Sky counts and saturation level
The first and simpler application of LIS is the estimate of the sky counts and saturation level expected at LBT-WFI, which have to be taken into account when theexposure times are evaluated.
Sky counts are shown in figure 11 (in units of ADUs per pixel) as a function of the exposure time, in several bands and for different days from new to full moon. In figure 12 the expected noise level (in the same units and for the same bands and days from new moon) is also shown. It is easy to see that even very short exposure times in the U band are already ``sky dominated''. In the I band, where the sky flux is considerably higher, typical exposure times of very few minutes (typically between 2 and 5) should be considered.
Figure 11: Sky counts (ADUs per pixel) as a function of the exposure time (sec), in several bands and for different days from new moon (from lower to upper: 0,3,7,10,15 days from new moon) . Efficiency curves adopted in the present study.
Figure 12: Standard deviation of the sky counts (ADUs per pixel) as a function ofthe exposure time, in several bands and for different days from new moon (from lower to upper: 0,3,7,10,15 days from new moon).
The saturation levels at these short exposure times are plotted in figure 13 for the V band, where the problem is most severe. It is shown that at integration times of about 5 minutes stars begin to saturate at magnitudes between 18 and 20, depending on the seeing. Although these performances are fully adequate to the scientific aims of the instrument that is intended for deep imaging well beyond this limit, they must be taken into account when preparing the flux calibration plan and require a very fast and accurate shutter for the observations of standard stars and fields.
Figure 13: Peak counts (ADUs) as a function of the exposure time, for stellar profiles of different V total magnitudes, under two different seeing conditions. Solid horizontal line shows the saturation level of present EEV CCDs.
As a conclusion, typical exposure times of few minutes (typically between 1 and 5) are to be expected when using this instrument. The extreme rapidity of this imager may pose some problem in perfoming a flux calibration using ordinary standard stars and fields, and may require a tailored calibration plan. We emphasize that these short integration times require a fast and accurate shutter, a fast CCD controller (to maximize the duty cycle) and will result in a large amount of raw data.