Flatness Intercomparison Measurements Made on an Optical Flat

Abstract

A 4 inch (100 mm) diameter 0.75 inch (19 mm) thick fused silica optical flat is being used as the master to calibrate the 50 mm long slideways of the China Lake Nanostep Surface Profiler. The average of profiles taken along the 50 mm center sections of two mutually perpendicular diameters has been used for the calibration of the slideways. It is hoped that the Nanostep instrument will be capable of measuring absolute flatness. Until recently, the flat had been defined as being "perfectly flat" in order to obtain a slideways correction. Now actual measurements of the flatness have been undertaken at three laboratories, all using interferometric measurements but of different types. The persons who have taken the measurements and their laboratories are the first three groups of authors listed above. At the Optics and Applied Technology Laboratory, the optical flat was mounted vertically in a nonrestricting ring mount and comparisons were made with the flat being made part of a cavity in a Fizeau-type interferometer and the same cavity but without the optical flat. At Brookhaven National Laboratory, the flat was placed on its back and measurements were made with a pencil beam interferometer1,2 which measures surface slopes and then integrates them to obtain a surface profile. At the National Institute of Standards and Technology, a classical three flat interferometric intercomparison was made with the flat mounted vertically in a V-block. One profile from these last measurements is shown in Fig. 1 for the flatness along the full 100 mm diameter of one of the marked diameters. The measurements are an average of six independent determinations with a standard deviation of 0.52 nm. The dashed line is a polynomial fit through the data. Note that, according to these measurements, the flatness over the central 50 mm is very good indeed, with a peak-to-valley deviation of less than 2 nm.