-115M which is similar to Zerodur in optical and physico-mechanical properties. During all the period numerous tests have been carried out that proved stability of Sitall's properties through the time on special purpose optics. Sitall properties in comparison with Zerodur are given in Table 1:

Table 1. Sitall and Zerodur properties

material	Sitall CO-115M	Zerodur
Mean linear coefficient of thermal expansion within temperature range -60oÑ to +60oC, (K-1)	±1.5 x 10-7	±1.0 x 10-7 temperature range - 0oC to +50oC
Refraction ratio (ND)	1.536	1.542
Density (g/cm3)	2.46	2.35
Young's modulus (MPa)	9.2x104	9.3x104
Poisson's coefficient	0.28	0.24
Thermal capacity (J/g K)	0.92	0.80
Thermal conductivity (W/m K)	1.18	1.46
Thermal diffusivity (m2/s)	0.52 x 10-6	0.72 x 10-6

Our production facilities allow manufacturing optics from casting and annealing of Sitall blanks to their final figuring and polishing. LZOS's Sitall capacities allow to produce over 100 tons a year. Now Sitall is widely used for the manufacturing of high-precision astronomical mirrors at LZOS as well as at other companies. Figures 1 and 2 below present the main steps of Sitall blanks production process.

LZOS has machinery providing a full cycle of optics manufacturing from glass melting to final polishing. It consists of equipment for:

• blanks pre-machining including cutting, rounding, ends and flat surface machining, sphere figuring on blanks up to 6 meter in diameter;
• geometrical dimensions machining (coordinate boring, surface-grinding and other machines) to an accuracy up to 10m on blanks up to 4 meter in diameter;
• computer-controlled surface shaping of large optical components up to 4 meter in diameter.

Figure.1 Casting of Sitall blanks.

The blanks geometrical dimensions machining includes standard operations of cutting and milling of cast blanks on a special equipment using diamond tooling (Figure 2).

Figure.2 Sitall blanks milling.

The most significant of our current works is the manufacturing of 96 hexagonal 1096 mm x 55 mm blanks for the SALT primary mirror⁵. To meet successfully the SALT CTE specification new generation interference dilatometers were developed and manufactured. They ensure a CTE measuring error of no more than 5 x 10^-9 K^-1. The CTE homogeneity at 18 measured points of a blank is not more than 15 x 10^-9 K^-1. Distribution of CTE over 72 manufactured blanks is shown in Figure 4. Table 2 lists some CTE and birefringence requirements for the SALT blanks.

Table 2. SALT blank CTE and birefringence specifications

Average CTE	0 ± 0.15 x 10-6 K-1
CTE variation through blank thickness	0.015 x 10-6 K-1
Average birefringence	< 3 nm/cm
Maximum birefringence	< 10 nm/cm

The maximum birefringence measured at the distance of 50 mm from the edge of each of six flats of a blank segment is less than 10 nm/cm and an average birefringence is less than 3 nm/cm.

The SALT segments are being manufactured according to the bubbles/inclusions specification shown in Table 3. The manufacturing process of the SALT segment blanks is shown in Figure 5.

Table 3. SALT blank bubble/inclusion specification

Bubbles/Inclusions:	Maximum number of bubbles (inclusions) with mean diameter more than 0.2 mm depending on blank zone
	Critical zone	Non-critical zone
Maximum area of bubbles (inclusions) within 100 cm3	2 mm2	2 mm2
Maximum mean diameter	2 mm	6 mm
Maximum average number of bubbles (inclusions) within 100 cm3	5	5

Thus LZOS has manufacturing facilities, technology and qualified staff for the manufacturing of blanks of demanded dimensions, for instant for the 100 m OWL Telescope (Overwhelming Large Telescope). To provide the required quantity and delivery schedule it will be necessary to enlarge and develop the facilities and first of all in Sitall casting. The calculations we have made show that we will need to set in operation extra 5 bath furnaces and approximately 40 annealing furnaces that can ensure required production volumes. It is important to emphasize these innovations will not lead to extra construction of new production shops because the existing production infrastructure allows to place necessary equipment to available production areas. Table 4 shows proposed by LZOS specification suitable for the OWL segment blanks. This proposal could be valid for the manufacturing of blanks for other Extremely Large Telescopes.

Table 4. Proposed OWL blank specification

CTE and Birefringence:
Average CTE	0±0.1x10-6K-1
CTE homogeneity	for single segment 0.01õ10-6K-1 for all segments 0.05õ10-6K-1
Difference in CTE between back and front surfaces	0.01õ10-6K-1
CTE measuring accuracy	0.005õ10-6
Number of CTE samples on a blank	13
Birefringence caused by striae or inclusions	within critical zone < 25 nm outside critical zone < 50 nm
Permanent stress sign	minus
Average birefringence	< 8 nm/cm
Maximum birefringence	< 15 nm/cm
Bubbles/Inclusions:
Mean size	< 5.0 mm
Maximum size	< 8.0 mm
Maximum number within 10 cm-3	< 8.0
Average number	< 0.5 cm-3
Maximum number of mean diameter > 1 mm at depth up to 4 mm from working surface	< 10.0
Maximum size of inclusion outside critical zone	< 20.0 mm

The flowchart below describes a standard production process proposed for the OWL segment blanks.

2300 mm x 450 mm disk casting

Preliminary tests of material quality

Cutting into 2300 mm x 100 mm blanks

Bubble and birefringence measurement, blank forming and working surface selection

Preliminary cutting into 1850 mm hexagonal

Both-sides grinding up to 80 mm thickness

Final geometrical dimensions milling

Central blind hole manufacturing

Fabrication of CTE test samples

Spherical surface figuring

Final test of blank parameters, blank certification

Packing of blank

LZOS can manufacture both blanks for optical components and finished optics of Sitall, Zerodure and other optical materials.

Figure.3 Computer-controlled polishing of optical components.

At present the manufacturing of finished sub-mirrors of the primary mirror for the LAMOST (The Large Sky Area Multi-Object Fiber Spectroscopic Telescope, China) is going on at LZOS. The sub-mirrors are made from Zerodur supplied by Schott. The LAMOST telescope is a reflecting Schmidt telescope. There are two large segmented mirrors in the LAMOST: one is the Schmidt plate M_A, and the other is the spherical primary mirror M_B. The dimension of M_B is about 6.7m x 6m. It is composed of 37 hexagonal sub-mirrors.

Table 5. M_B Sub-mirror Specification

Material	Zerodur
Number	40 blanks
Shape	hexagonal
Surface shape	concave, sphere
Diagonal dimension	1100 mm
Thickness	75 ± 0.25 mm
Radius of curvature	40000 ± 40 mm
Surface rms	< 20 nm
Surface p-v	< 150 nm
Difference of sub-mirror radius (with respect to a reference mirror)	< 1.5 mm

The technological process of sub-mirrors manufacturing includes the final geometrical machining (Figure 9), back surface grinding, milling of slots and chamfers, central hole milling and then bonding of support Invar elements (Figure 10), and after final polymerization of bonded elements the sub-mirror is set on a machine for the optical surface grinding and polishing. The polishing of the sub-mirrors is carried out in a thermo-stable optical shop at a temperature of 22 ± 1 ^oC.

After a session of surface treatment the sub-mirror is set on the membrane technological cell into a test bench. This cell allows deformation changes of the sub-mirror surface shape during technological and certification tests in permissible limits lesser by amplitude than the required surface error. The cell has an automatic system of mirror position stabilization when changing environmental conditions (atmosphere pressure, moisture) in the course of the surface shape testing and provides constant surface shape with necessary precision. Consequently with more stable mirror position we have reached the high quality of optical surface.

In the course of the testing process the sub-mirror is installed on 18 supports. The analysis of the sub-mirror support system was made using a model of the sub-mirror with its actual dimensions (with holes and slots). The model is divided into 9453 elements of Solid95 type. Mounting on the 18 supports is made under pressure of 81.3633 N (8.294 kg). The sub-mirror model is constrained against displacement on three points coincident with inside circle supports. Support reactions are not more than 0.02 N (2 g). Figure 11 presents the sub-mirror model and topography of the surface shape deviation from the required one. The achieved wavefront deformation is 0.014 (8.86 nm) rms and 0.076 (48 nm) p-v.

The main complexity of the spherical surface testing of the sub-mirrors of the segmented primary mirror M_B is a large radius of 40000 mm and a tolerance of a radius deviation of ± 1.5 mm for all the sub-mirrors. The required specification is met only with using a vertical Fizeau test set-up. This method allows to reduce a test path up to 10184 mm as well as to carry out the testing with respect to one reference surface. Defocusing of more than ± 0.5 will be an evidence of a radius deviation of more than ± 1.5 mm. The basic element of the optical test setup is a Fizeau lens one surface of which has a radius of curvature of 40000 ± 20 mm and represents as a reference surface. The other surface of the lens is hyperbolic and serves to direct rays of a homocentric beam along the normal to the spherical surface. The CTE value we measured for the Fizeau lens material (fused silica KV) was 0.3 x 10^-6K^-1.

Figure.4 Sub-mirror model and surface topography on membrane-pneumatic support system.

To implement this test method a test bench was designed and mounted (Figure 13). The test bench is used for the testing of both the Fizeau lens and spherical sub-mirrors. To test the spherical lens a 1200 mm reference spherical mirror with a radius of curvature of 40000 ± 7 mm was produced of Sitall. The certification of the reference mirror was carried out in a special vertical test tower on the basis of a 70 m height vacuum chamber. Figure shows the interferogram of the Fizeau lens tested using the spherical reference mirror. An rms is less than 0.05 (=632.8 nm). The sub-mirror is set under the Fizeau lens on a definitely fixed distance in order to insure required tolerance for a difference of sub-mirror radius within a range < 1.5 mm.

LZOS manufactures both optical blanks including blanks of Sitall and finished optics of Sitall, Zerodur and other optical materials. LZOS has great experience in designing and manufacturing of large astronomic optics, development of new lightweighted mirrors, computer-controlled forming of high-accuracy optical non-spherical surfaces of lightweight, thin and off-axis components of arbitrary shape.

Figure.5 Fizeau test setup used for testing of segmented mirror elements:
1 - Sub-mirror to be tested; 2 - Fizeau lens with reference surface;
5,6,8 - Elements of interferometer 3; 4 - CCD camera; 7 - Diagonal mirror.

The most important work on the Sitall blanks manufacturing is our contract for delivery of 96 segment blanks for the SALT primary mirror. The most important of our existing works on the segments figuring is the contract for the polishing of 40 LAMOST primary mirror sub-mirrors made of Zerodur. At the moment the test bench is designed and mounted and the figuring of the first sub-mirrors is in progress.

LZOS's production potential after increasing its Sitall melting capabilities will allow manufacturing of blanks for primary and secondary mirrors of Extremely Large Telescopes on existing production areas.

REFERENSES

1. 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.
2. 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.
3. M. A. Abdulkadyrov, S. P. Belousov, A. N. Ignatov, V. E. Patrikeev, V.V. Pridnya, A.V. Polyanchikov, V. V. Rumyantsev, A. V. Samuylov, A. P. Semenov, Y. A. Sharov, Manufacturing of primary mirrors from Sitall CO-115M for European projects TTL, NOA and VST. Proceedings of SPIE, 4451, pp. 131-137, 2001.

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