Items zeros of the images of primary calibration

Items zeros are expressed for the fluxes measured either in a number of blow of ADU or ADU a second (as in the case of the CFHT), or in a number of photoelectrons a second (as in the case of the CTIO). We thus propagated zero point for each image of primary calibration by adding a term depend on the duration of the image:

$$zero_{image} = zero_{instrument} +2.5\log_{10}(t_{expo})$$ (8.17)

The images observed with the CTIO were reduced same manner as the standard star images. Item zero for these images is thus usable just as it is. On the other hand, zero points provided by the CFHT and ELIXIR are expressed for fluxes in ADU and are standardized with a CCD of mosaic (the CCD4). This standardization is carried out at the time of the quantum correction of effectiveness. In this procedure, the level of the quantum chart of effectiveness this CCD is fixed at 1, one applies same standardization to other CCDs which will be thus expressed in the same photometric system.

All the images resulting from observations CFHT were corrected for the quantum effectiveness individually. In particular, each chart of quantum effectiveness was standardized to 1, fluxes of different CCDs are thus not expressed in the same photometric system .

A subtle point of the calibration is the source of the charts of quantum effectiveness: we built them starting from the images of science by using the bottom to ciel(voir chapter 7 ). The calibration ELIXIR was made by using charts of quantum effectiveness built starting from images of the zenith taken in the twilight or the dawn which contain primarily diffused sunlight (thus enriched in blue compared to the solar spectrum). As we saw previously, these two types of quantum chart of effectiveness correspond to very different spectra: bottom of sky whose principal contributions are in the red part of the band considered and primarily the sun whose principal contributions are in the blue part of the spectrum. To caricature, these two sources of calibration concentrate in extreme cases two extreme of the same filter.

Figure 7.7 presents the distribution of the basic reports/ratios of sky for the CCD 4 and 10. The funds of sky were calculated on the rough images and are in theory a direct measurement of the quantum report/ratio of effectiveness (under the terms of gains near). The histogram presents two populations, they correspond respectively to the images of science and the images of twilight zenith. The reports/ratios of quantum effectiveness between the two CCDs are thus not constant according to the wavelength: the CCD10 sees its quantum effectiveness decreasing more quickly in the red than the CCD 4.

It is thus not possible to use the reports/ratios of quantum effectiveness to express zero points ELIXIR in our photometric system.