Here's a minor, but maybe useful mod to a Stellafane type Foucault tester.
After use I try to remember to tip the dial indicator up so that there's no pressure on it but that loosens the nut holding it. To save getting a wrench and maybe getting some of whatever metal chips/oil might be on that on the tester and maybe mirror I switched the bolt and nut for a scavenged bicycle seat post quick release. (This one might actually be for another component, but looks the same to me) You can adjust tension easily by turning the nut that's just visible under the dial indicator.