Four monitors WATCH (Lund 1986), designed at the Danish Space Research
Institute, are mounted on board the orbital observatory GRANAT. The field of
view of a monitor is a circle 74
radius. The scintillation
detector is made of alternating stripes of NaI and CsI. The full geometrical
area of the detector is 47 cm2. The effective area of the detector is
dependent on the incident angle of the arriving photons as shown in
Fig. 1 (click here): at small angles it declines as a cosine, starting at
more steeply, and for a source 65 off-axis it is one
fourth of that for an on-axis source. The fields of view of the four
instruments cover different quarters of the sky and partially overlap.
Because at the very beginning of the experiment one of the monitors went out
of order, 
% of the sky can be simultaneously viewed under most
favourable conditions. The performance of the WATCH instrument is
based on the rotation modulation collimator principle. A point source,
provided it is bright enough for at least one rotation of the
modulation collimator (
s), can be localized with an accuracy of
arcmin (at the 
confidence level), where n10
is the source signal in units of 10 standard deviations (Brandt 1994). The
detector count rate is recorded in two energy bands, the boundaries of which
were reset several times during the mission and roughly correspond to
8-20 keV
and 20-60 keV.
Figure 1: Detector efficiency as a function of source off-axis angle (same
for both WATCH energy bands)