[kragen]

It's very difficult, but it's quite common for amateur astronomers to grind mirrors to a precision of 0.1 microns or better, using century-old techniques. (Typically the mirrors are smaller, but that's a matter of the available budget for materials and man-hours, not the metrological precision.) I think it's reasonable to assert that if a company doesn't have such amateurs on staff, it will not be able to fabricate optics to such demanding precisions. There are not currently any assembly lines for such things.

The century-old techniques are a little better now that you can use a laser instead of a candle, and there are improved techniques you can use, but amateur telescope fabrication techniques are totally capable of hitting λ/8 precision.

[AstralStorm]

The difference is size, the simple grinding process does not keep accuracy for big mirrors and lenses.

[kragen]

The accuracy isn't in the process of grinding; it's in the process of measurement. All the grinding process needs to do is to remove only a small amount of material, which is itself a nontrivial problem; a single large grain of grit can ruin weeks or months of work, so amateur astronomers may get very angry at you for things that seem unreasonably minor, like opening a door without washing your hands. But that's not a problem that scales superlinearly with mirror size.

Really large mirrors are segmented anyway.