Detection

It is necessary to confront vague ideas with clear images.

Jean-Luc Godard

The search for supernovæ is a discipline which was artisanal a long time. Until in the Eighties, it was done by comparing photographic plates with the eye. This art required a drive and a patience out of the commun run. One of the pioneers, Fritz Zwicky, discovered during its life, using a simple camera assembled on a telescope with great angle with the Palomar go up, 122 supernovæ among the 281 discovered within the framework of the project that it initiated.

In 1988, only 600 supernovæ had been observed.

It is the progressive replacement of the photographic plates by CCDs cameras which allowed the automation of research and thus to lead to many discoveries. Indeed, the digitalization of the images allows the subtraction pixel pixel of images taken at different periods. The residuals on the subtraction correspond to objects whose luminosity varied. In addition to the automation of detection using average data processing, this technique makes it possible to detect the supernovæ drowned in the light of their galaxy host.

However, until in the middle of the Nineties, the telescopes equipped with CCD covered angular surfaces on the sky much less significant than than allowed it the photographic plates. It was thus necessary to multiply pointed to lead to equivalent covered surfaces.

The appearance of the first large mosaics of CCDs made it possible at the beginning of years 1990 to reach sufficiently significant surfaces to allow systematic research programmes of supernovæ.

A second significant aspect in research is the depth of the images: long integrations allow the discovery of supernovæ weak and very remote (with great redshift). The installation of large mosaics on large telescopes (4 meters and more), made it possible to discover supernovæ until redshifts around 1 (which corresponds to a supernova which exploded there is approximately 10 billion years).

In the continuation of this chapter, we describe how discovered and the supernovæ thanks to these large telescopes are studied. In particular, we detail the various strategies of observations which make it possible to optimize the discoveries. Then we explain the image processing from the observation to detection.

According to detection, the spectroscopic follow-up on the large telescopes of the class of the 8m of the objects discovered makes it possible to confirm their nature (supernova, AGN...), their type for the supernovæ and finally to measure their redshift.

Finally the most interesting supernovæ of the Ia type (great redshifts, discovered in an early phase of their evolution) are selected and followed photométriquement during several months.