Converting an old floppy drive to a precise Foucault tester
Gut everything except the read-write head carriage base (junk the head itself):
Attach piece of metal angle to head carriage base (one below came from bend floppy drive mounting bracket). Tape safety razor blade (or something similar) to the upper end - this is testing knife edge. If you are using razor blade, put pieces of tape over top and bottom ends of the blade - its better not to risk slicing your eyelid (or worse) when testing! The carriage base usually pivots a bit - this can be utilized to move knife edge sideways.
Mount depth gauge to a wodden block and attach that assembly with wood screws (from below). Gauge's probe should still press against knife edge carrier bracket when it is completely backed-off. Make sure that probe is parallel to carriage tracks! My gauge has 1 inch travel which is plenty for testing a 20 inch f4 mirror.
Attach wooden block with tread insert to the edge of the base and tread 1/4x20 bolt through if. Align block so the bolt presses knife carrier bracket exactly opposite depth gauge:
Attach lamp (mine is made from a piece of acrylic tube with a section frosted with sandpaper and a miniture bulb is inserted inside - the rest of tube is wrapped in black electrical tape):
Attach battery holder and switch. To keep with a spirit of scavenging, I converted broken flashlight! Also make a small hole with a needle in a piece of aluminium foil and mount this pinhole over the frosted section of the lamp.
Here it is in use:
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© Jan Wisniewski