urn:nasa:pds:context:instrument:pvo.ouvs 1.0 1.7.0.0 Product_Context 2020-08-26 1.0 extracted metadata from PDS3 catalog and modified to comply with PDS4 Information Model urn:nasa:pds:context:instrument_host:spacecraft.pvo::1.0 instrument_to_instrument_host Stewart, A.I, 'Design and Operation of the Pioneer Venus Orbiter Ultraviolet Spectrometer', IEEE Transactions on Geoscience and Remote Sensing, GE-18, 65-70, 1980. reference.STEWART_1980 PIONEER VENUS ORBITER ULTRAVIOLET SPECTROMETER Spectrometer not applicable not applicable INSTRUMENT OVERVIEW =================== The University of Colorado's Ultraviolet Spectrometer instrument carried on the Pioneer Venus Orbiter spacecraft is a 125-mm f/5 Ebert-Fastie design with a 250-mm Cassegrainian telescope. The detectors are two EMR miniature photomultiplier tubes with a combined sensitivity range of 1100 to 3300 Angstroms. The diffraction grating is controlled by an optical device using the moire fringes generated by two offset radially ruled gratings. A grating motor moves the grating through 27 degrees in 512 equal steps. Each step size corresponds to 4.4 Angstroms. The motor may be operated in a scan-and-flyback mode or in a mode in which it steps to a commanded position and holds there. Further description of the instrument is found below. A detailed description of the instrument, its operations and its derived data products is given in STEWART_1980. The instrument was manufactured by the Laboratory for Atmospheric and Space Physics and the 'build date' is taken to be 1976-01-01. The spectrometer dimensions were 0.0213 by 0.0390 by 0.0169 (height by length by width in metres) and the mass was 2.72 kg. Instrument ID : OUVS Instrument Host ID : PVO PI PDS USER ID : AIFSTEWART Instrument Name : Ultraviolet Spectrometer Instrument Type : Ultraviolet Spectrometer Build Date : 1976-01-01 Instrument Mass : 2.72 Instrument Length : 0.0213 Instrument Width : 0.0390 Instrument Height : 0.0169 Instrument Manufacturer Name : Laboratory for Atmospheric and Space Physics PRINCIPAL INVESTIGATOR ====================== The Principal Investigator for the Ultraviolet Spectrometer was A. Ian F. Stewart. SCIENTIFIC OBJECTIVES ===================== The objective was to study Venus in the ultraviolet from all ranges between 66,000 km, far outside its tenuous corona of atomic hydrogen, and 140 km, inside the fringes of the atmosphere itself, and to do so continually at all local times throughout an entire solar cycle. During the 14 year mission continuous images, spectral scans and limb profiles of Venus were made. In addition, observations were made of several comets and stars. CALIBRATION =========== The OUVS instrument response was calibrated at LASP using tungsten, deuterium, and argon miniarc sources derived from NBS standards. Also measured were the response of off-axis light, the nonlinearities in the detector systems, and the instrument polarization. Periodic calibrations during flight were performed using a calibration lamp and by viewing selected stars. OPERATIONAL CONSIDERATIONS ========================== A data buffer was essential to reconcile the basic instrument data rates of between 2048 and 256 b/s (depending on the integration period) with the OUVS allocation in the spacecraft telemetry stream, which is typically 80 b/s (although it could be as high as 512 b/s). This low duty cycle also requires careful timing of the acquisition of data (a data arc), so that the scientific content of the data can be maximized. The acquisition of spectral data also poses a problem, since the spacecraft (and therefore the OUVS line of sight) rotates through 60 degrees during the 2 s required for a complete spectral scan; this motion causes gross changes in the viewing and illumination geometry during the acquisition of the spectrum and renders interpretation difficult. Data-arc timing is accomplished by accepting spatial reference pulses from the spacecraft and by providing a commandable time-delay device. The spatial reference pulses are issued when the spacecraft X-axis passes through the planes containing the spin axis and the velocity vector (RAM pulse), the planet-spacecraft vector (NADIR pulse), and the 270 degree ecliptic longitude vector (RIP pulse). The instrument limb sensor also generates pulses at the leading and trailing edges of the planet's illuminated disc. The spacecraft issues 1024 sector pulses per rotation, and the time delay is accomplished by counting a commanded number of these pulses after receipt of the selected spatial reference pulse, before beginning to load data words into the buffer. The length of the data arc is determined by the integration period; at 4 ms the buffer fills in 1 s, or 30 degrees of spacecraft rotation; while at 32 ms it takes 8 s or 240 degrees. The size of the data buffer means that spectra are acquired in two halves, with an interval between the halves for the telemetry system to read out the buffer. The problems caused by the variation in line-of-sight during the data acquisition are minimized by taking spectra only when the spacecraft is near periapsis, so that the disc subtends much more than 30 degrees at the instrument, and by using the NADIR pulse to begin the data arc 15 degrees before the line-of-sight crosses the nadir plane (the arc then ends 15 degrees beyond this plane). This procedure reduces the variations in lighting and viewing angles during the acquisition of the spectrum. DETECTORS ========= OUVS is mounted on the orbiter via a mounting wedge, so that the line-of-sight is offset 59.95 degrees from the spin axis. The spin axis points nominally at the south ecliptic pole. The detectors are two EMR miniature photomultiplier tubes operating in the F and G channels with a combined sensitivity range of 1100 to 3300 Angstroms. The 510G-09 tube has a cesium-iodide cathode, a lithium-fluoride window and is sensitive from 1100 to 1800 Angstroms. The 510F-09 tube has a cesium-telluride cathode, a fused silica window and is sensitive from 1600 to 3300 Angstroms. The sensitivities at several wavelengths are given in Table II in STEWART_1980. Each tube has its own pulse amplifier and discriminator unit and the dark-count level for both tubes is about 4/s. ELECTRONICS =========== The electronics consist of logic box, low and high voltage power supplies, and a pulse amplifier and discriminator unit for each photomultiplier tube. The logic box contains the spacecraft interface units, the command decoding logic, the pulse counters, word compressors, and data buffer used to process the detector outputs, and the instrument status logic. Additional logic provides for such functions as voltage regulation, and temperature compensation of devices such as the grating motor control units. Pulses from the detectors enter a 15-bit counter with integration periods of 4,8,16 or 32 ms. The data word thus formed is compressed by shifting it until the leading bit reaches position 15; a new 8-bit word is then formed consisting of the next 4 bits plus four bits indicating the number of shifts. The stream of 8-bit words is stored in a 256-word buffer; once the buffer is full, no new words are generated until the spacecraft telemetry system has read out the entire buffer. OPTICS ====== Light enters the instrument through a 250-mm f/5 Cassegrainian telescope, heavily baffled against scattered off-axis light and protected by a cylindrical sunshade. The sunshade has one internal baffle and is lined with a honeycomb material. The telescope focuses light from the source on the 6 mm by 0.6-mm entrance slit of the monochromator, giving a 1.4 degree X 0.14 degree field of view. The monochromator is an Ebert-Fastie design of 125-mm focal length, using a 1-in diffraction grating ruled at 3600 grooves/mm, giving a dispersion of 22 Angstroms/mm in first order at the exit slit plane. The grating is blazed at 1300 Angstroms. Two exit slits of the same size as the entrance slit are used; the spectral resolution is thus 13 Angstroms. All the reflecting surfaces in the optical train are coated with magnesium fluoride, and all scattering surfaces are painted black. OPERATIONAL MODES ================= In orbit around Venus the spacecraft spin rate is 5 rev/min with the spin axis pointed nominally at the south ecliptic pole. The orbit is highly eccentric (e=0.843); periapsis and apoapsis altitudes are approximately 150 km and 66,000 km, and the latitude of periapsis has moved from 18 degrees to -10 degrees over the 14 years of the mission. The OUVS line-of-sight is offset 60 degrees from the spin axis and the planet is in view for two periods: the first, from P - 125 min (i.e., 125 min before periapsis) to P - 35 min is used for global imaging, since the planet subtends less than 40 degrees, while the second, from P - 15 min to P + 10 min, is used for high-resolution mapping, spectral work, and limb profile measurements, i.e., those observations that must be made at close range. When the planet is not in view, the atomic hydrogen corona can be observed from P - 5 h to P + 1 h, and outside this period, stars and interplanetary hydrogen can be observed around the small circle at 30 degrees south ecliptic latitude. For observations of comets the spacecraft spin axis is maneuvered so that the OUVS line-of-sight passes through the target during each spin. Two instrument modes were available. The wavelength mode used the grating motor in its fixed-position mode, and the data channel, integration period, and data-arc timing were all commandable. In spectral mode the grating position was changed by one step after acquiring the previous sample. The data buffer was used to store and read out acquired samples. Data-arc timing and the data channel were selected by command. A full 512-grating position spectrum was acquired in two parts since the buffer holds 256 data words. The minimum sampling time was 0.004 seconds and the maximum was 0.032 seconds. Wavelength resolution was 13 Angstroms and the wavelength step size was 4.4 Angstroms.