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16-10-200716-10-2007
Filter Wheel #3 Assembly (FW#3)
Light emerging from the exit slits passes through Filter Wheel #3 which contains Spectral "Order"
filters. When the Brewer is in the Ozone Mode, a UG-11/NiSO4 filter combination prevents
radiation above 325nm from passing. In the NO2 mode of operation, a BG-12 filter blocks UV
radiation.
During a UV measurement FW#3 switches from the a UG-11/NiSO4 filter combination to a UG-11
filter at 325nm.
The Filter Wheel is driven by a stepper motor when appropriate commands are received from the
Brewer software.
Photomultiplier Detector (PMT)
Light passing through the exit slits is collected on the cathode of a low-noise EMI 9789QB05 PMT
detector [PM 1]. The photon pulses are amplified, discriminated, and divided by 4, before being
transmitted to a counter. The resulting photon count is registered in one of six wavelength channels.
Radiation through the exit slits is focused onto the cathode of the PMT by a 38.1 mm focal-length
quartz Fabry lens [LE 6].
The cathode of the PMT is located at the ultraviolet focus of the Fabry lens. The spacing between
the lens and the PMT takes into account the apparent change in focal length due to the presence of
the filters. The PMT is enclosed in a magnetic shield which is maintained at cathode potential
(approximately -1300 V) to minimize dark noise. The PMT and its shield are spring-loaded against
a teflon ring which locates the cathode at the center-of-focus of the Fabry lens and also insulates
the magnetic shield from the PMT housing.
The housing has been designed to allow access to the photon amplifying and discriminating
circuitry without disturbing the optics.
Electronics
The Brewer Spectrophotometer contains its own microprocessor, an 8051 based microprocessor,
which controls all internal instrument operations. Specifically, the microprocessor is responsible for:
• positioning the diffraction grating - the microprocessor controls a stepper motor which
drive a micrometer which is connected by a lever arms to a rotating grating.
• positioning the slit mask to allow light to pass through one of six exit-plane slits to the
photomultiplier: this mask is driven by a second microprocessor-controlled stepper
motor.
• synchronizing the photon-count circuitry with the movement of the slit mask.
• turning the halogen and mercury calibration lamps on and off.
• reading optical position sensors.
• reading analog monitor voltages.
• moving motors to track the sun.
• moving neutral density, diffusing, and polarizing filters into the optical path.
• moving "order" filters into and out of the optical path.
• opening and closing a field-of-view defining iris.
• providing a RS-422C communications link to an external computer.
The microprocessor is programmed to accept commands from the external Computer, execute the
commands, and return results to the Computer. An IBM compatible computer is used as the control
console to facilitate programmed command sequencing as well as automatic data logging and
processing. Raw data is recorded on hard disk drive, and real-time results may be printed on hard
copy or printed to disk for later printing.
MKIV OPERATOR'S MANUAL
2. SYSTEM DESCRIPTION
19
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