Copernicus Orbiting Astronomical Observatory-3
Copernicus Orbiting Astronomical Observatory-3
Space MissionsMission Overview
The Copernicus satellite, otherwise known as the Orbiting Astronomical Observatory 3 (OAO-3), obtained a series of high resolution far- (900-1560 Å) and near- (1650-3150 Å) ultraviolet spectral scans of 551 objects, primarily bright stars, from 1972 to 1981.
Copernicus OAO-3.
Copernicus OAO-3.
The Copernicus satellite was launched into a nearly circular 7123 km radius orbit, inclined at 35 degrees, on 21 August 1972. The main experiment was an ultraviolet telescope and spectrometer. However, it also contained a cosmic X-ray experiment provided by University College London/MSSL. The main body of Copernicus measured 3 x 2 meters. The solar pannels were fixed at an angle of 34 degrees to the observing axis, and were kept within 30 degrees of the Sun. This restriction resulted in certain parts of the sky being visible only at certain parts of the year. The astronomical instruments were co-aligned, with the UV telescope residing in the central cylinder of the satellite and the X-ray experiment in one of the bays surrounding it. While the UV telescope was observing, the X-ray detectors primarily took background measurements. Occasionally, the X-ray detector observed an X-ray source in the field of view of the UV target. It operated until February 1981.
Instrumentation
Ultraviolet Telescope-Spectrometer
Copernicus OAO-3.
Copernicus OAO-3.
The ultraviolet observatory was built to record the spectra of stars over the wavelength range extending from the Lyman Limit (912 Å), below which interstellar space is mostly opaque (over distances greater than about 100 pc), to the wavelength where the Earth's atmosphere begins to transmit light (around 3100 Å). The lead institution for designing and building the system was Princeton University Observatory. The resolution of the spectrometer ranged from 0.05 Å to 0.4 Å. The primary purpose for building this observatory was to record spectral absorption features created by various atoms and molecules in the interstellar medium, which would be seen as a pattern of very narrow features on top of the spectra of very hot (early-type) stars. The ultraviolet region is a far better spectral domain for studying the interstellar medium than visible wavelengths accessible from the ground, since it contains a much richer and more useful collection of transitions from important interstellar species. A secondary objective was to study the spectral features arising from the stars themselves.
The instrument configuration consisted of a Cassegrain telescope (focal ratio: f/3.4) with an aperture diameter of 80 cm, within which was mounted a Paschen-Runge type spectrometer with a 1-meter Rowland circle. This spectrometer was positioned in front of the primary mirror of the telescope with a 24 μm wide entrance slit at the focus. The pointing of the telescope was controlled by a fine-guidance sensor that captured starlight that was reflected off the entrance slit of the spectrometer. The sensitivity of this sensor allowed the telescope to point very accurately on stars with visual magnitudes brighter than 6th magnitude. Fainter stars could not be observed. At the time the spectrometer was designed, ultraviolet image sensors had not yet been developed. Thus, the spectra were recorded with photomultiplier tubes that scanned the individual spectral elements in sequence. This process was very inefficient by modern standards, but nevertheless it produced spectra with very high photometric accuracy. Spectra at wavelengths longward of about 1500 Å were compromised by high noise levels created by fluorescence from cosmic ray particles impacting on the entrance windows of the long-wavelength detectors (the detectors that recorded the short wavelengths had no such entrance windows and thus had very low background count rates). As a result, most of the observations were carried out at wavelengths below 1500 Å, where the highest quality spectra could be recorded. Fortunately, most of the important spectral features appear at the shorter wavelengths.
Key Scientific Achievements from the Ultraviolet Instrument
The Interstellar Medium:
- Measurements of the abundances and rotational excitation of molecular hydrogen
- The abundance of atomic deuterium
- The discovery of O VI, indicating the presence of very hot gases in space
- An understanding of the depletions of free atoms as they formed dust grains
Stars:
- Mass loss from early-type stars
- Coronal emission from cool stars
- High resolution spectral atlases of narrow-line, hot stars
X-ray Experiment
There were 4 X-ray detectors in the UCL/MSSL experiment. The main detector was a proportional counter sensitive to the energy range 2.5-10.0 keV (1-3 Å). It had a simple collimation tube with a 2.5 x 3.5 degree FWHM field of view. The effective area was 17.8 cm2 with a sensitivity of about 3 mCrab. The others were two proportional counters and a channeltron, at the foci of grazing incidence telescopes. The channeltron suffered from high UV background and was not scientifically productive. The proportional counters covered the energy range 0.7-1.5 keV and 1.4-4.2 keV (6-18 Å and 3-9 Å). By the clever use of stops at the foci, the fields of view and the effective areas could be set to 10´ (12.3 cm2), 3´ (11.3 cm2), or 1´ (7.6 cm2) and 10´ (3.7 cm2), 6´ (2.4 cm2), or 2´ (1.0 cm2) for the high and low energy systems, respectively. These 2 systems became inoperable in July 1973 due to a failure of a background shutter. The basic accumulation time was 62.5 seconds, followed by 24 seconds of dead time. Thus, effectively, there was an 86.509 second sampling interval. There was a way to force better time resolution (to multiples of 1.62 s, this mode was named J20), which was used occasionally for observing bright sources, searching for bursts, etc. However this mode was used only on a few occasions and was not scientifically useful. A six channel Pulse Height Analyzer could be connected to any one of the 3 proportional counters and used to gather spectral information.
Diagram of the Copernicus Telescope-Spectrometer (From Rogerson, et al. 1973, Astrophysical Journal, 181, L97. ©1973 The American Astronomical Society, reproduced by permission.
Diagram of the Copernicus Telescope-Spectrometer (From Rogerson, et al. 1973, Astrophysical Journal, 181, L97. ©1973 The American Astronomical Society, reproduced by permission.)
External Links
- The Copernicus Satellite (OAO-3) - Goddard Space Flight Center & High Energy Astrophysics Science Archive Research Center (HEASARC)
Preview Image
Artists concept of the Copernicus Orbiting Astronomical Observatory-3. NASA History, SP-4312 "Dreams, Hopes, Realities." Chapter 5: Exploring the Earth. (Source: NASA.)
Citation
Jenkins, Edward (Contributing Author); Bernard Haisch (Topic Editor). 2008. "Copernicus Orbiting Astronomical Observatory-3." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 8, 2007].
<http://www.cosmosportal.org/articles/view/135481/>
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