This short article shows some details of T. Lum's digital readout Caustic tester.  Like all Caustic tester designs, this one can also be used for many of the other common ATM mirror testing methods.  Built to a high degree of accuracy, it may be used to perform the Caustic, Foucault, Wire, and Ronchi tests.  (The text of this article is extracted from an email from Mr. Lum.  Eventually, a detailed construction article will replace this one.)