Data Set Overview
=================
The OPE photopolarimeter was designed to measure
the polarized components of the light in seven
bandpasses or channels, ranging from the near
ultraviolet to the near infrared. Three channels
(so- called blue, green and red) were devoted to
the observation of the scattering of solar light by
cometary dust grains, in emission-free countinuum
bands. Four other channels (so called OH, CN, CO+,
C2) were devoted to the observation of light
emitted by cometary gases.
The imaging of a mosaic of interference filters
(placed in front of the objective lens and
organized into a mosaic to compensate for the
chromatic effects) onto a microchannel plate
allowed the spectral discrimination to be achieved.
A polaroid analyzer, placed in front of each
filter, except the CO+ one, allowed the
polarization to be determined by the rotation of
the analyzer with the spin of the spacecraft.
During a spin period (of approximately 4 seconds),
eight consecutive measurements of the polarized
intensity, Ii, Ii+1, Ii+2, etc, (of approximately
half a second each), were performed in the seven
channels at eight so-called clock angles.
Absolute calibration
Due to the reduced amount of time available for
calibrations after the flight detector was
available, the absolute-radiance sensitivity of the
instrument had been inferred by considering the
individual spectral transmission of each optical
component, the field of view of the instrument, the
photocathode spectral sensitivity and the total
responsivity of the microchannel plate
photomultiplier (Giovane, 1991). The results were
the following:
Channel Wavelength Bandpass Responsivity
nm 50% peak,nm 10-7 Wm-2sr-1mm-1ct-1
Blue 442 5 2.09
Green 576 10 0.88
Red 718 4 4.40
OH 310 6 16.81
CN 387 4 8.20
CO+ 424 4 1.34
C2 514 4 3.75
The on-board calibration tritium-phosphor source
(mounted on the back of the baffle cover), which
allowed the instrument to be tested during the
pre-Halley-encounter cruise, was released with the
baffle cover prior to Halley encounter.
The data for Optical Probe Experiment was submitted
by Andrew Weisenberger for the Frank Giovane at the
Space Astronomy Laboratory, Univ of Florida. The
data are grouped by 'phases' into tabular form; the
first four columns of the table are calibrated
fluxes identified as P2, P3, P4, and CO+ (no
contribution or only a very weak one, not separable
from the underlying continuum exists for that gas
channel). No time is specifically given but rather
the last column lists cometocentric distance
ranging from 192041 to 947 km. Each table
corresponds to a specific phase. Fluxes are
collected at 500 ms intervals, one phase after
another but are listed in each table at 4 s
intervals (at every rotation of the spacecraft or
about every 274 km along the spacecraft
trajectory).
Data for the gas are not given. Three phases gave
non usable data and are thus not provided here. In
all tables, the geometry relative to the comet is
the same. The distance to the nucleus can be used
as a clock to identify the phase at which the
observations were taken.