I want to build a very precise motion system(based on hydro/aerostatic rail),it;s a DIY project with serious budget constrain.
So I want buy a cheap system to measure parallelism of motion stages,Those sensor should be able to pick up at least 0.1u displacement.
My project still in the planning stage,I can't afford laser interferometer for such measurement.
After some googling I believe inductive displacement senor can fit my need.

system I intent to buy are used tesa or mahr inducitve displacement senor stsyem:
tesa tt 20 with a plunge typpe sensor cost 500-600 usd
or
mahr
c1298m+ plunge type sensor around 1000used
or
mahr millitron 1202+ 1302 prob 600usd
I'm very new to this kind of high preicsion metrology devices,any advice would be welcomed