(Feb 1997 - updated 30 Jan 98)

	We wish to remind observers that FOS instrumental sensitivity is a very
steep function of wavelength shortward of 1200 Anstroms.  Although instrumental
sensitivity is determined to 3% accuracy in this region, this steep sensitivity
gradient requires accurate knowledge of observed wavelengths to insure accurate 
flux calibration.  Common instrumental uncertainties can produce important 
systematic errors in pipeline processing.

	Both telescope pointing uncertainties and FOS filter-grating wheel (FGW)
position uncertainties can affect observed FOS wavelengths.  An error of
0.5 Angstroms or 2 pixels, produces little effect longward of 1200 A., but
yields systematic photometric errors of 2% at 1180 A. and 4% at 1150 A. Larger
miscenterings will yield proportionately larger systematic errors in this
region. Target centering inaccuracies of only 0.15 arcsec or filter-grating
wheel misalignment of 0.5 diodes will yield such 2 pixel offsets.  Note that 
ACQ/BINARY alone is not sufficient to avoid pointing errors of this magnitude. 
The target acquisition strategies recommended in the FOS Handbook version 6 
for programs requiring accurate photometry and/or wavelengths yield pointing 
accuracies of 0.04 arcsec (~0.5 pixels).  Three-sigma FGW uncertainties are of 
the order of 1.2 pixels.  

	Observers with a need for accurate absolute or relative flux
calibration in the 1150-1200 A. region should examine fiducials in the G130H
spectral range, such as Lyman alpha absorption, to determine the degree of
miscentering along dispersion.  Since the pixel-positions of flat field
features are not affected by target centering, any re-calibration should 
proceed normally through flat field correction (.c5 output products), next
the spectrum should be shifted by the appropriate amount, and then flux
calibrated.