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Copernicus Instrument Performance

The following sections have been extracted from the Final OAO-C Copernicus Operations Report prepared by Princeton University in September 1981. They concern the optical performance and instrument characteristics throughout the duration of the mission


Section 1: Summary of Instrument Performance

Guidance

While the Princeton Experiment Package (PEP) was gathering stellar data, the pointing was maintained by centering the stellar image on the slit jaws of the experiment using the Fine Error Sensor (FES). To ensure that the guidance characteristics were similar over the range of stellar brightnesses observed, the high voltage of the FES was determined by an automatic gain control (AGC) that depended on stellar magnitude. During the mission, stars between -1.5 and 7.0 visual magnitude were regularly observed.

[...]

Spectrometer Telescope

Throughout the mission of the PEP, both spectrometer carriages performed at nominal design specification. No mechanical anomalies were ever noted. Before the OAO-C launch, carriage 2 was discovered to have a low-rate leak in its sealed lead-screw bellows assembly. This was considered, however, to have little consequence over the anticipated 1-year lifetime of the PEP, and a decision was made to continue with launch preparations.

The commanded carriage motion was extremely predictable, and all deviations from expected behavior were traced to electronic glitches, operational programming errors, or a lack of understanding of the nonstandard operation of the carriage motion control subsystem (special programs, configurations, etc.).

The static position indicators for both carriages possessed a known inherent ambiguity that occasionally resulted in a scientific data loss because of incorrect interpretation of the carriage position status by operations personnel. On several occasions, the carriages were also inadvertently commanded to positions beyond their normal ranges of travel, but in each instance, the electrical limit switches were activated, and the motion of the carriages was halted be fore any physical stop was met.

The obscuration pattern of carriage 1 sensors by the carriage 2 collection mirror was determined early in the mission, and this effect was included, when necessary, in the observing programs. Stray light entering the vent ports of the far-UV sensors was also recognized as a problem. This effect was much reduced by judicious operations programs and data reduction correction procedures.

[...]

Optical Performance

The overall performance of the "optical" system (mirrors, phototubes, and associated electronics) was within expected limits for the first 5 years of the mission. Shortly after launch, telemetry indicated that the secondary mirror had not positioned itself properly. A check of the image size and shape in orbits 168-170 showed the image to be in focus. It was concluded that the secondary mirror was positioned correctly and that the telemetry was bad. A second check of the image size was performed in orbits 35250-35260 and showed no significant change in the image. A third set of image data obtained during the last month of the mission was lost. It is believed, however, that the image size and shape were maintained throughout the mission.

The only major change in performance occurred in the experiment far-UV sensitivity. During the first 5 years of the mission, the decline in instrument sensitivity was similar to that predicted before launch: the largest decline occurred the first year with the shortest wavelengths being the most affected. Beginning near orbits 25000-30000, the decline in sensitivity accelerated, dropping by a factor of 5 in 1 year, with the largest decline occurring at the middle, rather than shortest, wavelengths. This decline continued throughout the remaining 3 years of the mission. Investigation of the cause for this decline is still underway. The best explanation at this time is that the decline is due to contamination of the optical surfaces in the spectrograph by an unknown material. The onset of the rapid sensitivity degradation in 1977 corresponds to the onset of solar maximum, suggesting that a process similar to that found in the Television Infrared Observation Satellite (Tiros) is working (Reference: Tiros Project Memo of October 12, 1979). Details of the decline in sensitivity can be found in Section 2.

No failure of any component in the optical system was recorded during the mission. All six phototubes and their associated electronics were functioning at termination.

[...]

Section 2: Malfunctions and Resultant Impact

Mechanical Failures

The few failures that occurred within the PEP did not compromise or severely limit its basic scientific mission. These failures are itemized as follows:
a. Secondary Mirror/Focus Mechanism
During the immediate post launch checkout of the PEP, status data indicated incomplete uncaging of the telescope's secondary mirror/focus mechanism assembly. Subsequent data analyses suggested failure of the telemetry monitor circuits and not of the uncaging operation.

An attempt to change the telescope focus in orbit was also unsuccessful. Position status data of the secondary mirror implied that no motion was produced by commanding. Because the final focus adjustment prior to launch was calculated to include the effects of the launch environment, the telescope was believed to be near best focus. Image shape tests confirmed this status. End-of-mission attempts were made to move the secondary mirror, but again, no motion was observed.

b. Calibration Lamps
Postlaunch checkout of the calibration lamps confirmed launch survival and no significant changes in the spectrometer wavelength calibration. The lamps were infrequently used thereafter. During a special series of observations about one third into mission lifetime, however, both lamps failed to operate. End-of-mission attempts to ignite calibration lamp A were unsuccessful.

[...]

Sensitivity Degradation and Effects

The principal malfunction in the PEP was the rapid decline in the far-UV sensitivity. The decision to terminate the spacecraft was based partly on the loss of far-UV sensitivity. Figure 2-1 shows the relative sensitivity of the high-resolution far-UV phototube U1 from orbits 100 to 44000. In the first 25000 orbits (5 years), the decline was close to that predicted be fore launch. A rapid (approximately 50 percent) decline at shorter wavelengths was experienced during the first year followed by smaller declines in subsequent years. At longer wavelengths, the decline was more gradual and reasonably constant (approximately 10 to 15 percent per year) through the first 25000 orbits.

After orbit 25000, the character of the decline was significantly different. At the shortest wavelengths (<1000 Å), a slow decline continued until termination. At the middle wavelengths (1000 to 1300 Å), a dramatic decline occurred. Between orbits 30000 and 35000, the sensitivity of the U1 tube decreased by a factor of 2.25 at 1100 Å. By orbit 40000, another factor of 5 was lost. Thus, in 10000 orbits, more than a factor of 10 was lost. At longer wavelengths (>1300 Å), the decline was not as rapid (a factor of 4 in 10000 orbits), but it was larger than that observed before orbit 25000.

The low-resolution far-UV phototube U2 exhibited a behavior similar to that of U1. The only significant difference was that during the period of rapid decline (orbits 25000-44000), it showed a greater decline than U1. The U2 wavelength dependence was similar to that of U1.

The near-UV phototubes, V1 and V2, did not exhibit the rapid decline seen in the far-UV phototube. They displayed an initial rapid decline followed by a much slower decline until the end of the mission. At orbit 25000, both V1 and V2 were at approximately 70 percent of their launch sensitivity. By orbit 44000, they still retained 60 percent of their sensitivity at launch.

Clearly, something occurred between orbits 25000 and 30000 that greatly affected the sensitivity of the far-UV phototubes. This is also the period in which glitches began to occur in abundance (Section 2) [not included here]. This suggests that the two malfunctions may have the same cause or that the sensitivity decline was somehow caused by the effects of the glitches.

[...]

The initial decline in sensitivity, orbits 0-25000, is similar to that seen in other devices. The cause is thought to be primarily due to the decay of the photocathodes. The rapid decline occurring after orbit 25000 is, however, quite anomalous. Investigations conducted shortly after the onset of the rapid decline absolved the power supplies and other control units as the cause of the sensitivity loss. This left only contamination of the optical surfaces and/or photocathodes as the cause of the decline. This suspicion is further supported by the fact that U2 showed a greater decline than U1 (U2 undergoes one more reflection than U1).

At the same time that Copernicus was experiencing this rapid decline in sensitivity, Earth sensors on Tiros-N, NOAA-A, and 5D/1 were also noted to display significant sensitivity losses. Analysis revealed that their sensitivity loss was due to contaminants on the optical surfaces. The contaminants came from outgassed material reacting with atmospheric oxygen to produce polymers. The increased solar activity in 1977 raised the mean density of oxygen in the upper atmosphere and caused the polymer production to increase dramatically (see report from Tiros Project for details).

Analysis of the data suggests that a similar reaction may have occurred in the PEP. The most likely location for the contaminant is in the spectrograph. Ample materials exist for outgassing, and the spectrograph is open to space so that atmospheric oxygen can enter (direct observation of oxygen atoms confirms their increased abundance at the altitude of the spacecraft in 1978 through 1981). U1 and U2 show greater declines because of the peculiar absorbing properties of the contaminant. The peak absorption appears to occur between 1050 Å and 1150 Å. U2 shows more loss because the incoming light undergoes an extra reflection to enter the phototube. Further analysis is continuing. A final report on the decline and possible causes will be issued when the analysis is complete.

The impact on Copernicus operations caused by the loss of sensitivity was to lengthen all observations. Programs were run several times to get the data quality obtained with a single run before 1977.

Impact of V Tube Fluorescence and Short Frame

As explained in Section 1 [not included here], the background levels in the near-ultraviolet tubes (V1, V2, V3) were much larger than expected. It has been determined that passage of energetic particles (cosmic rays, particles, etc.) through the windows covering the tubes causes a short-term fluorescence and a long-term phosphorescence. The sum of these two components results in a background count rate of approximately 7000 counts/14 seconds in V2 and approximately 10,000 counts/14 seconds in V1. Therefore, V1 and V2 were useful only for fairly bright stars, where the stellar signal was much larger than the background. With the advent of the short frame program in 1975, the V1 tube began returning higher quality data. However, short frame observations required the use of the OBP [On-Board Processor] (except for the technique implemented in 1980, whereby the spacecraft was commanded from the ground). From 1975 to 1980, the OBP had numerous minor failures and three major failures. The major failures resulted in the OBP being inoperable from April 8 to August 7, 1977; August 30, 1978 to June 26, 1979; and June 28 to October 3, 1980, a total of 17 months.