Sloan Digital Sky Survey Telescope Technical Note 19960805
Jessica Granderson University of Washington
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.
In the first attempts to match the curvature of the plug-plates to that of the best focal surface, the bending fixture was set upon 3 6.630mm metal supports that were affixed to a table top and spaced 120°apart. The sky side of the plate was facing up. The adjustable parallel that served as the central constraint was also affixed to the table top. The results indicated that a central constraint of 27.36 mm (1.077") was slightly too large. See previous report . The difference between the shape of the plate and the desired shape near zero radius indicates that a change of approximately 0.25 mm, or 0.01" would yield a much better fit. (It should be noted that in the previous report, the focal surface of superceeded optical design kent005 was plotted as the desired shape. In this report, the focal surface of the current design, kmg001 is used. However, since the focal surface remained unchanged, the two designs represent identical curves.)
The bending procedure was repeated using a central constraint of 27.04mm (1.0645"). The deflection of the plate used in the initial bending (uw0111) was measured, as well as that of a second plate (ke0111). The results are plotted in Figures 2 and 3, respectively. The indicators used to measure the deflection of the plate were zeroed with respect to the tooling balls that support them, therefore measurements were also taken to determine the deflection of the plate surface along which the tooling balls were positioned (R = 323.85 mm) . These results can be found in Figure 4.
It can be seen from the graphs that by bending the plates and adding a central support, the curvature of the plug-plates can be made to match the surface of best focus quite closely. The two plates tested deformed into two different shapes. Since they were fabricated by two different groups, the 3.175mm aluminum plates differed in thickness by approximately 0.076mm". This difference in thickness is the most likely cause of the difference in deformation of the two plates. The ke0111 plug-plate was the thicker of the two, and correspondingly it did not vary as much in deflection along the inner radius.
As indicated in Figure 4, the deviation of the plate from flatness at R= 323.85mm was approximately 0.127 mm, or 0.005". This variation in surface flatness is certainly large enough to explain the scattering of the data points along a given radius. For example, in both Figures 2 and 3, there is approximately a 0.1 mm scatter of the data points at R=150 mm.
The results of the second set of plate bending trials confirm that forcing the plate into a pre-determined slope along its outer edge, 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 indicate that using a 27.04mm central constraint produces a curvature in the plug-plates that very closely matches the surface of best focus. Variations in the thickness of the plate, such as those that appear when different manufacturers are used, do affect the shape of the plate. The slightly thicker plate, ke0111, matched the desired curve most closely with a standard deviation of 0.025 mm; the thinner plate, uw0111, was fit with a deviation of 0.047 mm. The tolerated deviance of the plates from the target curve is 0.025 mm. An area-weighted standard deviation was also calculated, to account for the radial spread in data points that is obscured in a point by point, unwieghted calculation. The area weighted deviation for plates uw0111 and ke0111 were closer the allowed deviation of 0.025mm, at values of 0.029mm and 0.025mm, respectively.
Date created: 8/06/96 Last modified: 8/14/96 Jessica Granderson
