[William Tell]

Beelzebubba said: "Just put it on a bearing mount, and spin it about, using an interferometer like a machinist’s dial indicator, noting min and max radius."

At the level of precision desired in this case, how does one avoid measuring runout and imperfections of the bearing system?

I was thinking that maybe there was a way to measure a particular diameter from one side of the sphere to the other, in absolute terms of a certain number of wavelengths of some infrared standard.

Perhaps one can accomplish an absolute measure of diameter by fabricating a length standard first to calibrate the bearing system?

[Atlas Sneezed]

>>At the level of precision desired in this case, how does one avoid measuring runout and imperfections of the bearing system?

Hire technical experts. Anything is possible. Air bearings? A slightly larger cup filled with an oil film might do it.

>Perhaps one can accomplish an absolute measure of diameter by fabricating a length standard first to calibrate the bearing system?

Yes, you’d probably look for cyclic variations corresponding to the characteristics of the bearing system.