Once the candidates selected for the identification, we determine starting from astrometrical catalogues USNO the transformations which make it possible to pass from the co-ordinates on the image to the co-ordinates on the sky. This stage then enables us to build an astrometrical chart which will be used during the spectroscopic observations to align the spectrograph on the candidate.
In practice, pointed telescopes are not sufficiently precise to allow a direct alignment, one thus has recourse to pilot stars in the field, more brilliant than the object (of the order magnitude 19-20 for a telescope of 10m) which one calculates the shifts compared to the candidate on the images. At the time of the spectroscopic observations, the telescope is brought on one of these stars guides, visible in the camera of guidance, then one shifts the telescope towards the position of the supernova.
The spectra of these objects make it possible to determine their nature like their type for the supernovæ. These measurements also make it possible to make precise measurements of the spectral shifts of the supernova. If the subjacent galaxy is sufficiently brilliant, the redshift is estimated on the emission lines of the galaxy which are narrow and make it possible to reach precise details of the order of the thousandths. If the lines of the galaxy are too weak, the redshift is estimated on the lines of the supernova which are broader, the precision to the measure is then of the order of the hundredth.