# 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.,

`S`_{1,avg}(Lambda ± 10)
= `S`_{2,avg}(Lambda ± 10)/`a`
. |
(1) |

The ratio of the counting rate at the bottom of the line to the average
continuum level, `S`_{1,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

`N`_{min}/`S`_{1,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