An introduction to the WFC instrument

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

Scientific objectives

The scientific objectives of The Wide Field Camera (WFC) instrument on the BeppoSAX satellite are to The instrument consists of two indentical wide field cameras that are active at few keV photon energies (or about 1 to 10 Angstrom). The field of view of each camera is unprecedently large for an astrophysical X-ray telescope: 40 X 40 square degrees. The angular resolution of 5' is well matched to the source confusion limit of the relevant population of modestly bright X-ray sources. The cameras are pointed in opposite directions. Combined they cover 7% of the sky. However, because of the low-earth orbit, the earth will block the field of view of either camera for a few tens of percents of the time.

BeppoSAX was launched into a low-earth low-inclination orbit on April 30, 1996, from Cape Canaveral, Florida. Scientific operations started in July 1996 after an instrumental checkout period. The observations were shut down during May-August 1997 in order to implement a 1-gyro mode after 4 of the 6 gyros had failed. Apart from this observation-less period, operations have been running smoothly.

The scientific return of WFC has been very rewarding and exceeds expectations. In particular, the contribution of WFC to gamma-ray burst research has been pivotal, see WFC's scientific highlights. You are invited to browse through the list of scientific publications to obtain a good idea of scientific results so far.

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 both WFCs, the mask pattern is based on a 'triadic' residue set with u=21846 which is folded 2-dimensionally row wise and completed to a 256 X 256 pattern with 3 closed pattern elements. The bitmap of the pattern is shown below (white = open areas, black = closed). Click on image for enlarged version.

Detectors and general WFC parameters

Both WFCs are completely identical in design. The detectors are 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 each SAX-WFC

Detector type Multi-Wire Proportional Counter
Detector area (=mask area) 255 X 255 mm2
Detector gas Xe (94%), He (1%), CO2 (5%)
Detector gas pressure 2.1 atm
Active detector area 530 cm2
Effective area 430t cm2 at 6 keV
Distance Aperture-Detector 703 mm
Field of View (FWZR) 40 X 40 sq. degrees (0.47 sr)
Mask element size ~1 X 1 mm2
Mask open fraction 0.33
Angular Resolution (FWHM), on-axis 5 arcmin
Active photon energy range 1.8-30 keV
Photon energy resolution (FWHM) 18% at 6 keV
Photon detector depth 32 mm

Since the sealing of the detectors in 1995, the detectors have remained stable within a few percent with regards to gain.
Jean in 't Zand, SRON, March 12, 1999