...1,

Research supported by fund ex 60% D.P.R. 382/80 and F.S.E. of European Community.

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...6,

Deceased September 2000.

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...5,

On leave of absence from Dept. of Astronomy Ohio State University, Columbus, OH43210 USA.

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... (USA)

Data shared with the Columbia-Vatt collaboration.

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... conditions

We have verified that it is possible to exploit the relation (10) with ${\Phi}_{\rm curr,a}$ replaced by $\Phi_{\rm curr}$ (i.e. using non photometrically aligned images) in order to perform in one single step the photometrical alignment and the seeing correction. In the two cases we get the same final results.

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... as

In order to simplify the notation, hereafter we write $\Phi$ for the corrected superpixel flux $\hat{\Phi}_{\rm curr}$ as given in Eq. (11).

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... maximum

Evaluated starting from the excess of flux with respect to the background $\Delta\Phi_{\max}$.

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... simulations

The finite size effect is computed exactly from the formulæ by Witt & Mao (1994). For the limb darkening we use an approximation method based on Gould (1994).

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