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Light weight mirror blanks

STROPEK - Glastechnik manufactures light weight mirror blanks for optical instruments. With our patented technology we offer a mirror substrate near net shape, nearly closed back, with low internal stress and low weight. Our main domain are mirror blanks for astronomical telescopes. However these substrates can also be used in other fields of optics were low mass, a high stiffness to weight ratio, low thermal inertia, fast and secure processing of optical components are important.

Gerhard Stropek

Breslauer Str. 30

73457 Essingen


Tel: ++49 163 1752785

Mail: [email protected]

Specially designed for telescope makers

To serve the astronomical community and instrument makers we offer a standard line of well tested mirror blanks suitable for primary mirrors in diameters from 10 to 26 inch. These blanks are already pregenerated to mach the desired ROC. They offer a concave front plate and a plano backplate like traditional mirror blanks. They are available in two different glass types with characteristic thermal expansions. All glass types are suitable for a precision optical surface as needed for an astronomical telescope. Edge thickness ranges from 40mm to 51mm. Focal ratio F3.7 to F5. The areal density of our standard mirror blank is 30Kg/m2 targeting for an areal density of ~25Kg/m2 for the finished primary mirror. The high number of support elements eliminates print through issues during conventional polishing to a small amount. High stiffness and fast thermal equilibrium shortens the time needed for testing the optics and greatly improves productivity. The distribution of support elements is FEM optimized for minimal print through and wavefront abberation in the finished mirror. Carefully figured, these mirrors deliver sharp images of unprecedented clarity in the final instrument.


Material: Soda-lime glass [density ~ 2.5g/cm3, thermal Expansion 7.6x10-6m/m/K [-18C°+20C°]

Plate thickness: 6mm for front and back plate


Material: Borosilicate glass [density ~ 2.2 g/cm3, thermal expansion 3.2x10-6 m/m/K [-18C°+20C°]

Plate thickness: 6.5mm for front and back plate

Dimensions and mass for standard line of telescope mirror blanks

Diameter [inch]: 10", 12", 14", 16", 18", 20", 22", 24", 26"

Thickness at the edge: 40 mm to 53 mm depending on the diameter

Focal Ratio: Near net shape. Typical F5 (smaler blanks) to F3.7 (larger blanks).

Number of support Elements FEM optimized 61 to 219 depending on the diameter

Mass: approx 30 kg/m2 (SG-L); approx 29 kg/m2 (SG-B)

Example: 10" Blank SG-L F5.0: Thickness 43 mm. Mass 1.5 kg. Finished mirror approx.: ~1.3 kg.

Example: 14" Blank SG-L F3.7: Thickness 45 mm. Mass 3.0 kg. Finished mirror approx.: ~2.6 kg.

Example: 20" Blank SG-L F3.7: Thickness 49 mm. Mass 6.1 kg. Finished mirror approx.: ~5.3 kg.

Example: 26" Blank SG-L F3.7: Thickness 52 mm. Mass 10.3 kg. Finished mirror approx.: ~8.9 kg.

Custom mirror blanks & OEM

We can produce nearly all shapes and areal densities from small pieces to large optics and from prototype to volume. - Please contact us.

  • Diameter up to 26", but not limited to
  • Thickness up to 52 mm, but not limited to
  • Different focal ratios
  • Plano, concave, convex front- and backplates
  • Mirror Blanks with areal densities down to 15kg/m2
  • Support elements: different number, diameter, distribution
  • Outside contour: elliptical, hexagonal, etc.
  • Central openings
  • Tilted backplates
  • Contured backplates

We allow optics to become light weight. Our patented technology allows optical engineers to keep track with demanding requirements in terms of low weight, thermal management, stiffness, easy handling and higher productivity in processing and metrology of optical mirrors.

Main Characteristics

Low weight: Our standard mirror blank consists of a front and back plate with a thickness of 6 mm (soda-lime glass) or 6,5 mm (borosilicate glass) with an density of 2,5 g/cm3 or 2,2 g/cm3 producing a mirror blank with an areal density of around 30 kg/m2. Later stages of optical processing remove typically ~2 mm glass in total. The areal density of the finished mirror will be in the range of about 25 kg/m2. Using coarse abrasives further glass removal on the front and back plate can be achieved and will bring areal density significantly down. We do not recommend going below 17,5 kg/m2 areal density (3,5 mm on the face plate) since print through can become an issue if not great care, low polishing pressure or sophisticated polishing technologies are used. It is possible to produce a mirror blank with more support elements on special request for even lower weight targets while keeping print through to a low level.

Semi closed back plate: The back plate of our standard mirror blank is flat and closed to over 70%. This sandwich design provides considerably more stiffness for the substrate than traditional web backed mirrors. The semi closed back plate facilitates support on polishing machines, it facilitates manual processing and it facilitates the axial support in common mirror cells. The semi closed back plate protects the support structure in case of rough handling.

Fire polished support structure: The light weighting structure in our mirror blanks is fire polished and free of micro cracks. This means high structural strength and tension resistance. The wavy nature of the support elements reduces points of stress concentration.

Constant thickness front and back plate: This means equal cooling and equal areal density from edge to edge

100% pure monolithic: No different materials, no bonding: STROPEK - Glastechnik guarantees that the mirror blank is 100% pure monolithic with exact one and the same material throughout. No bonding or mixing of different materials. If properly supported and in thermal equilibrium our blanks will deliver the optical quality as tested in the optical shop.

100% free of bubbles: A flawless product without bubbles, striae, impurities in the critical layer or anywhere else in the mirror blank

100% vacuum applicable: Full ventilation.

High degree of transparency for stringent inspection and quality control.

Fine annealed with only low internal stress.

Near net shape: All our mirror blanks are near net shape. Vastly reducing time and work for generating the final ROC by the optician.

Material: Soda-lime glass or borosilicate glass are available. For astronomical purposes we don't hesitate to recommend soda-lime glass provided that it is in combination with our light weight technology. The driving force for quality images is low deflection of the mirror under gravity and fast thermal equilibrium and not the type of glass material. In both respects our blanks excel. The average linear thermal expansion rate of soda-lime glass is 7.6x10-6 m/m/K within a temperature range of -18 °C to +20 °C. Density is 2.5 g/cm3. For our mirror blanks in borosilicate glass we use "BOROFLOAT" (Schott). Borosilicate glass offers a lower expansion rate of 3.2x10-6 m/m/K in the above mentioned temperature range. Density is 2.2g/cm3. Please refer to the website of Schott for further details on "BOROFLOAT" which is a registered trademark of Schott AG, Mainz, Germany.


Thermal properties

The diagram shows the drop in temperature of one of our light weighted mirror subtrates. A warm (~30 °C) mirror was placed in a cool room (~19 °C). A thermocouple inside the central support element and in contact to the backside of the face plate was read every 10 min. The mirror was a 10" F5 mirror with an areal density of 22,5 kg/m2. The thickness of the face plate was 4.3 mm. After 50 minutes the temperature differential was <1 C°. This experiment was done in a lab with only minimal air currents and no fans. Please mark that the temperature was measured inside of the mirror blank on the most unfavorable position.

Mechanical properties

In order to determine the bending characteristics (astigmatic mode) a three point bending test was conducted. The test piece was a 10" F5 mirror with an edge thickness of 43 mm and a total weight of 1.2 kg. The lightweighted structure consists of 61 support elements in an FEM optimized distribution for minimal wavefront abberation due to gravity loads. The mirror was supported on two points separated by an angle of 180° on a 90% radius. Increasing Forces in the centre were applied by weights with a mass of 1.0 kg, 5.0 kg, 10.0 kg, 15.0 kg, 20.0 kg, 25.0 kg. For reasons of comparison a disc of solid glass with an diameter of 10" and a thickness of 8 mm was used. The flexture of the disc was measured to be 0.61 mm when a force of 243 N (25 kg) was applied were as the flexture of the lightweight mirror was <0.02 mm and beyond reasonable measuring with this kind of test.

Print Through

Print through or quilting are mid spatial frequencies produced by different bending characteristics of supported and unsupported areas of the optical surface during polishing. The most effective way to reduce print through is a reduction in polishing pressure. Application of computer controlled polishing technologies in the final stages of figuring like ion beam figuring (IBF), fluid jet polishing (FJP), small tool polishing (STP) or magneto rheological figuring (MRF) are also applicable.

The picture shows an interferometric contour phase map of a 10" F5 mirror with an areal density of 22.5 Kg/m2 and 61 support elements. Thickness of the face plate was 4.3 mm. The mirror was polished on a polishing machine to a sphere using conventional pitch polishing with a sub aperture tool of 75% diameter. Polishing pressure was 0.2 psi. Polishing pitch was Gugolz #64, Polishing temperature was 21,5°. Long frequencies aberrations were removed in order to isolate mid spatial frequencies. The residual wave front aberration due to print through is ~0.03 wave RMS (532nm) this translates into a strehl ratio of >95%. Please mark that this result was easily achieved with traditional pitch polishing and without any use of sophisticated polishing methods.