Correction of color

We present in details the corrections of color in appendix A . Atmospheric absorption has a weak dependence according to the wavelength, one can thus consider that the terms of color of the instruments are constant for a given instrument. Moreover, it is generally difficult to make a measurement of the terms of color with the standard star observations over only one night for lack of statistics. We will use in the continuation the nominal terms of color of each instrument.

For each point of measurement, we make a correction for the color. This correction depends on two things: terms of color and the color of the object considered.

Our corrected magnitude is written then for the magnitudes in filter I:

$$I = i + \alpha_I (R-I)$$ (8.21)

where $R-i$ the color of the object considered.

The color generally comes from the observation of the same object in various filters (in the filters R and I in our case). Here, we do not lay out, most of the time, of a measurement of the color; the images of research are only out of I and the images of follow-up were generally made in only one filter (except for the observations with the NTT which were made in R and I).

We must thus evaluate the value of the color a priori. For that, we used a simulator of curve of color giving the evolution of value (IH) according to time for our supernova

Using the lightcurves not corrected for the color, we could determine the phase of the supernova for each observation on the ground. We thus could determine the color and thus make the correction. As figure 8.31 assembles it , the variations of color over short periods are relatively weak, in particular before the maximum of luminosity. Moreover, the made corrections are in general small (about 0.02 magnitudes) us thus did not reiterate the process.

Appear: Curve of color $R-i$ of a supernova of the Ia type to a redshift of 0.55

We thus have now the lightcurves in the system standard magnitude corrected for the terms of color. The calibration comprised many stages: the choice of the primary image of calibration, correction of airmass, propagation of zero points towards the system of photometry used for the lightcurve (the coadditionnée image of the night of better seeing) and finally corrections of color.

It thus seems essential to test all the procedure to avoid the errors of calibration. For that, we used the images taken with the space telescope, for which the `` way of calibration '' is completely independent of that that we described here.