Why a coronograph?

We plan to image Venus when it is close to the solar limb, around the transit event occuring on June 5-6, 2012, when twilight phenomena become visible. The use of coronagraphs will enable us to obtain the best possible contrast between the faint features shown by the atmosphere ofthe planet and the bright background in the Sun proximity.

In 2004, the aureole - essentially due to refracted light in Venus atmosphere - was imaged by modern tecniques for the first time (both from the ground and from space), and a refraction model was shown to be able to explain its main properties.

Requirements

The following properties that the instrument should have were identified:

• The image of the coronagraph occulting mask coincides as close as possible with the limb of the Sun, for exploring the region where Venus aureole reaches the photosphere.
• The imaging system must be supported by very good equatorial mounts, for accurately positioning the image of the Sun relative to the occulter, and for keeping it fixed during the observation.
• The optimum scale of the image is around 0.5 arcsec/pixel, and anyway not larger than 1.5 arcsec/pixel, for a confortable analysis.
• The instrument allows centering the field of view close to the solar limb where Venus will be located, i.e. ~15 arcmin from the Sun's center. Further decentering of the field of view (up to 1 Solar diameter or more) should be possible for studying the scattered light ring.
• Each coronagraph is equipped with one filter.
• None of the filters corresponds to high contrast features on the solar disk which could produce a detectable signature on the aureole, introducing variations that are not related to atmospheric structures. This includes chromospheric structures, prominences etc. All this features should be suppressed as much as possible.
• The imaging system (filters, instrument, camera) has to provide a good SNR on the wide brightness range involved (see Scientific Background). Camera dynamics and filter densities are important in this respect.
• Since the light flux remains important, short exposures will be adopted. The signal will thus be dominated by photon noise and by the read-out noise of the cameras.