FOS WAVELENGTH SCALE
BELOW THE CALIBRATION LAMP CUTOFF AT 1239k
(Lab oratory Calibration Plan 1 SB)

M. SIRK and R. BOHLIN
SPACE TELESCOPE SCIENCE INSTITUTE

Instrument Science Report CAL/FOS 038
October 1986


Ab8tract

Since the FOS internal PtCr--Ne calibration lamps ha.ve no useful
emission lines below 1239k, the standard wavelength calibration at the
shortest wavelengths around 1150k is uncertain by about 10 times the
typical error at longer wa.velengths. A new technique is developed to
extend the calibrations down to the blue Digicon cutoff at 1150k for
gratings G13OH and G16OL. Estimated uncertainties at 1150k a.re 0.14k
(37 km ~1) for G130H and i.ok (260 km s1) for G160L, while the
corresponding uncertainties at La (1216k) are 0.068k (17 km ~1) and
0.8k (200 km ~1), respectively. This is an improvement of up to a __
factor of 2 for G13OH and up to a factor of 5 for G16OL. However, the
above errors at iisok are still worse tban the typical RMS error found
a.t longer wavelengths by about a factor of 5. At La the uncertainty
is only about 3 times the RMS sca.tter at tbe longer wavelengths.


Features on the G18OL calibration spectrum have been assigned new
vacuum wavelengths by determining line centers of features on higb
resolution FOS spectra that are smoothed to tbe G16OL resolution.