The two identical Wide Field Cameras, developed by SRU/SRON (Jager et al. 1989, 1997), consist of two position sensitive proportional counters filled with 2 atm of Xenon, which view the sky through a coded mask perpendicularly to the axis of the NFI and 180 degrees away from each other. They combine a large field of view (40øx40ø per unit, and a quarter of this at FWHM) with an angular resolution of 5 arcmin (FWHM). The energy band ranges from 1.8 to 30 keV with a resolution of 18% at 6 keV.
The principle of the WFC is the Dicke camera. A two-dimensional position and energy sensitive detector (multi-wire proportional counter) is placed behind an opaque metal screen with a pseudo random array of holes, a so called coded mask. X-ray sources in the field of view cast shadows of the mask on the detector, displaced according to the position of each source. A crosscorrelation afterwards reconstructs the position and flux of all sources in the observed part of the sky. The mask pattern is square, with 256 x 256 elements of 1mm2 each. The open fraction of the pattern is 33% (effective 26.4%), that increases the S/N ratio for faint sources.
|Energy range||2-28 keV|
|Geometrical area||250 cm2 (1 unit, through mask)|
|FOV||40ø x 40ø(20 x 20 deg FWHM)|
|Angular resolution||5 arcmin|
|Window||150 um Be|
|Gas||2 atm Xenon (5% CO2, 5%He)|
|Energy resolution (FWMH)||18% @ 6 keV|
|Energy binning||31 channels|
|G monitor in real time by collimated sources of||Fe55, Cd109|
|Temporal resolution||0.5 msec|
|Sensitivity in 3E4 sec||1 mCrab|
|Max. count rate||2000 cts/s|
|Crab count rate||300 cts/s|
|Cosmic XRB count rate||110 cts/s|
Scientific capabilities of WFC
The sensitivity depends in principle on the pointing direction in the sky, because each source in the FOV contributes to the overall background. Towards high-galactic latitude the Cosmic Diffuse X-Ray Background is the main contributor to the background. In this case the sensitivity is at the order of a few mCrab in 10000 s. This will allow the monitoring of faint sources like AGN, along with their prime objective, that is the survey of the galactic plane and the search of X-ray transients for follow-up studies with the narrow field instruments.
On the basis of the logN-logS distribution of gamma-ray burst and assuming fx about 1/100 of fg we expect to detect about 3 X-ray counterparts to gamma-ray bursts, thus positioning the events within 5' and gathering broad band information with the simultaneous observation of the gamma-ray burst monitor.
More detailed information about this instrument is available from the WFC Frequently Asked Questions pages and from the WFC pages at SRON.
Last update: February 20, 1997