FOS/BLUE Wavelength-Zeropoint Correction Procedure

Summary:
	A significant time-dependent systematic shift of up to 7 quarter-stepped 
pixels (1.75 diodes) in the zero-point of the FOS/BL wavelength scale has been confirmed 
in FOS Instrument Science Report (ISR) CAL/FOS-149 (Rosa, Kerber, and Keyes).  
Relative wavelength offsets within individual spectra are not affected by this shift.  
We provide an IRAF/STSDAS cl script (foswcorr.cl) and descriptive help file (foswcorrhlp.txt) 
for use as a quick-fix to correct the wavelength files (.c0* files) for any FOS/BL 
observation.  Figures 1 and 2 (PDF - 14 Kbytes) illustrate the magnitude of the 
observed shifts and the effectiveness of the quick-fix algorithm provided here.  
Wavelength zero-point offsets caused by filter-grating wheel positioning uncertainties 
are not removed by this correction algorithm.  Currently corrections are not available 
for FOS/BL PRISM nor for any FOS/RD observation.

 Obtain the cl script:  foswcorr.cl (ASCII-text - 9 Kbytes). 
 Obtain the help file for the cl script:  foswcorrhlp.txt (ASCII-text - 2 Kbytes). 
 Obtain the FOS ISR:  CAL/FOS-149 (PDF - 53 Kbytes). 

Discussion:
	FOS/BL wavelength zero-points shifted in a more-or-less monotonic
fashion from launch (April 1990) through de-commissioning (February 1997).  An
approximate 7-pixel (1.75 diode) shift occurred over this time period.  At any
given epoch the peak-to-peak scatter about the mean trend is +/- 1-pixel
(i.e., 10 times the measurement error).  For high-dispersion gratings, 1
quarter-stepped FOS pixel corresponds to approximately 60 km/sec.
	FOS/RD wavelength zero-points over the same time period present an
apparently random distribution with a peak-to-peak range of 7 quarter-stepped
pixels.  Therefore, changes in FOS/RD wavelength zero-points are NOT confirmed
at the present time.  Work continues to determine whether improved
characterization of the geomagnetic field and the onboard GIM correction can
remove any of this measurement scatter and thereby facilitate a possible
correction algorithm for FOS/RD.
	Please note that no correction algorithm exists for FOS/BL PRISM, due
to the non-linearity of its wavelength scale, nor for FOS/RD observations.

	It is expected that the correction algorithm contained in the cl script
presented here will eventually be incorporated in standard calfos 
processing.  The availability of an updated calfos will be announced both 
on the FOS WWW page and in the Spectrographs STAN when available.  For the 
present the cl script presented here is the ONLY procedure provided by STScI 
for the correction of FOS/BL wavelengths.  
	Please refer to FOS ISR 149 for a thorough discussion of the correction 
algorithm and other characteristics of the observed zero-point shifts.  

	Any additional correction algorithms, improvements to the FOS/BL 
quick-fix procedure, or assessments of the impact of the observed shifts on 
other instrumental calibrations will be announced on the FOS WWW page.


30 September 1998 - version 1