SPACE TELESCOPE SCIENGE INSTITUTE
        INSTRUMENT SCIENCE REPORT CAL/F0S-012
	
	

TITLE: FOS Filter-Grating Wheel Repeatability (FOS CAL. Plan 10D)

AUTHOR: G. Hartig, R. Bohlin, H. Ford and R. Harms	
DATE: Dec. 1984


ABSTRACT:


Recent measurements of the FOS filter grating wheel (FGW) repeatability 
indicate that this mechanism is not operating in a completely satisfactory 
manner. Implications of the non-repeatability, most importantly with 
regard to target acquisitlon are discussed. The measurements imply that 
the cause of non-repeatability is some combination of wheel wobble about 
its central bearings, distortion of the wheel, and imperfect detenting (in 
order of problem importance). Evidence is presented that the repeatability 
for at least some of the FGW elements is dependent on operating temperature; 
measurements made at ambient will underestimate the severlty of the problem. 
There is little indication that the performance has deteriorated with FGW 
use; the major part of the problem may be inherent in the fundamental 
limitations of the design. Refurbishmerit of a motor that had a damaged 
pinion does not appear to have improved the performance. Some possible 
work-arounds are proffered, in the event that the mechanism cannot be 
improved prior to launch.