Design and construction of instrument rotator for the Sloan Digital Sky Survey (SDSS) telescope

Abstract

This paper will describe the concerns, parameters and restrictions in the design and construction of the instrument rotator used on the SDSS telescope. The rotator provides support for two 600 Lb. Spectrographs, through all axes motion, without causing harmful radial moments to be translated to its inner ring which supports the mosaic imaging camera. This is accomplished using an outer-inner ring design. The outer ring is a thin-walled box structure incorporating the drive surface and is attached to the inner ring through a steel membrane. This rotator design requires the telescope’s primary support structure to provide final structural integrity. Due to this feature, a special fixture was needed to transport the rotator from the vendor and to install it onto the telescope. Positional accuracy and feedback is provided by an optical tape and read-head system manufactured by Heidenhain and attached to the inner ring. The drive motor was designed to use the same motor as those employed for the other two telescope axes, thus minimizing the spare-parts inventory and maintenance. Its drive pinion is of a pinch design, with the pinion axis parallel to rotator radius. A great deal of attention and planning was required in the construction of the box frame outer ring and the induction heat-treating of the drive surface. Drive surface tolerances were maintained within +/-0.001 inches, and internal stress cracks from heat-treating were minimal.