Hi everybody!

Last time I saw an instrument needing excellent mechanical stability (a tunnel effect microscope) it was built of metal.

But microscope mirrors, which have similar needs, are made of a special ceramic. One known example is zerodur by Schott.

So could such instruments be made of zerodur or similar?

Besides the much better expansion coefficient, I ignore the properties and fabrication abilities. Mirrors are ground but make big efforts to cast the near-net shape, so deep grinding is unclear. The cost is an interrogation as mirrors take several months to cool down. Chemical etching woud be nice. Assembly is unclear, electric sealing would be nice but is said to need sodium ions.

Marc Schaefer, aka Enthalpy

invar

fused quartz

I guess there are others too.

 

In general, it's easier to thermostat the room.

Edited December 31, 20196 yr by John Cuthber

Hi JC, thanks for your interest!

Zerodur is several magnitudes better than Invar and quite better than silica, that's why telescope mirrors prefer it.

Not every instruments needs this stability, but some do, and for them Zerodur could be an advantage.

Thermostat, yes. Sometimes it's not a complete solution, for instance if the instrument produces heat or if operators can touch.

For some things, zerodur isn't up to the job.

This stuff is about 10 fold better

https://en.wikipedia.org/wiki/Ultra_low_expansion_glass

 

However, I'd not want to have to cut a screw thread into zerodur.

 

 

 

If you are lucky, you can "engineer out" the effects of expansion

I always thought this was a really neat trick.

 

https://en.wikipedia.org/wiki/Gridiron_pendulum

Zerodur claims 10^-8/K too, allegedly over a wider temperature range. I guess the choice depends on where you live.

Screws are the wrong choice with hard and brittle materials, sure. Other solutions.

And I spent quite some time observing the Gridiron pendulum at my grand-mother's clock. But for 2D stability, or when you need zero expansion at varied positions, it gets complicated.

I just wonder if the Gridiron pendulum (1726) inspired achromatic optics (known in 1750) or the other way round, or if both inventions were independent.