FOS WAVELENGTH CALIBRATION EXPOSURE TJIMES
(Laboratory Calibration Plan 1SG)

R. BOHLIN and M. SJIRK
SPACE TEL ESCOPE SCIENCE INSTITUTE

Instrument Science Report CAL/FOS028
April 1986



The FOS wavelength calibra.tion .trategy is based on refereuce
template spectra for the smallest apertures (Sirk and Bohlin 1986a and
1986b). The most accurate wa.velength scales for the larger apertures
are obta.ined by cross-correlating the internal Pt-Ne-Cr lamp spectra
from the larger a.perture with the sma.Il aperture template spectrum
after the later is broadened by the appropriate aperture width.


The recoramended exposure times for the sma.ll aperture templa.te
spectra. are in Table 1. These spectra need to be obtained
infrequently throughout the mission. The exposure times a.re in
seconds and a.re the sum of the time for ea.cb quarter step of the
standard Operating mode. To do both apertures of a pair, double the
listed time. The exposure durations are ba.sed on the requirement to
obtain about 30000 counts in the peaks of the brightest lines. This
criterion provides sufficient number of lines above the faint line
threshold of 300 total counts to ensure accurate wavelengtb
calibrations. In the case of tbe 0.1-Pair of apertures on the red
side, the lower aperture is 1.7 times larger tban the upper aperture,
so that peak counts may exceed 50,000 in the lower 0.1 aperture.