OPERATIONAL CONSIDERATIONS FOR OPE ENCOUNTER WITH 26P/GRIGG-SKJELLERUP CHARACTERISTICS OF THE CHANNELS 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+, C_2) were devoted to the observation of light emitted by cometary gases. An eighth channel for the ultraviolet continuum is disussed in some early papers but this channel was not implemented at the time of launch. 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, I_(i), I_(i+1), I_(i+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 et al., 1991). The results were as follows: Channel Wavelength Bandpass Inverse 50% of peak Responsivity [nm] [nm] [10^-7 W/m^2/sr/um/ct] 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 C_2 514 4 3.75 NB: In the above table, um stands for micrometers, nm for nanometers, sr for steradians, ct for counts. The same notation is used subsequently in this discussion. 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. A subsequent calibration on the sky after the encounter with 1P/Halley is described below. INTENSITY AND POLARIZATION MEASUREMENTS AT GRIGG-SKJELLERUP (Levasseur-Regourd et al., 1993) To retrieve the intensity from the measured signals, one has to use the linearity factor (almost negligible), to remove the average dark count, to take into account the cross-talk, to remove the background, and finally to apply the absolute calibration factor. The intensity can be retrieved (except for the CO+ channel) from the sum of two polarized measurements, (I_(i)+I_(i+2)), at 90deg from one another. Since straylight (possibly from reflections on the despun antenna) was found on clocksectors 0, 1, and 2, the combinations that were used were (3+5), (4+6), and (5+7). The degree of polarization can be retrieved from combinations of consecutive measurements in clock sectors free of straylight, e.g.: P = 2((I_(i)-I_(i+2))^2 + (I_(i+1)-I_(i+3))^2)^0.5/I_tot where I_tot = I_(i)+I_(i+1)+I_(i+2)+I_(i+3). It has to be noticed that, since the polarized intensities are not obtained at a given angle, but are averaged in 45-degree wide clock sectors, the derived degree of polarization may be underestimated. SENSITIVITY AT GRIGG-SKJELLERUP ENCOUNTER With the possible decay in sensitivity of the instrument at the Halley encounter, the sensitivity has to be checked through observations of the astronomical background in the field of view of the instrument. This background is built up of three components: the zodiacal light from sunlight scattered by interplanetary dust which, with the planetary light, builds the solar system component, the galactic light from the stellar light scattered by interstellar dust which, with the light from the stars, builds the diffuse galactic component, and the very faint extragalactic light. Prior to the flyby of comet 26P/Grigg-Skjellerup, the OPE experiment was aimed at a = 19h 21m, d = -22deg 06' in Saggitarius (T. A. Morley 1992, private communication). The ecliptic coordinates were about Lo-Lo_sun = 90deg and b = 0deg, i.e. the observations were made in the vicinity of the ecliptic plane, at 90deg from the Sun; planet Neptune (mag 7.7) was marginally in the field of view. The galactic coordinates were about Lo = 16deg, b = -17deg i.e. the observations were not too far from the galactic plane; approximately 6 stars of magnitude close to 6 were within the field of view. Table 1 summarizes the contributions of the various background sources. The zodiacal light contribution is accurately determined through the table of Dumont and Levasseur-Regourd (1980); the integrated starlight from stars fainter than magnitude 6 is estimated through the compilation of Roach and Gordon (1973); the diffuse galactic component is obtained from its galactic latitude dependence in Toller (1990). The brightness is expressed in S10(V) or equivalent number of tenth visual magnitude solar-type stars per square degree. In physical units, 1 S10(V) equals 1.26 x 10^-8 W/m^2/sr/um at 0.55 um, that is to say about 1.25 x 10^-8 for the blue, green and CO+ channels, 0.43 x 10^-8 for the OH channel, 0.9 x 10^-8 for the red channel and 1.3 x 10^-8 for the C_2 channel. Background components (S10(V)) Zodiacal light 200 +/-10 Planet and bright stars 12 +/- 3 Integrated starlight 300 +/-50 Diffuse galactic light 20 +/- 7 Total background 532 +/-70 S10(V) blue, green, CO+ 67 +/- 9 x 10^-7 W/m^2/sr/um OH 26 +/- 3 x 10^-7 W/m^2/sr/um red 48 +/- 7 x 10^-7 W/m^2/sr/um C_2 70 +/- 9 x 10^-7 W/m^2/sr/um The signals detected prior to and after the flyby, in the clock sectors likely to be free of any stray light, were the following: Channel Signal Dark count Background (counts/half second) Blue 17 (+1, -4) 4 +/- 1 13 (+2, -5) Green 33 (+1, -9) 5 +/- 1 28 (+2, -10) Red 09 (+1, -3) 8 +/- 2 01 (+3, -5) OH 05 (+1, -2) 2 +/- 1 03 (+2, -3) CO+ 13 (+1, -4) 2 +/- 1 11 (+2, -5) C_2 11 (+1, -2) 3 +/- 1 08 (+2, -3) The following table presents the sensitivity at the encounter with 26P/Grigg-Skjellerup and compares the results with the pre-flight estimation. Channel Inverse Responsivity Inverse Responsivity [10^-7 W/m^2/sr/um/count] pre-launch estimate Blue 5 +/- 2 2.1 Green 2 +/- 1 0.9 Red 4.4 OH 9 +/- 7 17. CO+ 5 +/- 2 1.3 C_2 9 +/- 4 3.8 The comparison with the pre-launch estimate suggests that the relative response remains of the order of 0.4 between the green and blue channels, and of the order of 0.2 between the green and the C_2 channels, and that the red channel suffered a drastic loss of sensitivity at closest approach to 1P/Halley. GEOMETRICAL CIRCUMSTANCES The experiment was designed to look backwards along the trajectory of the spacecraft during the encounter with 1P/Halley. During the encounter with 26P/Grigg-Skjellerup, communication with Earth required that the experiment view in a direction controlled by the antenna on the spacecraft. The geometrical circumstances during the encounter are shown diagrammatically in the figure "opegeom.tif" in the same directory with the data on the CD ROM. This figure was prepared by the OPE team for inclusion in a manuscript which was being prepared for submission at the time of preparing this dataset for PDS. The well-defined parameters of the geometry include the velocity relative to the comet, 13.99 km/s at an angle of 100.6 deg relative to the comet-sun line, the orientation of the line of sight relative to the trajectory, 68.8 deg from the sun-Giotto-trajectory plane and lying nearly in the plane perpendicular to the comet-sun line, and the scattering angle for the observed photons, 90.4 deg. The absolute position of the spacecraft relative to the nucleus at closest approach is very uncertain. Shortly after the encounter, this was estimated in large part on the basis of the OPE results which, when compared with theoretical models for the expected variation of brightness with time, implied that the comet passed on the anti-sunward side of the comet with a distance of closest approach near but probably less than 200 km and that the field of view of OPE passed very close to the nucleus. The files for the ephemeris yield the best estimated position of the spacecraft relative to the comet but it must be remembered that this result is very uncertain. More recent work by the OPE team has revised these conclusions somewhat and the figure showing the geomtery reflects the newer understanding which is discussed in their manuscript being submitted to Icarus. REDUCTION ALGORITHM The reduction should correspond to the following steps: 1. Linearize the response (Giovane et al. 1991, Figure 11) 2. Remove the cross-talk using the cross-talk matrix (Giovane et al. 1991, Table III; see files 'opecross.lbl,.tab') 3. Remove the dark current, using the measured dark current in the same clock sector and correcting for the relative cathode areas used (Giovane et al. 1991). 4. Remove the scattered light using the data measured before encounter (files opecal.lbl/opecal.tab). 5. For CO+, multiply by responsivity; for other channels, combine sectors with orthogonal polarization (i and i+2) and then multiply by responsivity. 6. Extract polarization (except for CO+) as described above. In general, steps 1 and 3 have negligible effect for the data during the encounter. ACKNOWLEDGEMENTS This document is based on text provided by the PI, Dr. A. Ch. Levasseur-Regourd, as edited and expanded at the Small Bodies Node. REFERENCES Giovane, F., G. Eichorn, J. McKisson, J. L. Weinberg, A. Weisenberger, P. Lamy, A. Llebaria, M. Detaille, A. C. Levasseur-Regourd, and J. M. Le Blanc 1991. Photomultiplier for optically probing Comet Halley. Applied Optics 30, 2579-2591. Dumont, R. and A. Ch. Levasseur-Regourd 1980. Inversion of the Zodiacal Brightness Integral: A New Geometric Approach Suitable for Out-of-Ecliptic Zodiacal Light Programme. In "Solid Particles in the Solar System" (IAU Symposium 90), Ed: I. Halliday and B. McIntosh (D. Reidel Publishing: Dordrecht, Holland), pp 67-70. Levasseur-Regourd, A. C., B. Goidet, T. Le Duin, C. Malique, J. B. Renard, and J. L. Bertaux 1993. Optical Probing of Dust in comet Grigg-Skjellerup from the Giotto Spacecraft. Planet. Space Sci. 41, 167-169. Roach, F. E. and J. L. Gordon 1973. The Light of the Night Sky. (D. Reidel Publishing: Dordrecht, Holland). Toller, G. N. 1990. Optical Observations of Galactic and Extragalactic Light: Implications for Galactic Structure. In "The Galactic and Extragalactic Background Radiation" (IAU Symposium 139), Ed: S. Bowyer and C. Leinert (Kluwer. Acad. Pub.: Dordrecht, Holland) pp 21-34.