The space station was launched in 1986 and named "Mir". COMIS was part of the first module to the station - called "Kvant" - which was launched in April 1987. This module contains a station attitude control system and an astrophysical observatory called "Roentgen". It consists of four experiments that cover the medium and high X-ray regime part of the electromagnetic spectrum between 2 and 800 keV. COMIS was the sole imaging instrument.
Observations with COMIS started in June 1987, and the timing could hardly have been more favorable. A few months earlier a relatively nearby supernova went off, SN1987a, at a distance of only 170,000 light years. This was the first time such a supernova could be observed with modern techniques. COMIS was the only imaging X-ray device in orbit at the time. Unfortunately, despite hard efforts, COMIS did not detect SN1987a.
A few months later, a malfunction occurred in the high-voltage control of COMIS. No scientific data could be extracted anymore. Despite the fact that COMIS was not built to be repaired in-flight, the Soviets offered to carry out an exchange with the flight spare detector during an extra vehicular activity (EVA). Thus, a repair program was initiated that involved refurbishing and testing the spare flight detector in Utrecht, the design of repair tools in Utrecht, the construction of these tools in Birmingham, the set-up of repair procedures at the cosmonaut training center in Star City and the training of the cosmonauts for the necessary EVA-procedures. A first repair attempt was made on June 30th 1988, but could not be completed. This was achieved during a second EVA on October 20th. Observations started the next day of the Galactic center and showed the repair to be a complete success. The two repairs took 9 hours in total.
In November 1989, observations with Kvant were stopped temporarily because the Mir station was reconfigured with two additional modules, Kvant-2 and Kristall. The on-board activity associated with this change of the station's configuration lasted almost a full year. An attempt was done in October 1990 to restart the observations again. However, it was soon recognized that slewing the revised station cost too much energy: instead of only using the gyroscopic flywheels, rockets were necessary to aid the re-orientation of the station. This situation was solved in about a years time and operations started again on a reasonably regular basis early 1992.
All four X-ray instruments of the Roentgen observatory are rigidly fixed to Kvant and point towards the same point in the sky. In order to move from one target to another, the entire space station has to be re-oriented. This is accomplished by the use of gyroscopic flywheels. Kvant is provided with 6 of these, two in each of three mutually perpendicular planes. To supply the necessary amount of electric power to the gyros, a third blanket of solar cells was installed.
There are a number of non-trivial details in the design of coded aperture cameras and it is not surprising that, after the first ideas in the 1960s by Dicke and Ables, a complete branch of research in the area developed. This not only includes applications in astrophysical research but also in imaging of radioactive sources in laboratory as well as non-laboratory environments. For comprehensive information on astrophysical applications we refer to the web pages on coded aperture imaging
In COMIS, the mask pattern is based on a 'pseudo-random' array generated with a irreducible primitive polynomial. The array of 0s en 1s is folded 2-dimensionally row wise and completed to a 255 X 257 pattern. The bitmap of the pattern is shown below (white = open areas, black = closed). Click on image for enlarged version.
|Detector type|| Multi-Wire Proportional Counter |
|Detector area (=mask area)||255 X 255 mm2 |
|Detector gas||Xe (95%), CO2 (5%) |
|Detector gas pressure||1 atm |
|Active detector area||540 cm2 |
|Effective area||210 cm2 at 6 keV |
|Distance Aperture-Detector||1846 mm |
|Field of View (FWZR)||16 X 16 sq. degrees (0.06 sr) |
|Mask element size||~1 X 1 mm2 |
|Mask open fraction|| 0.50 |
|Angular Resolution (FWHM), on-axis|| 2 arcmin |
|Active photon energy range|| 1.8-30 keV |
|Photon energy resolution (FWHM)|| 20% at 6 keV |
|Photon detector depth|| 50 mm |