ZERODUR surfaces resulting from machining to spherical or aspherical forms have been investigated by visible light microscopy to identify subsurface damage (SSD). The rotation table peripheral grinding method has been used. Surface roughness (SR) of the different finished surfaces has been measured by a profilometer. We then examine the correlation between subsurface damage and roughness. This correlation allows for a precise determination and reduction of the SSD, enabling subsequent process steps such as polishing to continue with finer tool sizes resulting in a more efficient overall process.
Extreme light weighted ZERODUR Mirrors (ELZM) have been developed to exploit the superb thermal characteristics of ZERODUR. Coupled with up to date mechanical and optical fabrication methods this becomes an attractive technical approach. However the process of making mirror substrates has demonstrated to be unusually rapid and especially cost-effective. ELZM is aimed at the knee of the cost as a function of light weighting curve. ELZM mirrors are available at 88% light weighted. Together with their low risk, low cost production methods, this is presented as a strong option for NASA Explorer and Probe class missions.
We address the problem that larger spaceborne mirrors require greater sectional thickness to achieve a sufficient first eigen frequency that is resilient to launch loads, and to be stable during optical telescope assembly integration and test, this added thickness results in unacceptable added mass if we simply scale up solutions for smaller mirrors. Special features, like cathedral ribs, arch, chamfers, and back-side following the contour of the mirror face have been considered for these studies. For computational efficiency, we have conducted detailed analysis on various configurations of a 800 mm hexagonal segment and of a 1.2-m mirror, in a manner that they can be constrained by manufacturing parameters as would be a 4-m mirror. Furthermore each model considered also has been constrained by cost-effective machining practice as defined in the SCHOTT Mainz factory. Analysis on variants of this 1.2-m mirror has shown a favorable configuration. We have then scaled this optimal configuration to 4-m aperture. We discuss resulting parameters of costoptimized 4-m mirrors. We also discuss the advantages and disadvantages this analysis reveals of going to cathedral rib architecture on 1-m class mirror substrates.
The tip and tilt M5 mirror of the European Extremly Large Telescope (E-ELT) requires a demanding approach in light
weighting. The approximately 3 m x 2.5 m elliptical plano mirror is specified to a weight of less than 500 kg with high
Eigenfrequencies and low deformation under different inclination angles.
In 2011 SCHOTT has presented a study to develop a design for the M5 mirror blank of the ESO E-ELT. The design
presented was based on a radial square design to achieve the best compromise between performance and
manufacturability. With the fabrication of a prototype section SCHOTT demonstrated its capability to manufacture the
demanding features including pockets with 350 mm depth, thin walls and sloped pocket bottoms.
Now 3 years later SCHOTT presents an iso-grid based design that is in accordance with the manufacturability progress
that has been demonstrated in various ELZM (Extremely Lightweighted ZERODUR Mirrors) publications in the last two
years. The achievements on the specified mechanical parameters are compared to the first approach from 2011. In this
paper the results are presented and the performance parameters are discussed.
In a continuous effort since 2007 a considerable amount of new data and information has been gathered on the bending
strength of the extremely low thermal expansion glass ceramic ZERODUR®.
By fitting a three parameter Weibull distribution to the data it could be shown that for homogenously ground surfaces
minimum breakage stresses exist lying much higher than the previously applied design limits.
In order to achieve even higher allowable stress values diamond grain ground surfaces have been acid etched, a
procedure widely accepted as strength increasing measure.
If surfaces are etched taking off layers with thickness which are comparable to the maximum micro crack depth of the
preceding grinding process they also show statistical distributions compatible with a three parameter Weibull
distribution. SCHOTT has performed additional measurement series with etch solutions with variable composition
testing the applicability of this distribution and the possibility to achieve further increase of the minimum breakage
stress.
For long term loading applications strength change with time and environmental media are important. The parameter
needed for prediction calculations which is combining these influences is the stress corrosion constant. Results from the
past differ significantly from each other. On the basis of new investigations better information will be provided for
choosing the best value for the given application conditions.
Previously we established that the combination of new machining parameters for openbacked
monolithic lightweighted ZERODUR® mirror substrates, coupled with new methods for optical
finishing of aggressively lightweighted mirrors, have relieved classical mirror design constraints imposed
upon 1.2m diameter lightweight mirrors. We demonstrate that openback mirror substrates now offer
comparable mass, Eigenfrequency and substructure print-through performance to the sandwich-mirror
architecture, but with considerably less manufacturing effort than for sandwich mirrors. Here we extend
the analyses of the first paper from 1.2m diameter down to 0.6m in diameter and up to 2.4m diameter and
4m diameter mirrors.
Recent game-changing technology greatly extends the design possibilities and range of applications for aggressively lightweighted open-back Zerodur® mirrors. We have compared several lightweighting design approaches under this new technology. Analytic comparisons are for 1.2m mirrors, all constrained to have a free-free first Eigenfrequency of 200 Hz. Figures of merit include resulting mass, thickness and relative cost. Much more aggressive masses are now available in open-back mirrors, competitive with the more expensive closed-back sandwich mirrors. These breakthroughs are relevant to spaceborne implementation of lightweight mirrors ranging from a few tenths of a meter in diameter to up to 4 meters in diameter.
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