N II 1 Diamond turning won't do everything!

Given the recent impressive gains in diamond turning pre- cision optics it is fair to ask why we have virtually given up trying to use diamond turning methods to make mirror segments for the proposed US National 15m Telescope. The scale of this giant, ground-besed optical instrument is such that its primary mirror must be sub-divided into smaller segments; one of the leading design concepts we are now considering would use up to 100 precisely-made 1-2m mirror elements. When nested together in a mosaic and put under appropriate position servo control, these mirrors would function as a single, nearly-parabolic, optical sur- face. Manufacturing these elements would seem to be a natural application for the budding capabilities of diamond turning. Regrettably, this is not the case, even though the repetitive precision of diamond turning would be an enor- mous advantage in making identical mirror segments.

The first problem is that all but the central element will be off-axis, aspheric mirrors, individually not rotat- ionally symmetric, and the outer segments wil l be more than 7m off-axis. This suggests the need to generate the optical surface with the workpiece offset on a huge spindle (impractical); or the abil ity to work on axis while rapidly and precisely controlling the cutting tool motion in and out of the workpiece surface at least once/revolution; or the abil i ty to pre-bend the workpiece such that, after machining, presumably to some manageable on-axis con- figuration, and the workpiece is released, it will spring into the desired off-axis shape. We are currently attempt- ing to use the latter technique to polish a 2m diameter low-expansion glass mirror using a conventional optical polishing machine and have appropriately dubbed it the 'bend-and-polish' method. However, the technology for using any of these approaches on a diamond turning mach- ine is presently well-hidden from my view, if it exists at all.

Secondly, the size of the workpiece (1-2m) mandates excellent workpiece support on the turning spindle if the required 25-50nm (1-2/~in) surface accuracy is not to be lost. Telescope designers have grappled for many years with the flexible nature of large mirrors and although our methods work well and might adapt to diamond turning, we have not worried much about centrifugal forces or distortions due to machining stresses. Today 'optical quality' mirrors up to 0.5m can be diamond turned, but a new philosophy for workpiece support must be developed to do as well at 2m. Mirror surface flexure problems worsen more or less as the third or fourth power of mirror radius and it seems unlikely that present holding methods can be improved 64- fold or more by any simple modifications. (My apologies to the structural engineers for the cavalier interpretation of plate bending theory implicit in this statement.) The point is that workpieces above 0.5m deserve as much careful analysis as the diamond turning machine itself and I see no widespread recognition of this problem.

A third problem is how to measure the optical quality of the finished surface. A 15m telescope mirror segment wil l have a basic surface radius (ie radius of the 'best-fit' spherical approximation to the actual surface) in the range 50-60m. The centre of curvature is the point where optical mirror testing is most often performed. In this case, the optical centre of curvature could also be 7m off-axis. The dimensions of the testing set-up would thus be formidable. Conceivably, one could perform interferometric tests or a Hartmann test with the workpiece in situ on the turning machine, but I suspect that air turbulence wil l force testing to be done with the optical axis vertical (to minimize thermal layering effects). Facilities and technical expertise of the type required for this form of testing are uncommon: a lesson we are learning painfully from our 'bend-and-polish' experiment.

Finally, as i f achieving the final product were not sufficiently diff icult, the long-term dimensional stability of diamond-turned metal mirrors wil l have to be conclu- sively demonstrated to sceptical astronomers. Cast, nickel coated, aluminium mirrors were in vogue for telescopes in the 1955-65 period (for economy), but have lost favour since due to optical surface changes brought on by thermal expansion differentials and stress relaxation in the substrate. Since telescopes are almost always built one-at-a-time from non-renewable funds and without a back-up, astronomers will be understandably reluctant to opt for metal mirrors again without positive proof that these problems have been solved. There is little evidence to show that the marvellous accuracies achieved from diamond turning are matched by long-term stability of the workpiece. It is not easy to demonstrate. The stability of the test method comes into question and some methods of storing data (eg holograms) have dubious long-term value. Mirrors may, in fact, be the appropriate test vehicle because they can be re-tested with confidence, given an appropriate method. If an invariant wavelength of light is used for testing, then any changes can be blamed on the mirror substrate. However, this work will require, at least, several years to complete even after the proper approach is selected.

As a mechanical engineer, I was fascinated several years ago with the prospect of using diamond turning to make telescope mirrors, but my early enthusiasm has been replaced by the gloomy realisation that it may require ten years or more for solutions to emerge for the problems cited above. We would rather not wait that long before deciding how to build our telescope.

Kitt Peak National Observatory, Tucson, Arizona, USA

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