OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = GIO INSTRUMENT_ID = OPE OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "Optical Probe Experiment" INSTRUMENT_TYPE = "Photopolarimeter" INSTRUMENT_DESC = " The instrument is a photometer-polarimeter which views along the spin- axis of the spacecraft. Unlike most other instruments on Giotto which view in the ram direction, OPE views in the anti-ram direction, i.e. in the direction from which the spacecraft has come. It thus measures the integral brightness along the track as a function of position along the track. The derivative of this signal is, therefore, the polarized emissivity at each point of the track. The instrument has no moving parts. An f/1.7 objective of 18-mm diameter images the 'sky' onto a field stop with an effective field of view of 3 degrees. The seven interference filters to isolate various spectral regions are deposited in a mosaic directly on the front element of the objective and the linearly polarized foil is sandwiched between the elements of the objective. Immediately behind the field stop, a field lens reimages the objective and its mosaic of filters onto a micro- channel plate. The micro-channel plate provides anodes for each of the seven filters. The mosaic of filters is laid out in such a way as to provide approximately equal output signals in each bandpass and to partially correct for aberrations of the optical system. Polarization is measured by the rotation of the spacecraft (which rotates the linearly polarized foil together with the whole instrument) at 15 RPM. This allows determination of the first three Stokes parameters once every 2 seconds (once every half rotation). The data are sampled every 0.5 seconds, i.e., every 45 degrees of spacecraft (polaroid) rotation. The instrument is mounted on the top platform of the Giotto spacecraft, near the periphery of the spacecraft where it is in the shadow of the solar panels and can see past the de-spun high-gain antenna. The instrument is baffled against stray light from the coma outside the field of view and against light reflected from the high-gain antenna or the tripod. The baffle is 270 mm long with 7 vanes. A self-luminescent white source is mounted on the back of the baffle cover for testing and calibration during the cruise phase. This baffle cover was ejected by a pyrotechnic device in October 1985. Instrument Manufacturer : Service d'Aeronomie du CNRS, Verrieres le Buisson, France Instrument Mass : 1.32 Science Objectives ================== - Determine the changes in both number density (including inhomogeneities) and the grain-size distribution as a function of Giotto'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 emission as a function of distance from the nucleus. - Investigate the dynamical coupling of gas and dust by determining the gas-to-dust mass production ratio as a function of distance from the nucleus and the possible correlation with grain size. - 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. Calibration Description ======================= The pre-flight calibration was carried out primarily at the Max-Planck- Insitut-fuer-Astronomie in Heidelberg. The linearity of each of the seven channels was calibrated by a boot-strap method, starting with two incident light signals, C1 and C2, of comparable intensity and comparing the output of the sum of the two signals, C3, with the sum of the outputs for the two separate signals. The Intensity-Linearity-Ratio (ILR) for the counting level (C1+C2)/2 is then given by C3/(C1+C2). The results of a series of measurements on each channel were fitted by least squares and the resultant curves were used to calibrate the data. Cross-talk between the channels, which could arise either from optical scattering within the filters or misalignment of the optics with the detector or electronic cross-talk, was measured with a beam from a monochromator having a spectral width of approximately 0.1nm. The response of each channel of the instrument was recorded at a variety of wavelengths. The results were integrated across each bandpass and compared with the integrated signal received in the out-of-bandpass channels. The out-of-band and in-band counts were corrected by applying a preliminary ILR derived as a composite of all channels. The signals determined in this way were used to determine the preliminary crosstalk. This procedure was iterated until it converged. The final cross-talk was found to be less than 3%. A final calibration was obtained on the background sky, zodiacal light plus diffuse starlight, immediately prior to the encounter. General steps in going from raw counts to flux include the following: 0. Correct for non-linearity (this correction is completely negligible for Halley far from the encounter). It is small for Halley at closest approach (where signals are strongest) compared to other uncertainties. 1. Take dark current from same clock-sector reading and multiply by ratios of areas (obs channel/dark channel) on the MCP and subtract 2. Estimate scattered light, as a function of clock-sector, from the observations before and after encounter. 3. Correct for cross-talk between channels using matrix in Giovane et al., 1991. 4. For CO+ channel, multiply by absolute inverse responsivity. 5. For other channels, either add clock sectors separated by 90 degrees or multiply by two (very approximate), to go from the polarized intensity to total intensity. Operational Considerations ========================== During the flythrough of Halley's coma, the instrument was pointed in the direction opposite the direction of motion. Direct differen- tiation of the signal, with a full measurement every two seconds, should lead to the mean polarized emissivity in a cylinder of length about 137 km (2 seconds of travel) and diameter 6 km (2.62 deg field of view at a distance of 137 km). During the entire encounter with Halley, the scattering angle was 107.2 degrees. Although the instrument was not damaged during the encounter, the viewing direction after a dust particle impacted Giotto was not anymore parallel to the direction of motion, and not well determined, and thus the data could not be analyzed. Measurements were performed every 45 degrees in rotational phase. Only five phases gave a usable signal; these phases can readily be identified in the data tables by the chronology of the cometocentric distance: each geometrical configuration repeats itself every 4 sec or every 274 km along the spacecraft track. Due to the strength of the underlying continuum signal in the CO+ filter, the weak CO+ emission was not truly detected and that channel can be considred as another continuum channel. Detectors ========= Detector ID : OPE Detector type : Micro-Channel Plate Filters (see Giovane etal, 1991) =============================================== Continuum 1 Filter ------------------ Center Filter Wavelength : 0.4423 micron Interference filter with bandwidth at 50% of peak = 0.0047 microns and bandwidth at 1% of peak = 0.0090 microns. Peak transmission of 0.46. Continuum 2 Filter ------------------ Center Filter Wavelength : 0.5771 micron Interference filter with bandwidth at 50% of peak = 0.0098 microns and bandwidth at 1% of peak = 0.0090 microns. Peak transmission of 0.73. Continuum 3 Filter ------------------ Center Filter Wavelength : 0.7175 micron Interference filter with bandwidth at 50% of peak = 0.0034 microns and bandwidth at 1% of peak = 0.0075 microns. Peak transmission of 0.39. OH Filter --------- Center Filter Wavelength : 0.3113 micron Interference filter with bandwidth at 50% of peak = 0.0054 microns and bandwidth at 1% of peak = 0.0100 microns.Peak transmission of 0.23. CN Filter --------- Center Filter Wavelength : 0.3875 micron Interference filter with bandwidth at 50% of peak = 0.0037 microns and bandwidth at 1% of peak = 0.0100 microns. Peak transmission of 0.37. CO+ Filter ---------- Center Filter Wavelength : 0.4249 micron Interference filter with bandwidth at 50% of peak = 0.0036 microns and bandwidth at 1% of peak = 0.0080 microns. Peak transmission of 0.51. C2 Filter --------- Center Filter Wavelength : 0.5145 micron Interference filter with bandwidth at 50% of peak = 0.0036 microns and bandwidth at 1% of peak = 0.0060 microns.Peak transmission of 0.70. Optics ====== Telescope Focal Length : 0.0306 m Telescope Diameter : 0.018 m Telescope F Number : 1.7 Platform Mounting Description ============================= The instrument is mounted on the top platform of the Giotto spacecraft, near the periphery of the spacecraft where it is in the shadow of the solar panels and can see past the de-spun high-gain antenna. It views parallel to the spin axis. At the encounter with P/Halley, this is the direction opposite the ram direction. Instrument Section Description ============================== Section ID : OPE Data Rate : 723 bits/sec FOV Shape : CIRCULAR " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = 'REGOURDETAL86' END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = 'GIOVANEETAL91' END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END