Mission Overview

The objectives of the OSO satellite series were to perform solar physics experiments above the atmosphere during a complete solar cycle and to map the entire celestial sphere for direction and intensity of UV light, X-ray radiation, and gamma-ray radiation. The OSO 8 platform consisted of a sail section, which pointed two experiments continually toward the sun, and a wheel section, which spun about an axis perpendicular to the pointing direction of the sail and carried five experiments. Gas jets and a magnetic torquing coil performed attitude adjustment. Pointing control permitted the pointed experiments to scan the region of the solar disk in a 40- by 40-arc-min to 60- by 60-arc-min raster pattern. In addition, the pointed section was capable of being commanded to select and scan a 1- by 1-arc-min or 5- by 5-arc-min region anywhere on the solar disk. Data were simultaneously recorded on tape and transmitted by PCM/PM telemetry. A command system provided for at least 512 ground-based commands.

Launch Date: 1975-06-21 at 11:43:00 UTC
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Decay Date: 1986-07-09

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1975-06-21 00:00:00 UTC

Orbital Parameters
Periapsis 544.0 km
Apoapsis 559.0 km
Period 95.69999694824219 minutes
Inclination 32.900001525878906°
Eccentricity 0.0010819999733939767

Instrumentation

High-Resolution Ultraviolet Spectrometer Measurements

This University of Colorado experiment measured solar ultraviolet lines between 1050 and 2300 A and their variation with time and position on the disk. Spectroheliograms were also generated at selected wavelengths. The instrument consisted of an extended-focal-length Cassegrain telescope, an Ebert monochromator, and a small computer. The telescope focused sunlight on the entrance slit (variable from 1- by 5-arc-s to 1-arc-s by 15-arc-min) of the monochromator. The 3600 lines/mm grating in the monochromator was used in second order. The grating drive mechanism was capable of being programmed to scan: (1) the entire spectrum, (2) selected portions of the spectrum, or (3) selected single wavelengths. Two photomultiplier tubes, one covering the range from 1400 to 2300 A and the other covering wavelengths less than 1400 A, detected the radiation. The small computer controlled the experiment and allowed flexible observing programs through automated, data-dependent observing sequences.

Chromosphere Fine-Structure Study

The experiment was designed to measure solar chromospheric spatial and wavelength structure for the following spectral lines in the 1000-A to 4000-A region -- Lyman-alpha, Lyman-beta, the H and K lines of magnesium II, and the H and K lines of calcium II. The instrument, which was composed of a cassegrain telescope and a grating spectrometer, was capable of operating in two modes -- (1) it could hold a fixed solar location and scan the spectral lines, (2) it could simultaneously fix on three of the six spectral lines and scan a 1-arc-min by 1-arc-min region of the solar disk. The instrument was capable of angular resolutions from 1 by 1 arc s to 1 by 40 arc s and a spectral resolution of 0.02 A (except Lyman beta, 0.06A). Instrument sequencing was controlled by ground command only.

High-Sensitivity Crystal Spectroscopy of Stellar and Solar X Rays

This Columbia experiment was designed to monitor continuously the sun's emission in the 2-8 keV range, to obtain high-resolution spectra of many celestial X-ray objects, and to measure the polarization of X-ray emission from stellar sources. This instrument package was mounted in the wheel section and made use of the wheel rotation to obtain complete Bragg energy spectra of the sun every 10 s during flares. The spectrometer was oriented perpendicular to the spin axis and used large area panels of crystals (1100 sq cm of graphite, 194 sq cm of PET) to reflect, via Bragg reflection, monochromatic solar X-rays into proportional counter detectors. The detectors were double-sided proportional counters with 0.025-mm beryllium windows on each side and contained an argon-xenon gas mixture chosen for its high efficiency over the 2-8 keV range. A ground wire-grid plane through the center of the bank of counters effectively divided the instrument into two isolated spectrometers. The polarimeter was oriented parallel to the spin axis and utilized Bragg angle reflection to measure polarization in X-rays from celestial sources.

Mapping X-Ray Heliometer

This Lockheed Mapping X-ray Heliometer experiment was designed to measure the location, spectrum, and intensity of moderate-energy X-rays (2 to 30 keV) from individual solar active regions (including flaring regions) and from extrasolar X-ray sources. The instrument consisted of three X-ray collimator-detector systems, a power supply, and a data accumulation/readout system. The collimators were identical but oriented differently and had fields of view of 2.1 arc-min by 10-deg FWHM. One collimator was oriented so that the 2.1 arc-min field of view was parallel to the spacecraft spin axis; the other two collimators were inclined plus and minus 60 deg relative to the spin axis. The detectors were proportional counters of various areas and window thicknesses allowing a wide dynamic range of activity to be observed. The experiment was operated from June 1975 through September 1978 in a period of mainly moderate to low solar activity.

Soft X-ray Background Radiation Investigation

This Wisconsin experiment was designed to measure galactic latitude dependence of the X-ray background radiation in the 0.150 to 45 keV region, emphasizing the soft X-ray portion. Two sets of three proportional counters mounted on the OSO wheel viewed parallel and antiparallel to the wheel spin direction. All detectors used collimators that were of hexagonal honeycomb construction with 3.5- by 3.5-deg FWHM. Sensitivity was expected to be about 1% statistical accuracy near the galactic poles, and energy resolution was provided by selected filters. Since two of the counters had thin polycarbonate windows through which methane diffused, a high-pressure methane reservoir carried on the spacecraft replenished those counters through a gas flow system. The remaining detectors were permanently sealed. The sealed counters had an additional thin film thermal barrier in the X-ray path of 551 micro-g sq cm aluminum.

Cosmic X-Ray Spectroscopy

This GSFC experiment was designed to determine the spectra of sources and the diffuse cosmic X-ray background in the energy range 2 to 60 keV, and to measure intensity variations and possible emission lines of discrete X-ray sources. Proportional chambers (multianode proportional counters) were used as detectors. One detector complement, consisting of a propane-neon-filled chamber and a xenon-methane-filled chamber (240 sq cm), was located behind a 5-deg collimator and oriented parallel to the spacecraft spin axis. A single-volume, argon-methane-filled chamber (75 sq cm) was located behind a 3-deg collimator and was offset slightly from antiparallel to the spin axis. A xenon-methane-filled chamber (270 sq cm) was located behind a 5-deg collimator and was oriented antiparallel to the spin axis. Data were accumulated in a buffer memory for 1-min intervals and the data from the offset detector were sectored in azimuth.

High-Energy Celestial X rays

The purpose of this GSFC hard X-ray telescope was to measure the energy spectra of all known X-ray sources above the intensity threshold of 1.E-6 photons/sq cm-s-keV in the energy region .02 to 3 MeV and to search for time variations, both periodic and irregular, in the intensity of the sources detected. The detector consisted of two optically isolated CsI(Na) central crystals shielded by a large, active collimator also made of CsI(Na). The sensitive area was 27.5 sq cm and the FOV was 5-deg FWHM. One of the two central crystals was completely shielded and served as a monitor of the total internal detector background spectrum. The instrument was mounted in the wheel section of the satellite with the axis of its FOV offset by 5 deg from the negative spin axis of the wheel. The minimum detectable intensity of a point source that was brought to within 5 deg of the negative spin axis for greater than 1 day was about 10E-5 photons/sq cm-s-keV at 100 keV. This was determined from the background spectrum measured in orbit.

EUV From Earth and Space

This NRL experiment, mounted in the wheel section, obtained spatial and temporal measurements of extreme ultraviolet (EUV) emissions of hydrogen, helium, and oxygen in the earth's atmosphere and in interplanetary and galactic space. Three photometers were designed to measure EUV resonance radiation in various wavelengths from 170 to 1080 A and in portions of the 1125 to 1230 A band. Each photometer consisted of a continuous-channel electron multiplier used as a photon detector, together with a thin metal film or a magnesium fluoride-oxygen cell to serve as optical bandpass filters. There were four such bandpass filters: (1) a thin film of 1000 A-thick aluminum and 500 A-thick carbon (bandwidth of 170 to 440 A), (2) a thin film of 1000 A-thick aluminum (bandwidth of 170 to 800 A), (3) a thin film of 1500 A-thick indium (bandwidth of 730 to 1080 A), and (4) a cell with a magnesium fluoride window (bandwidth of 1130 to 1500 A). These bandpass filters were mounted on a wheel in front of the photon detectors and were rotated at regular intervals to change the filters. This made three of the indicated wavelength ranges operational at any given time. The instrument was mounted with the photometer axes at a small angle to the satellite-sun line and with sufficient baffling that the photometers would never see the sun.

Science
Summary