An introduction to the COMIS/TTM instrument

(for a layman introduction in Dutch, go to Nederlandse beschrijving)

History

In 1981, Kees de Jager, one of the founders of SRON and at that time president of COSPAR, was asked by Roald Sagdeev, director of the Space Research Institute (IKI) in Moscow, whether he might have any suggestions for experiments to be included in a new space station. Two years earlier, SRON had started the development of large-area position-sensitive proportional counters, for an application in a coded aperture camera. The prospect for a free ride to the Russian space station, at liberal mass constraints, seemed very attractive for the large detectors. The governing body of SRON ("GROC") was asked for a decision. They were somewhat apprehensive about the idea to cooperate with the Soviets, and were quite worried about whether SRON could benefit scientifically from such a collaboration. A vote was taken and the project was accepted with the smallest margin of majority. The project was named "COded Mask Imaging Spectrometer" or COMIS in short. The Space Research Group of the University of Birmingham soon joined the collaboration and the hardware was built by SRON and Birmingham. More specifically, the detector and its electronics were built by SRON, while the coded aperture and star sensor were by the University of Birmingham. The project was led by Bert Brinkman at SRON and Gerry Skinner at the University of Birmingham. The Dutch-English-Soviet collaboration was the first of its kind, and turned out to be very succesful. The design and building took only about 4 years, the instrument was operated for 12 years, and it cost less than 1 million dollars to the Dutch tax payer.

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.

Scientific objectives

The scientific objectives of COMIS/TTM on the Mir space station are to

Imaging principle

Since it is not possible to build an X-ray telescope with such a large field of view on the principle of classical optical systems, one has to resort to a variation of a theme which is much older, namely the camera obscura or pinhole camera. The angular resolution of a pinhole camera can be arbitrarily chosen and is determined by the size of the pinhole: the smaller this hole, the better the resolution. However, the smaller the pinhole, the less the light-collecting area. This would be detrimental to astrophysical applications where sources are usually very faint. This can be solved by not using a single but multiple pinholes at the same time, thus preserving the angular resolution but multiplying the light-collecting area. Thus is the principle of a coded aperture camera.

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.

Detectors and general WFC parameters

The detector is a large multi-wire proportional counters with an open area of 25 X 25 cm2. Several frames of hundreds of thinner-than-a-human-hair tungsten wires provide positional sensitivity. Building these was one of the largest technical challenges. Follows a table of the technical parameters of interest to the scientist.
Characteristics of COMIS

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

Jean in 't Zand, SRON, February 7, 2001