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Automated ground-based star-pointing UV–visible
spectrometer for stratospheric measurements
Howard K. Roscoe, William H. Taylor, Jon D. Evans, Andy M. Tait, Ray Freshwater,
Debbie Fish, E. Kimberly Strong, and Rod L. Jones
A novel automated ground-based star-pointing spectrometer system has been constructed for long-term
deployment in Antarctica. Similar to our earlier stellar system, a two-dimensional detector array
measures the spectra of the star and the adjacent sky, so that auroral emission from the sky can be
subtracted from the stellar signal. Some new features are an altitude–azimuth pointing mirror, so that
the spectrometer does not move; slip rings to provide its power thereby avoiding flexing of cables and
restriction of all-around viewing; and a glazed enclosure around the mirror to ensure protection from rain
and snow, made from flat plates to avoid changing the focal length of the telescope. The optical system
can also view sunlight scattered from the zenith sky. The system automatically points and tracks
selected stars and switches to other views on command. The system is now installed at Halley in
Antarctica, and some preliminary measurements of ozone from Antarctica are shown. © 1997 Optical
Society of America
1. Introduction
We have developed a new UV–visible instrument
~UVIZ!for measuring atmospheric constituents from
spectra of stars at Halley ~formerly station Z!in Ant-
arctica. Similar to our earlier instrument,
1
it has
the potential of measuring O
3
,NO
2
,NO
3
, and OClO
simultaneously. It significantly extends the range
of sources and elevation angles at night, providing
increased measurement opportunities, because it has
sufficient sensitivity to observe stars and planets.
This is particularly useful for measurements near the
poles in winter when it is dark, when conditions that
lead to possible ozone depletion occur. At other sea-
sons it also has potential for measurements of OClO,
because OClO amounts increase significantly during
the night, and of NO
3
, because NO
3
exists only at
night.
Similar to our earlier instrument, ozone is mea-
sured at visible wavelengths between 450 and 565
nm, where depths of absorption are less than in the
UV but signal levels are greater and less light is
scattered out of the beam when the star is close to the
horizon. This is especially important at Antarctic
latitudes, where Sirius, the brightest star in the sky,
is too close to the horizon for UV measurements with
a good signal-to-noise ratio. Spectra of a star are
acquired at low- and high-elevation angles; the ratio
of these spectra is independent of absorption in the
atmosphere of the star.
Unlike our earlier instrument, the UVIZ can also
observe the zenith sky, which allows the full capabil-
ity of the Systeme Automatique d’Observations Ze´-
nithales ~SAOZ!spectrometers
2
but with improved
signal-to-noise ratio because the detector has less
dark current. Because zenith-sky measurements of
ozone in the visible can be made at solar zenith angles
as large as 91°, these observations can be made for
much more of the year in Antarctica than are possible
with the Dobson spectrophotometer that observes at
UV wavelengths.
3
Many details of the spectrometer and detector are
unchanged ~e.g., entrance slit of 2.4 nm FWHM, a
200-groovesymm grating, 0.55-nm pixel width!, but
other new features include an adjustable focus, a fil-
ter wheel, and a tungsten lamp. The UVIZ is also
fully automated and is enclosed in an environmental
housing so that it can operate outdoors in extreme
conditions. It is now installed at Halley ~75.58 °S,
H. K. Roscoe, W. H. Taylor, J. D. Evans, and A. M. Tait are with
the British Antarctic Survey, Natural Environment Research
Council, Madingley Road, Cambridge CB3 0ET, United Kingdom.
R. A. Freshwater, D. J. Fish, E. K. Strong, and R. L. Jones are with
the Cambridge Centre for Atmospheric Science, Department of
Chemistry, Cambridge University, Lensfield Road, Cambridge
CB2 1EW, United Kingdom.
Received 20 March 1996; revised manuscript received 10 Octo-
ber 1996.
0003-6935y97y246069-07$10.00y0
© 1997 Optical Society of America
20 August 1997 yVol. 36, No. 24 yAPPLIED OPTICS 6069