  Nowadays, astronomers want to observe gaps in exozodiacal disks to confirm
the presence of exoplanets, or even make actual images of these companions.
Four hundred and fifty years ago, Jean-Dominique Cassini did a similar study on
a closer object: Saturn. After joining the newly created Observatoire de Paris
in 1671, he discovered 4 of Saturn's satellites (Iapetus, Rhea, Tethys and
Dione), and also the gap in its rings. He made these discoveries observing
through the best optics at the time, made in Italy by famous opticians like
Giuseppe Campani or Eustachio Divini. But was he really able to observe this
black line in Saturn's rings? That is what a team of optical scientists from
Observatoire de Paris - LESIA with the help of Onera and Institut d'Optique
tried to find out, analyzing the lenses used by Cassini, and still preserved in
the collection of the observatory. The main difficulty was that even if the
lenses have diameters between 84 and 239 mm, the focal lengths are between 6
and 50 m, more than the focal lengths of the primary mirrors of future ELTs.
The analysis shows that the lenses have an exceptionally good quality, with a
wavefront error of approximately 50 nm rms and 200 nm peak-to-valley, leading
to Strehl ratios higher than 0.8. Taking into account the chromaticity of the
glass, the wavefront quality and atmospheric turbulence, reconstructions of his
observations tend to show that he was actually able to see the division named
after him.
