The performance of a capacitance-wire wave recorder
A capacitanoe-wire wave recorder circuit was used in conjunction with a p.t»f,e. wire probe to establish what errors are associated with using such a system for the measurement of small waves. In the experiments the probe was moved relative to the water, rather than vico versa and both photographic and electrical methods were employed to investigate the errors introduced by the meniscus, the wetting of the wire and the effects of wire thickness on the wave shapes. The electrical methods involved accurate recording of the probe movement by means of a linear potentiometer and a comparison of this movement with the wave form produced from the circuit measuring the chasige in capacitance of the probe caused by its movement in the water. The effects of increasing the operation! frequency of the recording circuit was also investigated.
‘ A water wave recording instrument for use in hydraulic models
Abstrsset. Consideration is given to the requirements of a water waveform recording instrument and to the details of an instrument designed to meet thtie requirements. The instrument has a linear calibration, and tests show that over a wide range of conditions its accuracy is to better than 2 ~. A capacitance type sensing element is used, together with transistor circuits and a pen recorder. The device is suitable for recording water waves in hydraulic models.
Active profiling and polishing for efficient control of material removal from large precision surfaces with moderate asphericity
We present the development of an active fabrication technology for controlling material removal on large precision surfaces of moderate departure from a sphere. The underlying philosophy was established as an efficient solution to the challenging problem of fabricating secondary mirrors of up to nominally 2.5 m in diameter for modern 8 m class telescopes and beyond. The facility described comprises a CNC profiler, two contact profilometers, and a full size active polisher. The trial work-piece was a convex surface of 830 mm in diameter on a zero-expansion ceramic (`Cervit') blank – a 1/3 scale hyperbolic mirror for a proposed 2.5 m diameter f/7 secondary mirror for the 8 m Gemini telescope. Using software error-correction from profilometric metrology data, a factor of 2 improvement in generating the convex aspheric profile was achieved. An active loose-abrasive polishing process is also described, in which a full-size tool is configured to deliver variable, edgeless, sub-diameter removal-footprints. Real-time monitoring of process variables is described, and approximately 10% convergence in each polishing pass is reported.