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When the instrument is aligned to view sunlight, an image of the sun is focused at the centre of the
iris. With the iris closed, about three solar diameters of skylight around the sun pass through the
iris aperture into the spectrometer. With the iris open, about 10° of skylight enters the
spectrometer.
On the spectrometer side of the iris there is another plano-convex lens [LE 3]. This lens is
positioned such that its focal point is in the plane of the iris. Light passing through the iris aperture
is therefore collimated along the optical axis.
Lenses [LE 2, LE 3] in the iris-diaphragm assembly are mounted with their plane side facing the iris.
Filterwheels
Filterwheels [FW 1, FW 2] are mounted on bronze bearings in a filter-wheel housing. Each wheel
has six 25.4mm diameter holes spaced at 60 degree intervals. Each hole can be selected to
intersect the optical axis by rotating its filterwheel. The filterwheels are rotated by two
microprocessor controlled motors.
FW #1 contains one open hole ( #3 ), two film polarizers ( #0 and #5 ), a ground-quartz disk ( #1 ),
an opaque blank ( #2 ) and a second ground quartz disk + a neutral density filter ( #4 ). The ground-
quartz disk is used as a diffuser for direct-sun and lamp measurements. The opaque disk is used
for dark count tests. The film polarizer ( #0 ) is used for all zenith sky and Umkehr measurements.
This film polarizer is mounted such that its axis is perpedicular to the entrance slit. The second film
polarizer ( #5 ) is mounted such that its axis is parallel to the entrance slit. It is presently used only
for specialized research purposes. The open position (#3) is used for moon and UV observations.
The second ground quartz / N.D. filter is used in the NO
mode of operation..
2
FW #2 contains an open hole ( #0 ) and five neutral-density filters providing ND=0.0, 0.5, 1.0, 1.5,
2.0 and 2.5 attenuation ( #0 through # 5 ). This filterwheel is used to automatically adjust the light
level entering the spectrometer.
[AP 1], an 11.18mm fixed aperture located on the spectrometer side of the filterwheel housing limits
the field-of-view of the spectrometer to f/6.
Both Filterwheels have black markings that are visible through the viewing window. The
number of markings indicate the position of the filter wheel (positions 0 through 5).
Foreoptic Viewing Ports
There are two viewing ports along the foreoptic assembly, through which may be viewed the iris
diaphragm and any images which are focused onto it, and the entrance slit.
At the bottom of each viewing port is a glass right-angle prism [VP 1, VP 2] mounted inside the
foreoptic tubing. The prisms are aligned so that the object to be viewed (iris or entrance slit)
appears centred in the viewing port. A plano-convex glass lens [VL 1, VL 2] is placed in each
viewing tube to magnify the image.
Rear Fore-Optics.
A plano convex lens [LE-4], whose plane suface faces the entrance slit of the spectrometers
focuses the collimated ultraviolet radiation onto the entrance slit of the spectrometer.
SPECTROMETER
The purpose of the spectrometer optical subassembly is to accept light through the entrance slit
and disperse it into a high-quality spectrum along the exit-slit focal plane. The spectrometer is a
modified Ebert type with focal length 16 cm, and aperture ratio f/6.
Six exit slits are positioned along the exit focal plane at the Ozone (or NO2) operating wavelengths -
303.2 nm (302.1 for mercury-wavelength calibration), 306.3/431.4 nm, 310.1/437.3 nm, 313.5/442.8
nm, 316.8/448.1 nm and 320.1/453.2 nm with 0.6/0.9 nm resolution. Wavelength is adjusted by
rotating the grating with a stepper motor which drives a micrometer acting on a lever arm. The
wavelength-calibration procedure is capable of measuring the wavelength setting with a precision of
0.0001 nm, and of controlling the wavelength setting to 0.006 nm.(in the UV region.
16
MKIV OPERATOR'S MANUAL
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