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The minimum detectable count
rate for the WFC depends mainly on the background count rate, and the effective
size of an unresolved source. The minimum detectable count rate, 
,
can be estimated from the following formula (e.g., de Korte et al., 1981,
Space Science Reviews, 30, p. 495-511):
where 
is the required significance threshold (i.e., signal-to-noise),
is the background count rate in the ``detection cell'', f is the fraction of
the total source count included in the ``detection cell'', 
is
the minimum number of photons required to
constitute a detection and t is the integration time.
The ``detection cell'' is the region of the image used to detect the source. For
the WFC, assuming an on-axis source, an appropriate cell area
depends on the filter used, since the point response function is
energy-dependent. For the S1, S2 and P1 filters the appropriate
radius enclosing 70% of the total count (i.e., 
) is
, whilst for P2, 
. The corresponding cell areas, 
, are thus 
arcmin 
and
arcmin respectively.
The count rates in the background cell are then calculated
as 
, where 
arcmin is the
area of the
whole field of view and 
it the total background count rate of the
whole field.
The minimum detectable count rate as a function of t is shown
graphically in Figure 12.1 
for 
, f=0.7, 
.
Three curves are shown, the two labelled `LOW' and `HIGH' are to be used
for the
S1,S2, P1 filters and are plotted for 
and
counts s 
, i.e. for the typical range of
background rates
expected. The low background rates are only achieved for restricted
time intervals, guest investigators should therefore assume the `HIGH' curve
when proposing.
The curve labelled P2 is to be used for observations with
the P2 filter
and assumes 
counts s , i.e. the night-time
background rate expected in this filter (see § 7.8.4 ).