A digital data recording system has been used on the six-inch transit circle since January 1961. By means of servo repeaters and analog-to-digital converters, readings of the two micrometer screws are displayed in light banks and may be recorded by means of an accumulating printer or a tape punch. For a screw turning up to 16 RPM, the indications are read and stored to an accuracy of one milli-revolution. The store operation, controlled by the break of the standard clock, takes place in about 50 microseconds. Readings may be recorded at 1, 2, or 4 second intervals by the clock signals. A digital clock identifies the second at which the record is made. At any instant, the observer may record the readings.
A servo repeater system was designed to drive the right ascension screw of the micrometer. A ball and disc integrator with cosine input device is used as speed changer. The speed scale is nearly linear. Without discontinuity, the output speed may be varied from 16 RPM (0° Decl.) through zero RPM (90° Decl.) to — 8 RPM (60° sub-polar Decl.). By means of a remotely-controlled differential synchro, the observer is able to perfect the guiding on a star. A small servo motor and control transformer are the only units of this system mounted on the telescope.
It is now possible to make observations at all declinations in the same time interval and with the same guiding technique.