Many papers were devoted to the study of diameters and specially to the
reduction to the standard system defined by the isophote at the
brightness of 25 B-
.
The conclusion of these studies was published by Paturel et al. (1991).
The diameters are expressed to 0.01' in log of 0.1' according to the convention of Second Reference Catalog (de Vaucouleurs et al. 1976). They are designated as logd25. For instance a diameter of 10' will be given as logd25=2.00. Axis ratios are expressed in log of the ratio of the major axis to the minor axis. They are designated as logr25.
The main catalogs are reduced to the 
-standard system using a relationship
where D is the diameter and R is the ratio of the major axis
to the minor axis in
a given catalog. The constants a, b and a' are given in
Paturel et al. (1991,
Tables 1a and 1b) for the most common catalogs.
Diameters and axis ratios extracted from LEDA images or from COSMOS database
were converted into the standard system using the same relationships but with
different coefficients (Paturel et al. 1996; Garnier et al. 1996;
Rousseau et al. 1996).
The completeness curve logN vs. the limiting logd25 is shown in
Fig. 3 (click here). The completeness is satisfied down
to the limit log
(i.e. 0.8' in diameter).
Diameters logd25 are available for 82033 galaxies.
The histogram of actual uncertainty slogd25 on apparent diameter logd25
is given in Fig. 4 (click here). More than 13 000 galaxies have a diameter with
an actual uncertainty smaller than 0.05 (in logd25).
The distribution of logarithms of axis ratios is shown in Fig. 5 (click here).
This distribution is close to the one expected if the orientation
of galaxies is randomly distributed.
Figure 3: Completeness curve for logd25. The completeness is satisfied down
to the limit log
(i.e. 0.8' in diameter)
Figure 4: Histogram of actual uncertainty slogd25 on apparent diameter
logd25
The position angle of the major axis is noted PA. It is
counted from North towards East, between 
and 
and
is almost randomly distributed (Fig. 6 (click here)). A small excess of
galaxies appears at PA = 
and PA = 
which seems to be
an artifact.
Figure 5: Histogram of log of axis ratio logr25. The solid
curve shows the distribution of logr25 for random orientation
Figure 6: Distribution of major axis position angles
In RC2 apparent diameters were corrected for galactic extinction and inclination effect according to Heidmann & de Vaucouleurs (1972a,b,c). Recently, this question was revisited after the result by Valentijn (1990, 1994) that galaxy disks are opaque. Our conclusion (Bottinelli et al. 1995) leads to the following correction:
where C=0.04, ag is the galactic extinction (see section about coordinates)
and 
is given by Fouqué &
Paturel (1985) as 0.094 for spiral galaxies and 0.081-0.016.t for
early type galaxies with morphological type code t<0.