Princeton University Observatory

Princeton, New Jersey

Scattered Light in U1

W. Cochran

March 2, 1973

Saturated lines of several stars were examined to determine the amount of scattered light in the U1 spectra. Lines were selected in which the stray light was fully occulted. It was then assumed that any residual light in the bottom of saturated lines was purely a result of scattered light.

The counting rates in the bottoms of heavily saturated lines were measured. Following York*, the continuum level, averaged over 10 Å, was measured on U2 and then divided by the quantity a (see Fig. 9 from York) to correct for the difference in counting rate between U1 and U2; i.e.,

S1,avg(Lambda 10) = S2,avg(Lambda 10)/a . (1)

The ratio of the counting rate at the bottom of the line to the average continuum level, S1,avg(Lambda 10), plotted as a function of wavelength, is shown in Figure 1. The data shows a linear relationship, which can be described by

Nmin/S1,avg(Lambda 10) = 0.047 + 0.000185(Lambda - 1000) , (2)

where Lambda is measured in angstrom units.

The ratio of the counting rate at the line bottom to the local continuum as measured on each U1 scan was also examined, but these data showed considerable scatter. Thus the above formula gives the best available representative for the scattered light in the U1 spectra. The average deviation of the plotted points from the mean line is 0.0059 of the computed U1 continuum.

* D. G. York, Stray Light in PEP, December 8, 1972

cc: Copernicus Astronomers