SCATTERED LIGHT CHARACTERISTICS OF THE
HUBBLE SPACE TELESCOPE FAINT OBJECT SPECTROGRAPH
(SV #1319)

F. Bartko, Bartko Science & Technology
G. Burks, University of Colorado, CASA
G. Kriss and A. Davidsen, Johns Hopkins Univer3ity
R. Cohen, V. Junkkarinen, R. Lyons,
Univer~ity of California, San Diego, CASS

Instrument Science Report CAL/FOS-073
April, 1992

Abstract

Observations of a standard star (BD+75D325) have been used to measure
tbe Hubble Space Telescope (HST)/ Faint Object Spectrograph (FOS)
scattering characteristics in the wavelength rauge 115 to 250
nin. Spectra of the standard star were obtained as the star was
progressively offset from the optical line of sight axis of the
telescope, both in the in-dispersion aud cross-dispersion directions.

These data have been reduced and analyzed to deterrnine the scattering
function of the telescope-spectrograph combination. Two primary
results have been obtained. (1) The resulting scattering function
exhibits three characteristjcs: (a) The inner core (theta < 4") is
domiuated by the large PSF of the HST; (b) the outer wings of the
scattering function (4" < theta < 32") show a theta dependence consistent
with predictions tor the liST Airy disc; and (c) The wavelength
dependence of this scattering function follows approximately 1/lambda,
suggesting that the UV rnicro roughness contribution to the scatter is
quite srnall, and hence the liST prinrnry rrlirror is very smooth at
ultraviolet wavelengths. (2) The FOS scattering contribution is
limited only by grating scatter, amt is consistent with pre-launch
grating calibration measurements.


As a result, both the scatter contribution from the HST mirror surface
micro roughness aiid tue FOS gratings appear to be within their design
specifications. We also describe the implications of these results tor
EOS on achieving its limiting performance for c.ertaiu typical
observations.