SPACE TELESCOPE SCIENCE INSTITUTE

INSTRUMENT SCIENCE REPORT CAL/FOS-026

FOS WAVELENGTH CALIBRATION
(Laboratory Calibration Plan 13B)

M.Sirk
R. Bohlin


February 1986


Abastract

A database of 157 Pt-Cr-Ne emission lines are used to perform wavelength
calibrations for all FOS dispersers through the lower aperture of the 0.1 
arcsec pair. Typical RMS errors of the fit are 0.03, 0.08, and 0.13 diode 
(pixels) for the high dispersion gratings, low dispersion gratings, and the 
prism, respectively. Accuracies of wavelength determination are most likely 
limited by non uniformities in the diode spacing at the 0.02 diode (1 micron) 
level, which corresponds to a velocity accuracy of 5 km/sec. Thw existing 
calibration spectra do not show enough lines in thw far UV to constrain 
calibtation fits from 1250A for H13, and 1500A for L15, down to the MgF2 
cutoff at 1150A. Estimated uncertainties at 1150A are 0.28A (73 km s-1) for 
H13 and 5A (1300 km s-1) for L15, while the corresponding uncertainties at 
1216A are o.1A (25 km s-1) and 3.3A (814 km s-1). Thermal effects introduce 
a differential shift in line positions of up to 0.3 diodes.

Since these data were obtained and analyzed, the red side digicon assembly 
was replaced in December 1985. Preliminary rusults indicate an increase in 
dispersion of the new red Digicon of about 2%. More definitive wavelength 
calibrations for the red side are to be obtained during the thermal vacuum 
tests at die Lockheed integration site.

The final section outlines the revisions that are needed for the overall 
wavelength calibration strategy.