The expected limiting magnitudes for stellar objects at LBT-WFI. For comparison, faint objects with extended exponential profiles have limiting magnitiudes about 0.4 mag brighter.

The results of the simulation that we outlined in the previous sections can be summarized in the following statements:

  • The large collecting area of the primary mirror and the extreme efficiency of the optical and electronic components of this instrument combine to produce one of the faster imager worldwide, surpassing by far the capabilities of any other instrument available (or planned) to the Italian community;

  • The expected limiting magnitudes are shown as a function of time in figure 22, in four different bands and for two seeing conditions. It is shown that an exposure time of 12 min is sufficient to reach stellar Johnson magnitudes U=25.5, B=27, V=26.8, I=25 at s/n=5 with a seeing of 0.6 arcsec;

  • The optical quality of the field corrector and the pixel sampling are adequate to prevent source confusion at the faintest magnitudes (V=27) even in relatively short exposure times (1hr), ensuring a good statistical completeness in the counts. More accurate simulations including ghosts and other optical defects are needed to assess the limits of the instrument with long exposure times.

  • The pixel sampling cannot be increased without degrading the performances at the faint magnitudes. With a scale of 0.26''/pxl, the instrument is able to resolve adequately the instrumental PSF under normal oprational conditions. Since most extragalactic objects are compact at V>24 (the typical half-light radius of faint galaxies is 0.2''-0.3''), this is the largest pixel scale allowable. As a result, chips with different pixel size or wider format arrays - i.e. extending over 15' of radius from the optical center - cannot be considered as valid alternatives.

  • With these performances, the LBT wide field imager is ideally suited to address all the scientific items that need multicolor information over large areas down to limiting magnitudes at or beyond the spectroscopic limit. It appears to fit the requirements of several proposed studies that either aim at selecting rare objects with peculiar colors (e.g. Lyman-break galaxies that require very deep U or B exposures) or at studying the multicolor properties of the objects beyond the spectrographs reach. Both these observational strategies are not feasible with wide field imagers on 2.5m class telescopes.