The capability of detecting an astronomical object in an image is limited by the spatial noise in the background. The brightness of the faintest object which can be detected is called limiting magnitude. This limiting magnitude depends on the structure of the light-source to be detected, the shape of the point spread function and the criteria of the detection. Because the image correction by the adaptive optics is highly depending on the seeing conditions, the limiting magnitude also differs from observation to observation. The numbers presented here were calculated for average observing conditions, where 50% of the flux from the object is encircled in the following 50%-radii R50 (from ``Report on Image Quality of the COME-On Plus Adaptive Optics Instrument (M13)'' by E. Tessier from June 27, 1995):
detection (50 % of the overall flux of the object is 5 times the noise in
the background signal within the area defined by the 50% radius).
Depending on wavelength and pixel scale,
the shot noise induced by the thermal background and dark current of
the detector exceeds the readout noise after a certain integration
time per individual frame. This integration times for background
limited observations are summarized in table 3.2.
Table 3.2: Integration times per frame for background
limited observations
For integration longer than the tabulated values, the brightness of the background limits the observations. The background brightness for the different filter bands are summarized in table 3.3.
Table 3.3: Brightness of the background
The figures 3.1 and 3.2 illustrate the results for integration times of one second and one minute per frame. The total integration time (abscissa) already includes the time to spend on sky observations.
Figure 3.1: Limiting magnitude for an integration
time of one minute per individual frame
Figure 3.2: Limiting magnitude for an integration
time of one seconds per individual frame