LOCATING SPECTRA ON THE FOS DK~ICONS AND THE PHOTOMETRIC CONSEQUENCES
OF ERRORS IN POSITION



J. WHEATLEY and R. BOHLIN

SPACE TELESCOPE SCIENCE INSTITUTE
	Instrument Science Report CAL/FOS022

October, 1985


1.	Inroduction

The optimum method of determining the locations and orientations of
FOS spectra on the two-dimensional Digicon photocathodes is
investigated. Tbe Y-center of a spectrum as a function of diode number
deviates from linea.rity by up to 15 microns because of small
distortions in the magnetic focusing fields in the Digicons. In
addition, the spectra a.re rotated by sma.ll angles (typically O.~O5
for the high resolution gratings) with respect to the diode
array. This report presents all spectral position and orientation
measurements since the replacement of the Digicons in the second
quarter of 1984. Average shifts in spectral position are computed
between calibratious at different temperatures a.nd a.t different
Digicon voltages.


The photometrie effect of errors in Y-position could not be determined
directly to better than 5 percent from tbe observed Pt-Cr-Ne emission
line speetra.. However, the light loss caused by an incorrect Y-center
ca.n be modeled using the mea.n. point source eross-sections a.nd the
mean loci of Y-centers. Exa.mples of observed light loss curves
derived from high signal-to-noise external Tung.ten lamp continuum
spectra a.re presented. These results will provide a useful cbeck on
the .ubsequent modeling.