Copernicus Instrument Description
Mission OverviewThe third Orbiting Astronomical Observatory (OAO-3, OAO3, or OAO-C) named Copernicus, was launched from Cape Kennedy on the 21 August 1972. The Copernicus satellite contained two co-aligned astronomical instruments, the "Princeton Experiment Package" (PEP), a telescope-spectrometer designed for high-resolution studies of absorption lines in the interstellar medium, and a cosmic X-ray experiment provided by UCL/MSSL (further information on the X-ray experiments can be obtained from HEASARC). The Copernicus satellite was when launched the heaviest unmanned space observatory, and was placed in a nearly circular 745 km altitude orbit inclined 35 degrees to the equatorial plane. The telescope-spectrometer was a 32-inch diameter reflecting telescope and ultraviolet spectrometer system designed, built, and operated by the Princeton University Observatory (Principal investigator Lyman Spitzer, Jr.), and was designed to obtain ultraviolet spectra especially in the rich 900-1200 A wavelength region. Between August 1972 and February 1982 a total of 549 different objects were observed (plus measurements of air-glow and geocoronal Lyman-alpha emission), and 687,960 separate scans were obtained.
Instrument DescriptionThe Copernicus telescope-spectrometer (described in Rogerson et al. 1973a, and the Guest Investigator's Guide available on-line) used a cassegrain telescope with a f/3.4 primary of fused silica with a clear aperture of 80 cm and a fused silica secondary with a clear aperture of 7.5 cm. The optical surfaces were Al over-coated with LiF. The equivalent focal length at the spectrometer entrance slit (the cassegrain f/20 focus) was 1589 cm, with the spectrometer placed in front of the primary mirror (blocking approximately 42% of the primary aperture); the following is a diagram showing the "Princeton Experiment Package":
The spectrometer was of the Paschen-Runge type with a concave grating focusing the spectrum on a 1-meter Rowland circle with a dispersion in first order of 4.2 Å per millimeter (a general discussion of such spectrographs is provided, for example, in Kitchin's book Astrophysical Techniques, chapters 4.1-4.2, in particular see Figure 4.1.12). The entrance slit (3 mm by 24.2 micron) corresponded to 39.0" x 0.314" on the sky. Two movable carriages each carrying two photomultiplier tubes were used to scan the spectrum, the U tubes were generally sensitive only in the second order, while the V tubes were restricted to the near-ultraviolet (V1 by a fused silica filter and V2 by a sapphire prism). The nominal instrumental widths (FWHM) are 0.10 Å in first order (V1 and occasionally U1) and 0.05 Å in second order (most U1 scans); for U2 and V2 the widths are about 4 times larger. There was a diagonal mirror in front of carriage 2 (holding the U2 and V2 tubes) to prevent physical interference. Another tube (U3/V3) was fixed at 3430 Å and used to monitor photometric errors due to image motion in the narrow entrance slit. The U1 and U2 tubes were generally restricted to wavelengths greater than the Lyman-limit (912 Å), although the carriages allowed observations to wavelengths as short as 710 Å. The U1 carriage could also be used in 1st order on bright stars to obtain spectra at wavelengths longer than 1500 Å, but even for the brightest stars this was generally limited to wavelengths between 1500 and 1560 Å.
Phototube Carriage Order Nominal Nominal Wavelength Number FWHM Coverage (Å) (Å)
The electronics system provided for pulse counting of the signals from the data tubes during a nominal 14-second integration period (the actual time was 13.76 seconds) while the carriages were motionless, the carriages were then moved to the next position and a new integration started. The U tubes had a slowly varying dark count rate approximately equal to the flux of cosmic rays through the front of these tubes, while the V tubes had a large rapidly fluctuating dark count rate that seriously limited the photometric precision obtainable with the V1 and V2 tubes (although this could be largely removed by using a special scan mode, see for example Barker et al. 1984). Stray light from the target reached the U1 and U2 tubes through vent (outgassing) holes in the spectrograph, but this could be eliminated for the U1 tube by positioning carriage-2 to block light just longward of the U1 exit slit (see Rogerson et al. 1973b; York et al. 1973). A residual background of about 10 percent of the continuum was present for observations using the U1 tube longward of 1000 Å, presumably due to scattering from the optics. At the shortest wavelengths the scattered light could be greater than the observed intensity from the star due to the reduced efficiency at short wavelengths. The wavelength dependence of the scattered light is similar for the V1 tube). In most cases the scattered light correction must be derived from the residual counts in the bottom of saturated stellar or interstellar lines (see Rogerson et al. 1973b; Morton 1975; Bohlin et al. 1983). The correction of U2 data for stray light is described by Bohlin (1975). During the short time required to record a single scan the telescope typically oscillated by an amplitude of about 0.02 arcsecond, which was good enough so that the counts were usually not noticeably affected by guidance changes (Rogerson et al. 1973a; Bohlin et al. 1983). To achieve the highest photometric accuracy with U1, carriage 2 could be held motionless during each scan and the data from U2 used to smooth out fluctuations in U1 which are due to spacecraft pointing changes (this technique was used primarily for observations of the Lyman lines of Hydrogen and Deuterium).
There was a wavelength dependent decrease in the sensitivity of Copernicus over the duration of the mission (Polidan 1981; Bohlin et al. 1983). Over the first 5 years (27,000 orbits) the U1 and U2 tubes declined in sensitivity as expected as surfaces were contaminated, electronics decayed, and similar effects took their toll; however, there was a rapid and unexpected decrease in efficiency (particularly near 1100 Å) during the sixth year of operations. Tests of the high voltage units and the instrument focus suggested that the most likely cause of the decay was contamination of optical surfaces (Polidan 1981). It appears that material, either from outgassing or from a lubricant leak, had coated the optical surfaces and then had polymerized when it came in contact with Oxygen. The period of rapid decline corresponded to Solar maximum, when the Earth's atmosphere is expanded and there is additional Oxygen at the height of the Copernicus orbit. Similar behavior was also observed in the TIROS and Nimbus spacecraft during the same time interval due to polymerized outgassed material. The decline in sensitivity of the U1 tube is shown in the following graph from Polidan (1981):
For the V1 and V2 tubes the decline was less severe, the V tubes retained 60% of their pre-launch sensitivity (Polidan 1981).
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Polidan, R. S. 1981, OAO-3 End of Mission Tests Report Sections 8 and 9, NASA Technical Memorandum 83824
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Above material compiled by jtl