PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "NULL" OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VEGA1" INSTRUMENT_ID = "TVS" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "TELEVISION SYSTEM" INSTRUMENT_TYPE = "CCD CAMERA" INSTRUMENT_DESC = " Instrument Overview =================== The television system consists of two telescopes, one narrow-angle camera (TVY) for high-resolution imaging of the nucleus, and one large-angle camera (TDN) for detecting and tracking the comet. The TVY optics have a reflecting objective with a focal length of 1200 mm and a detector which yields an average angular resolution of 3 arcsec, i.e. a spatial resolution of the order of 150 m at the nominal flyby distance of 10 000 km. The maximum angular dimension of the nucleus and its near environment is expected to be 5 arcmin at closest approach, and the pointing error is estimated to be +/- 5 arcmin. The field of view of the TVY must therefore be not less than 15 arcmin. The TDN has a refractory objective with a focal length of 150 mm. It is characterised by an angular resolution of 0.5 arcmin, required for early acquisition of the comet and its nucleus, and a 2deg field of view imposed by the constraints associated with control of the pointing platform. The light collected by each telescope is divided into two paths by a beam splitter, 0ne channel is fitted with a fixed filter, the other has a set of eight filters mounted on a rotating wheel, to yield a spectral analysis of the signal. The images are forms on area CCDs cooled by a passive radiator regulated by a Peltier plate. The commutable filters of the TDN have an additional function, namely to adjust the amount of light collected by the detector. This channel is operated autonomously and performs an independent analysis of the video signal, thus providing redunant information to the platform pointing system for the sake of reliability. The signals delivered by the three other TDN and TVY channels are handled by the same microprocessor system which analyses the images and generates the commands that control the motion of the platform. The TVS electronics include a 816 kbit memory to store both data and programs, The main characteristics of the TVS are given below. ------------------------------------------------------------------------- Table . Television System characteristics Camera system High resolution Low resolution ------------------------------------------------------------------------- Objective Reflector Refractor Focal distance 1200 mm 150 mm Aperture 240 mm 50 mm Relative aperture 1:5, effective 1:6.5 1:3 Channel Multispectral Integral Multispectral Integral ------------------------------------------------------------------------- Spectral range, micro m 0.4-1.1 0.63-0.76 (1) 0.4-1.1 0.63-0.76 (2)0.63-0.76 ------------------------------------------------------------------------- Number of filters 8 1 Range (1):1 1 Range (2):7 ------------------------------------------------------------------------- Field of view 26.4'x39.6' 211'x316' 211'x158' Resolution 3.1inx4.1in 24.75inx33in 99inx132in ------------------------------------------------------------------------- Shutter Mechanical Electronic ------------------------------------------------------------------------- Exposure time range 0.01-163 s 6-800 ms ------------------------------------------------------------------------- Detector area 5l2x512 pixels 512x256 pixels ------------------------------------------------------------------------- Data compression Floating window of 128 X 128 pixels Full image around brightest point of 128x128 pixels after integration of 4x2 pixels ------------------------------------------------------------------------- The resolution of the images was a trade-off. Scientists wanted nucleus images with the highest possible resolution, but that was limited by weight considerations and by selecting a trajectory for the spacecraft which was far enough away from the nucleus to guarantee a high survival probability. The compromise solution was to use a Ritchey-Chretien type telescope for scientific imaging with a focal length of 1200 mm, an effective aperture of f/6.5, and a field of view of 26.4x39.6 arcmin. It was immediately obvious that the field of view of this narrow angle camera (NAC) was too small to find the nucleus autonomously. It was also not possible to steer the telescope from the ground because the position of the nucleus in the coma was not well enough known. Therefore, a wide angle scanning camera (WAC) had to be added to the payload. A major concern was the target definition, namely what should the pointing system be aimed at. The sensors could identify only the centre of brightness, but it obviously was not identical to the centre of the nucleus. This was partly due to partial illumination of the nucleus by the Sun (comparable to the phases of the Moon) and partly due to jet activity. The pointing system might well have locked on to a bright dust jet and be steered away from the nucleus. As we found no way to define an offset reliably, we decided simply to aim at the centre of brightness. This targeting strategy worked well for Vega. The main navigation system for the platform was the imaging system itself. However, as a backup an eight-segmented light sensitive sensor was mounted on the pointing platform. Its working principle was extremely simple: if the centre of light from the comet would move away from the centre of the sensor, a change of the platform orientation would have been initiated to compensate for this offset. Fairly sophisticated computer simulations proved that this backup was reliable, but fortunately, it never had to be used. To have many backups was a major element in our design philosophy. A third navigational backup, which used analog signals from a completely independent CCD sensor, was added to the TV system. The output signal was processed in a similar way to the eight segmented light sensitive sensor but with much higher precision." 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