Sloan Digital Sky Survey Telescope Technical Note 19960708
Jessica Granderson
The plug-plates of SDSS project are responsible for locating the optical-fiber plugs spatially and for defining the plug tilt with respect to the surface of best focus. The plates are 795 mm (31.3") in diameter and 3.2 mm (0.125") thick. Approximately 670 holes are drilled in each plate. For drilling, the plate is held by a drilling fixture that deforms it elastically so that its upper surface is convex. The hole axes are drilled parallel. In the telescope, the plate is deformed to match the surface of best focus. When this is done, the hole axes are aligned with the principal rays from the optics.
The bending fixture consists of two rings with outer diameters approximately equal to that of the plate. In order to deform the plate, the inner surfaces of the rings between which the plate is clamped are slightly angled, rather than planar. See Figures 1, 2 and 3.
A finite element analysis was performed to compare the curvature of the plate to the slope of the focal surface, given the constraints imposed on the plate by the bending fixture. See Figure 4. A further displacement constraint can be added at the center of the plate to force the plate sag to match that of the best focal surface, once the the plate has been bent to the desired curvature.
To verify the design, a plate was inserted into a prototype and the subsequent deformation of the plate was compared to the desired shape. See Figure 5. For each of four angles, the displacement of the plate with respect to its outermost exposed edge was measured at five points along the radius. In order to match the sag of the plate to that of the focal surface, gauge blocks were inserted between the surface supporting the clamping device, and the center of the underside of the plate. The shape of the plate after two trials using different gauge block sizes can be seen in Figures 6 and 7 .
It can be seen from the graphs that after bending the plate and adding a central suport, the curvature of the plate was quite near that of the desired Kent005 curve. Close examination of the Kent005 curve and the plate curvature toward the outermost data points reveals that with no central support, the slope of the plate is slightly greater than that of the Kent005 curve. Upon adding a central support, however, the slope of the plate decreases to a value slightly less than that of the Kent005 curve.
From the results of the initial plate bending trials it appears that forcing the plate into a pre-determined slope along the outer edge of the plate, and applying an additional displacement constraint at the center of the plate, is a practical method by which to deform the plug-plates. The experimental data suggest that decreasing the size of the gauge block used as a central constraint would significantly improve the fit of the plug-plate to the shape of the Kent005 curve. Given that the slope of the unsupported plate is greater than desired, while the slope of the supported plat is less than desired, alteration of the extent to which the plate sag is adjusted after bending, is expected to produce a notable improvement in the shape of the plug-plates.
Date created: 7/08/96 Last modified: 7/19/96 Jessica Granderson
