What did I use?
1. A dial indicator. I bought one for €25,95 (= about $32,-). Accuracy: 0.01 mm (0.0004").
2. A piece of 6 mm thickness aluminium bar, 20 mm wide and about 150 long (lenght depending on the diameter of the measuring circle one has in mind).
3. About 100 mm of 6 mm thickness metal tubing (I used stainless) to make the legs.
4. Three small metal balls fitting on the metal tubing, to act as the feet of the legs, resting on the mirror.
5. Two bolts to clamp the dial indicator in place.
6. Some metal epoxy to glue the legs into the holes in the aluminium bars and to glue the balls to the legs.

The legs of the spherometer in the upper photograph are relatively close together. For a larger mirror it can be necessary to make one with the legs further apart for better resolution. I also made such a wider one. This time I used bolts and wingnuts for the legs and glued the balls to the bolts. The length of the legs can now be adjusted. A photograph is below.

Total cost of each spherometer: about €34,- ($41,-).

The distance between the two legs on the same bar as the dial indicator is the diameter of the measuring circle of the spherometer (diameter of mine is 90.8 mm for the narrow and 240.8 for the wide legged sperometer). The third leg, also resting on this circle prevents the spherometer from falling and holds the spherometer in the corect position.

The focal length of the mirror is calculated with formulae: F = 1/2(S^2+r^2)/2s, in which F is the focal length, s is the value as measured with the spherometer and r is the radius of the spherometer's measuring circle, or half of the distance bewteen the two legs on the same bar as the dial indicator.

If you make one: be sure the dial indicator cannot move in the aluminium bar, otherwise it will measure inconsistently.

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Email to: Jan van Gastel