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The definition of the AB magnitude is:

(5) 
where c is the speed of light. From equation (11) (see
Appendix B) and after some
algebra, it can be shown that:

(6) 
where
is the
``countstoenergy''
conversion factor and
is equal to
,
i.e. the equivalent width of the filter.
We can rewrite eq 6 as follows:

(7) 
where
is the zero point defined as

(8) 
In this way, the AB magnitudes can be determined from eq. 7
where X is the airmass,
the extinction
coefficient and
the zero point reported in Table 1.
Next: Field scattering effect: a
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Juan Alcala
20020205