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Single pointing observations

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 gif. 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.



ISOCAM Observer's Manual - V1.0
Tue Oct 31 12:06:23 MET 1995