Here we wish to observe an extended galaxy. From its IRAS flux we want
to be able to detect a surface brightness of 100 
Jy/''
with
a signal-to-noise ratio of at least 15. The observation will be
performed using the broad-band filter LW2, and at this wavelength the
background emission is estimated from the IRAS database to be
80 Jy/''.
In the simulator, the source and the background flux density spectra
are modelled as power laws with exponents respectively of 
= -1 and
-4 (where is in 
). The spectra are
normalized respectively at 12 and 7 m, corresponding to the
central wavelength of the IRAS and LW2 filters.
Since the source is quite bright and stronger than the background, with an extension smaller than the field-of-view, we selected the single pointing observing mode (CAM01).
We chose an integration time of 2 s, a pixel field of view of 6'', and decided to estimate the observing time starting from a situation where the detector is not illuminated (last flux is 0.0).
The results of this simulation are presented in figure 
.
On this figure we can see two curves, but only the lower one interests
us here since the upper one does not take into account either the finite
flat-field accuracy, or the stabilization time (here 43 readouts,
90 s).
From this curve, we see that in 200 s observing time, we will reach a S/N of 17, thus meeting our initial request. Furthermore this curve also shows that longer integration times would not bring any improvement to the quality of the observation.
Do not forget that 180 s have to be added to this observing time to account for telescope pointing.
Figure: Example of a micro-scan observation with the performance
simulator.