***** File GIOOPE.TXT
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and only the Figure captions are included. Two additional references
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Giovane, F., Eichkorn, G., McKisson, J., Weinberg, J.L., Lamy, P., Llebaria,
A., Detaille, M., Levasseur-Regourd, A.C., and Le Blanc, J.M. 1991,
Photomultiplier for Optically Probing Comet Halley, Applied Optics, 30,
No. 18, 2579
Levasseur-Regourd A C, Weinberg I L, Giovane F, Schuerman D W, Lamy P, Festou M,
Berteaux J L, & Dumont R 1981b, Halley optical probe experiment, The Giotto
Mission, ESA special publ 169, 121.
The Giotto Optical Probe
Experiment
A.C. Levasseur-Regourd, J.L.Bertaux
Service d'Aeronomie du CNRS, Verrieres le Buisson, France
R. Dumont, M. Festou
Observatoire de Bordeaux, France
R.H. Giese
Ruhr Universitat, Bochum, Germany
F. Giovane
Space Astronomy Laboratory, Gainesville, Florida, USA
P. Lamy, A. Llebaria
Laboratoire d'Astronomie Spatiale, Marseille, France
J.L. Weinberg
Space Astronomy Laboratory, Gainesville, Florida, USA
Abstract
The Halley Optical Probe Experiment (HOPE) has been designed to provide in-situ
photopolarimetric data on both the dust cloud and the gaseous atmosphere in Halley's
coma. The Optical Probe's concept is presented here, together with a description of
the instrumentation and the possibilities for cross-correlation between the HOPE
results and those of other space and ground-based experiments. The instrument was
turned on successfully on 13 September 1985.
1. Concept and
Methodology
Cosmic dust observations from space probes are traditionally classified as either
remote-sensing (essentially optical) or in-situ (typically mass-spectrometers or impact
detectors). A third type of observation is possible with the ESA Giotto mission to
Comet Halley, namely optical measurements of cometary dust and gas parameters in
volume elements centred on the moving probe (Levasseur-Regourd et al. 1981a).
The Optical Probe's concept is shown in Figure 1. An optical measurement of the
polarized spectral radiance in the direction of motion is made at point M1 on the
spacecraft trajectory; a second measurement is then made at point M2. If the two
observations are made parallel to the direction of motion, then their difference is the
spatial derivative of the radiance along the line of sight. It is a direct measure of the
light scattered by the dust or emitted by the gas in a tube along the trajectory (Levasseur-
Regourd et al. 1981b).
The only assumptions made are: (i) that the cometary atmosphere is optically thin
(which is highly likely more than 500 km from the nucleus); and (ii) that the rapid
motion of the spacecraft through the coma allows a quasi-steady-state from one
measurement set to the next.
The polarized spectral radiance can be inverted only if the viewing direction is
parallel to the trajectory, resulting in two possibilities, the direction of motion and the
opposite one. The corresponding measurements differ only in scattering angle, which
typically for the Giotto mission are 72.8 deg for the forward viewing direction and
107.2deg for rearward viewing. Typical scattering phase diagrams such as those obtain-
ed for the zodiacal cloud and in laboratory measurements are flat in the 70deg - 110deg
range (Dumont 1976, Giese et al. 1978), implying that the ratio of the two polarized
radiances would yield almost no information on the forward-to-backscattering ratio.
A rearward-looking instrument has been selected, in view of the more critical demands
of a forward-looking type.
Figure 1. Concept of an optical probe
2. Instrumentation
The Optical Probe Experiment requires the measurement of linearly polarized
brightnesses in various colours in the direction opposite to the direction of motion of
the spacecraft through the cometary coma. Since an instrument on board a cometary
probe had to be relatively small and extremely reliable, we have chosen to have no
moving parts, which has led to an instrument mass of only about 1.3 kg (Fig. 2).
The spectral discrimination in the seven-colour refracting photopolarimeter is
achieved by imaging uniformly illuminated interference filters deposited on the objec-
tive lens onto a micro-channel plate detector. The polarization is determined by the
spinning spacecraft rotation (15 r.p.m.) of the polaroid analyzer; simultaneous
measurements are made for the seven channels in 0.5 s. One set of measurements,
which allows the determination of the total intensities, the polarized brightnesses, the
azimuths of the polarized components (i.e. the Stokes parameters) and their errors,
corresponds to a 180 deg rotation of the analyzer; it is performed every 2 s.
Table 1. HOPE spectral response
Central Bandpass Bandpass
wavelength 50% peak 1% peak
Filter (nm) (nm) (nm)
Continuum 1 443.5 4.5 9.0
Continuum 2 575.0 10.0 20.0
Continuum 3 717.5 3.5 7.5
OH 307.5 6.0 10.0
CN 387.0 4.0 10.0
CO+ 426.0 4.0 8.0
C2 514.0 4.0 6.0
The layout of the optical system is illustrated in Figure 3. It is composed of an f/1.7
objective lens of 18 mm diameter, seven interference filters (Table 1), a polaroid foil,
a field stop, a field lens and a micro-channel plate photomultiplier. The filter mosaic
is designed to provide comparable intensities for the various channels and a partial cor-
rection of the aberrations. The field of view for each channel is 3 deg. Cross-talk may
exist between the various channels, due to diffusion inside the filter mosaic, misalign-
ment between optics and detector, or electronic cross-talk; total cross-talk has been
found to be less than 3%.
The linearity calibration was performed at the Max Planck Institute for Astronomy
in Heidelberg. It allowed determination of the degree of linearity for individual counts
by a boot-strap method. This Individual Linearity Ratio (ILR) is defined for a given
count as the ratio of counts measured to counts expected, i.e. if a stimulus results in
a count C1, and another stimulus results in a count C2 (where C1 is approximately
equal to C2), the combination of the first and second stimuli results in a count C3.
Then if the system is behaving linearly, C1 + C2 = C3. The ratio of C3 to the sum of
C1 and C2 defines the ILR for the count (C1 + C2)/2. These ILRs were determined for
all channels. A least-squares curve was then fitted to the ILR vs counts for each chan-
nel. The product of successive values of the ILR yields the linearity ratio, which is
then used to correct the measured counts of our system.
The cross-talk measurements were also mainly carried out in Heidelberg using a
Jarrell Ash dual 1 m monochromator. In this set-up, a light beam is passed through
the monochromator with a dispersion of about 0.1 nm and the response of the instru-
ment noted in all channels. These measurements were repeated for numerous
wavelengths. The results were integrated across each bandpass and then compared
with the integrated signal received in the out-of-bandpass channels. The outband and
inband counts were corrected by applying a preliminary linearity ratio derived as a
composite of all channels. The outband and inband counts corrected in this way for
linearity were then used to determine the preliminary cross-talk. This cross-talk was
then used to correct the linearity ratios. The improved linearities were then used to
correct the cross-talk. This cross-talk was subsequently used to correct the linearity
ratios, and finally the improved linearity ratio was used to finalize the correction to
the cross-talk. The application of this finalized cross talk to the linearity ratio did not
result in any material change and the iteration process was therefore terminated.
Figure 2. The HOPE instrument
3. Wavelength Choice
Sunlight diffused by the cometary dust will be measured over a wide wavelength
range, from the blue to the red, in three gas-free emission bands: 439-448 nm,
565-585 nm and 714-721 nm. Of great importance is the fact that there are two
wavelengths in common between the Giotto HMC camera and the HOPE instrument,
and one wavelength in common between the USSR Vega probe camera and HOPE
(Fig. 4), providing the possibility of cross-checking.
Light emitted by the cometary gases will be measured in four spectral bands. The
OH emission band at 307 nm has been chosen as a tracer of H2O molecules. CN at
387 nm and C2 at 514 nm are the dominant minor species emissions in the optical
range; they could be representative of some carbon chemistry in the coma. CO+ at
426 nm will allow the penetration of the solar wind to be studied throughout the coma,
Figure 3. The HOPE optical system
Figure 4. Comparison of the continuum filter
bands
and will provide new insight into ion formation processes. Polarization measurements
are mainly aimed at the study of the optical properties of dust grains. They also carry
information on the production processes of the gaseous species; ground-based obser-
vations have been found to be compatible with the fluorescence mechanism. Polariza-
tion measurements for OH, CN and C2 will help to assess the possibility of some of
those species being produced in an excited state in the inner coma.
The rearward-looking photopolarimeter is located on the top platform of the Giotto
spacecraft, in the shadow of the solar panels. It is, nevertheless, necessary to baffle
the instrument against light coming from the cometary coma outside the field of view,
and reflected by Giotto's main antenna or tripod during the flyby. A 270 mm long baf-
fle with seven vanes has therefore been designed and incorporated. The total system
stray-light suppression is about 10**-12, as long as the flight sources are at least 1.5deg
from the instrument's optical axis. The possibility that specularly reflecting sources
might direct sunlight into the baffle has been studied during solar simulation testing
of the spacecraft.
A self-luminescent white source of moderate intensity is mounted on the back of the
baffle cover; it has been used during the tests and during the Giotto spacecraft cruise
phase. The cover was released in October 1985 by means of a pyrotechnic device. In-
flight calibration is provided via this calibration source, and by the background sky
radiation (zodiacal light + star light) to be observed immediately prior to encounter.
4. Anticipated Results
Multicolour brightnesses and polarizations will be obtained in volume elements
along the trajectory of the Probe throughout Halley's coma. The tubes will be 140 km
long (corresponding to 2 s in the encounter data take) and 7 km wide (corresponding
to the 3 deg field of view of the instrument). For the dust, HOPE provides six values
(three wavelengths X two states of polarization) of the product of the local density and
the differential scattering cross-section. It should be possible to distinguish changes
in density from changes in cross-section and five values of the normalized cross-
section are obtained. One has, nevertheless, to be aware of possible ambiguities in
order to arrive at proper interpretation of the data, especially in the regime of large
irregular dust grains which might show similar relative scattering functions, somewhat
independent of size (Weiss-Wrana K, 1983). Generally, however, if the normalized
cross-sections do not vary from one point to the next, any change in the local polarized
radiance is a direct measurement of a change in the dust number density; discrete
features (jets) will be obvious if the optical properties do not change dramatically
in these jets. If the normalized cross-sections vary, severe constraints will be imposed
on optical properties. Dust number densities will be derived from laboratory results on
dust scattering properties and modelling, developed at Ruhr Universitat, Bochum, Laboratoire
d'Astronomie Spatiale, and the Space Astronomy Laboratory (Levasseur-Regourd et al. 1983).
Figure 5 shows how various grain-size intervals contribute to the brightness
integral. The lower curve obtained using Mie theory indicates that the size intervals
(0.121 - 1.5 micro m) and (1.5-70 micro m) contribute equally to the overall integral; the upper
curve, obtained using a combination of Mie theory for small grains (</= 70 micro m) and
the Perrin-Lamy model (Perrin J M & Lamy P L, 1983) of rough particles for larger grains,
suggests that grains larger than 70 micro m may contribute to enhance the brightness integral.
Table 2 shows how colour should behave for three HOPE dust channels, taking the fourth one
(445 nm) as a reference. The red channel indicates the possibility of distinguishing between
Figure 5. The cumulative brightness integral as
a function of grain radius
Table 2. Colour dependence (left: Mie model; right: Mie + rough model)
Distance to
the comet (km) Lambda = 368.0nm Lambda = 575.0 nm Lambda = 717.5 nm
1.08 x 10**4 100 1.02 1.19 1.19 1.30 1.23
1.06 x 10**3 1.01 1.02 1.19 1.19 1.30 1.25
5.06 x 10**2 1.01 1.04 1.19 1.19 1.30 1.25
smooth and rough grains and/or the possible presence of submicron-sized particles.
A simulation has been made to prepare the restitution method. For the dust, the
Perrin-Lamy model was used, while for gases a Festou-Zucconi model was
employed (Festou M & Zucconi J M, 1984). The solar flux scattered along the
rearward-looking line of sight was integrated every 0.5 s. Then, the zodiacal background
was added and the overall intensity was partially modulated to simulate the polarization.
Afterwards, the stellar background and, for the gas channels only, the estimated polarized
gas signals, were added. We also included stray light, cross-talk, telemetry noise and
telemetry breakdown. Typical results are presented in Figure 6. This simulation allows the
ultimate precision of the inversion technique to be estimated as a function of the global
noise level.
In conclusion, with the HOPE experiment we seek to:
- determine the changes in both number density (including inhomogeneities) and the
grain-size distribution as a function of the Probe's position inside the coma
- determine the spatial distribution along the trajectory of emissions due to OH,
CN, C2 and CO+, in order to derive the production rates of the parent species,
as well as their nature, and to obtain information on the physical processes leading
to the formation of the observed radicals by means of the study of the polarization
of their emissions as a function of the distance of the nucleus
- investigate the dynamical coupling of gas and dust by determining the gas-to-dust
mass production ratio as a function of the distance and the possible correlations
with grain sizes
- compare the optical properties of cometary dust with those of interplanetary and
interstellar grains, in an effort to understand the histories and mutual interactions
of all three dust complexes.
The experiment should also provide the necessary link between in-situ and remote
measurements with regard to spatial changes through the coma. It has been suggested
to the International Halley Watch (IHW) that ground-based observations should be
made at HOPE wavelengths: in addition HOPE team members will make supporting
grounf observations.
Figure 6. Typical brightnesses for two
continuum wavelengths and two gas
wavelengths
From a comparison of in-situ dust-detector measurements and HOPE experiment
results, the following relationships should be determined: dust number density and
bulk density, mass distribution and grain size, species abundances and gaseous emis-
sions. These relationships should allow remote (and future) observations of comets to
be interpreted in terms of physical processes.
Acknowledgements
HOPE has been developed under CNES Contract HOPE/Giotto/SA-LAS and
NASA Grant NASW-3578. The authors are grateful to J.M. Le Blanc, and to many
other colleagues, whose efficient cooperation facilitated rapid development of the
HOPE instrument.
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