Alignment of Schlieren and Rayleigh opitcal systems in the ultracentrifuge. 3. Design, construction, and placement of Rayleigh mask

Problems in the optimal design on the double-slit mask used in the adaptation of the Rayleigh interferometer ultracentrifuge are discussed. Consideration is given to the orientation of the mask either symmetrically about a radius or offset with one slit along a radius from the center of rotation, to the placement of the mask above or below the rotor, to the use of a second pair of slits in the cell to mask imperfections in the cell windows, and to certain aberrations arising from variations in the refractive index across the width of the slit and from deviation of light perpendicular to the slits by a component of the radial refractive index gradients. Optimal mask dimensions for different experimental conditions for both symmetrical and offset masks are given. Equations based on the geometry of the cells and mask are derived, permitting calculations sufficient to align correctly a symmetrical mask with minimal labor.