  Exoplanetary transit and stellar oscillation surveys require a very high
precision photometry. The instrumental noise has therefore to be minimized.
First, we perform a semi-analytical model of different noise sources. We show
that the noise due the CCD electrodes can be overcome using a Gaussian PSF
(Point Spread Function) of full width half maximum larger than 1.6 pixels. We
also find that for a PSF size of a few pixels, the photometric aperture has to
be at least 2.5 times larger than the PSF full width half maximum. Then, we
compare a front- with a back-illuminated CCD through a Monte-Carlo simulation.
Both cameras give the same results for a PSF full width half maximum larger
than 1.5 pixels. All these simulations are applied to the A STEP (Antarctica
Search for Transiting Extrasolar Planets) project. As a result, we choose a
front-illuminated camera for A STEP because of its better resolution and lower
price, and we will use a PSF larger than 1.6 pixels.
