FOS APERTURE REPEATABILITY AND FILTER-GRATING WHEEL REPEATABILTY 
(CALJBRATION PLAN 1OC AND 10D) 


J. WHEATLEY, H. FORD AND R. BOHLIN
SPACE TELESCOPE SCIENCE INSTITUTE


Instrument Science Report CAL/FOS-008 

MAY, 1984

ABSTRACT


Repeatability of the FOS aperture wheel was measured in the ambient 
calibration of March, 1983. Cross-correlation of emission line spectra 
indicates that the tangential motion of the aperture wheel repeated to 
better than 0.6um in the focal plane except for a shift of 2.3 ± 0.3um 
in the wheel position of aperture C2 when tbe wheel was cycled in
the following order: C2, B2, A4, B2, C2.

Repeatability of the grating wheel was measured by locating the position 
of the 0.1 arcsec aperture in an aperture map after the grating wheel had 
been moved to various positions. Shifts of up to 10 m in botb X and Y 
detector coordinates were detected. To check for repeatability over longer 
time intervals, additional measurements were derived from otber ambient 
calibrations. A shift of -0.45 ± .02 diode (23 ± 1um) in X on the 
detector occurred between two H57 A4 spectra made 7hrs apart. Furthermore, 
aperture maps made on different dates are shifted up to 0.28 diode (14um) 
in X. The shifts in spectral position that are associated witb rotation of 
the filter grating wheel are pushing the limits of accceptability for good 
photometric reproducability. The detent mechanism of the filter grating 
wheel and the seating of the camera mirror should be checked.