The reduction of apparent *B*-magnitude to the RC3-system of magnitudes
with photoelectric zero-point has been studied recently
(Paturel et al. 1994). Apparent *B*-magnitudes reduced to the RC3-system
will be designated as *bt*.
Several effects were taken into account. The reduction of a given
magnitude *m* to *bt* is given by:

where *a*, *b*, *c*, *d*, *e*, *f*, , <*t*>,
, <*de*1950>
are constant values given in Paturel et al. (1994, Table 6).
The mean *bt* magnitude is calculated as a weighted mean
where the weight is derived for each source of magnitude
as the inverse square of the mean standard deviation. The final
actual uncertainty *sbt* is derived from the total weight.

The cumulative completeness curve *logN* vs. *bt* is shown in
Fig. 7 (click here). The completeness in apparent magnitude is satisfied
up to *bt*=15.5.
Apparent total magnitude *bt* is available for 76760 galaxies.
The histogram of actual uncertainties *sbt* on *bt* is given in
Fig. 8 (click here). More than 7 000 galaxies have an actual uncertainty on *bt*
smaller than 0.02 mag.

Note that apparent diameter can be roughly converted into
a magnitude *m* assuming that the mean surface brightness is constant for
all galaxies. The conversion can be made using the relationship (Di Nella
& Paturel 1994):

The standard deviation on *m* is about 0.5 mag. Using this relation,
it is possible to obtain an estimate of the apparent magnitude for
93062 galaxies, 76760 magnitudes of which come from *bt* and 16302 from
log*d*25. This magnitude *m* will be used for drawing a more general
completeness curve (Fig. 11 (click here)).

**Figure 7:** Completeness curve for apparent magnitude *bt*.
The completeness is satisfied up
to the limit *bt*=15.5

**Figure 8:** Histogram of actual uncertainty on apparent magnitudes *bt*

Apparent *bt* magnitudes are corrected for galactic extinction,
inclination and redshift effects according to the relation:

where *ag* is the galactic extinction in *B* (see Sect. 4), expressed in
magnitude, *ak* is taken from de Vaucouleurs et al. (1976, RC2 p33, rel.25),
*v* is the heliocentric velocity in (Sect. 9)
and *ai* is given by Bottinelli et al. (1995) as:

where, is taken from Simien & de Vaucouleurs (1986),
as a function of the morphological type code.
is taken from Fouqué & Paturel (1985) as seen
before (Sect. 6),
*C*=0.04 (Bottinelli et al. 1995) and .
Note that this relation has been demonstrated for spiral galaxies only.
For early type galaxies (*t*<0) we assume *ai*=0, in agreement with
de Vaucouleurs et al. (1991).

Colors are given in the *UBV* system.
They are:
total asymptotic colors *ubt* for ; *bvt* for
and effective colors (i.e. colors within the effective aperture in which
half the total *B*-flux is emitted) *ube* for ; *bve* for
.
Total asymptotic colors are corrected for galactic extinction,
inclination and redshift effects according to RC3. The corrected
colors are *bvtc* and *ubtc* for and respectively.

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