Control calibration

To check the calibration, we used stars present in the images of follow-up of the HST. Planetary Camera being in general too reduced to contain more one or two stars, we used the images of the three other CCDs: WFC1, WFC2 and WFC3.

We thus selected stars of fields common to the space images and the images on the ground.

Photometry and the calibration were carried out using H stphot) (dolphin2000b). This software makes it possible starting from the images of the WFPC2 to make the photometry and the calibration of stars. This software in particular made it possible to carry out the calibration the most recent measurements of zero points and the most recent terms of color of the camera (dolphin2000a.

Photometry on the ground has as for it was made by SExtractor by using the calibration which we determined.

We controlled the calibration for the fields of both supernovæ for which we have good observations and a good calibration on the ground at the same time in R and I: 2001go and 2001gq.

For these two supernovæ, we determined the colors observed ($R-i$) of the objects selected (see the charts of field from these two supernovæ in the appendix B), then we applied the corrections of color to measurements made on the ground and with camera WFPC2.

Approximately half of the selected objects was detected by hstphot bringing the statistics to a half-dozen of common objects per field. The dispersion magnitudes for the same objects is about 0.05 magnitudes even if the announced errors are generally below 0.01 magnitudes. The precision of the comparison will thus be limited to this value.

Moreover, one certain number of objects presents significant variations, in particular, the objects close to saturation.

Finally, only some objects could be used for the comparison. We checked that the magnitudes of the same objects were comparable on the ground and for the WFPC2.

Table 8.1 presents the comparison magnitudes obtained for 4 stars of the fields of 2001gq.

Table 8.1: Comparison the magnitudes R and I of stars of fields for 2001gq, the magnitudes were given by using hstphot 1.1 (dolphin2000b) for the images WFPC2 and SExtractor for images CFHT

Star	R(CFHT)	I(CFHT)	R(WFPC2)	I(WFPC2)
With	20.43	20.13	20.43	20.12
D	20.38	19.18	20.48	19.24
E	20.92	19.07	20.97	19.22
G	23.08	21.92	23.08	21.91