As part of a project to research material processing in space, a Ronchi instrument will fly on the space shuttle Columbia in September 1995. The instrument is one element of the Surface Tension Driven Convection Experiment (STDCE) to study thermocapillary flow in fluids under the influence of microgravity and localized heating. Quantitative data on surface deformations of fluids will be gathered by the Ronchi instrument, designed and built by H. Philip Stahl under contract for the NASA Lewis Research Center (Cleveland, OH).
In the Ronchi test, a collimated beam reflects off the sample surface and is focused through a grating en route to the detector. Distortions in the reflected wavefront caused by surface deformations cause variations in the shadow pattern produced by the grating. The detector records a Ronchigram in which the shadow pattern is superimposed on the image of the sample surface. Pattern variations can then be correlated with surface distortion.
System design
The experiment involves the study of oil in chambers 12, 20, and 30 mm in diameter. A carbon dioxide laser will heat the oil, and subsequent fluid flow and surface distortion will be characterized. Because it is intended for use in space by nonspecialists, the Ronchi instrument is modular. It includes illumination, projection/imaging, polarization, measurement, and camera modules. Components were chosen for ease of use, though packaging and robustness were also concerns.
The illumination module consists of a diode laser with a Galilean beam-expanding telescope. The diode-laser output is expanded 30 times, then apertured down to produce a 22-mm-diameter beam at the entrance pupil of the optical system.
The projection/imaging module adjusts beam size to uniformly illuminate the three chambers, using two afocal lens pairs consisting of stock f/2.25 achromatic doublets 40 mm in diameter, with 90-mm focal lengths. The outer lens pair simply focuses the beam on the inner pair. This second pair forms a Galilean telescope that can be flipped to demagnify or magnify the beam, resizing the 22-mm beam for the 12-mm or the 30-mm chambers. For the 20-mm chamber, the flip lens is simply removed from the system. The chambers themselves are manually exchanged by the flight crew.
The polarization module eliminates ghost reflections from the projection/ imaging module while optimizing source throughput from the low-reflectivity sample surfaces. The system consists of a pair of polarizers and a quarter-wave plate. Ghost reflections are vertically polarized by the first polarizer, while the quarter-wave plate rotates the polarization plane of the oil reflection 90° to the horizontal. The second polarizer blocks the vertically polarized ghost reflections and passes the oil reflections. The polarization assembly is followed by a pupil relay module that relays the beam to allow insertion of the gratings.
Data collection
The measurement module contains the grating whose shadow is projected to form the Ronchigram. Local surface slope on the sample is proportional to the distance between adjacent shadow lines on the Ronchigram. Known as equivalent wavelength, this slope is defined as
leq = d/2(f/#)
where d is the grating spacing. For Ronchi test systems using amplitude sine gratings, sample surface-relief sensitivity is directly proportional to equivalent wavelength, which is in turn proportional to the grating spacing. To vary sensitivity, the mission specialists will simply switch gratings.
Sensitivity is independent of grating position per se, but the grating must be positioned in the beam path to ensure adequate spatial sampling across the beam. Ten separate slots are provided for this purpose. The experimental apparatus will include vertical, horizontal, crossed, radial, and circular zone gratings to characterize full surface deformation.
The camera module images the Ronchigram onto a diffuser plate, forming a real image that is captured by a 0.5-in.-format RS-170 video camera. Data analysis will take place after the mission and is expected to shed light on fluid-flow mechanisms. Says STDCE project scientist Alex Pline, "With this data, we`ll put to rest the controversy concerning the role of free surface deformation in oscillatory thermocapillary flow. An understanding of the physical mechanism of this flow will lead to a greater understanding of other space-based applications."
Kristin Lewotsky | Associate Editor (1994-1997)
Kristin Lewotsky was an associate editor for Laser Focus World from December 1994 through November 1997.