In order to carry out an exhaustive analysis of the NGC 2422
cluster, all 
photometric data available must be included in the
study. There are two other sources of 
photometric data which
have been already mentioned: Nissen (1988) and Shobbrook
(1984). A comparison between the different measurements was
necessary and desirable. To begin with, a homogenization of the
identification had to be done since each previous observer lists the
observations carried out with different identification numbers. The
observations by Nissen were given in x and y coordinates from the map of
Hoag et al. (1961); Shobbrook (1984), on the
other hand, gives his data in van Shewick's and Smyth & Nandy's numbers;
Smyth & Nandy (1962) report their own identification chart.
Hence, in view of this chaos, we have constructed a cross identification
table (Table 4 (click here)) in which all the observations with 
photometry and their different nomenclature have been summarized.
Nissen (1988) was looking for the metal content determination for different clusters; hence he observed only late type stars, much fainter than those measured here or by Shobbrook (1984). Therefore there was no overlap between these sets.
Table 4: Nomenclature of the stars observed in 
A detailed discussion of the comparison to Shobbrook's (1984)
data is necessary. To begin with, the intersection of both sets is
seventeen stars, a significant number given the size of the sets
(about 30 stars in each one). A comparison of the magnitudes and the
colour indexes of both sets was carried out for the stars in the
intersection, with the idea of taking mean values for these stars. The
following findings were obtained: for V, 
, 
and 
the
relationship was linear with correlation coefficients on the order of
0.9 and slopes close to 45 degrees. However, the relationship for the
index b-y, showed an abrupt jump; a difference between the sets was
found since the numerical values obtained by Shobbrook (1984)
were systematically lower than those obtained here. As a first approach to
confirming which set was correct, we compared the goodness of our
transformation of the photometric values for our standard stars with
those of Crawford (1970) in all the colour indexes and, in each
case, the correlation coefficients 
were better than 0.997.
We carried out further comparisons of the photometric data from Shobbrook (1984) and the present paper to the previous photometric data of NGC 2422, both photographic (van Schewick 1966; Smyth & Nandy 1962) and photoelectric (Hoag et al. 1961, and the compilation of Mermilliod 1976). The following was found:
A comparison of Shobbrook's V magnitudes with those of
Mermilliod's (1976) and Smyth & Nandy's (1962)
gave excellent linear relations. Correlation coefficients were of 
equal to 0.999 and 0.853, respectively. Problems arose, however, when
the b-y colour index of Shobbrook was compared to the B-V values of
Mermilliod (1976) and Smyth & Nandy (1962). In
each case the correlation coefficients were rather poor, 
equal to
0.647 and to 0.837, respectively. More puzzling is the fact that the
curve that best adjusts to the b-y colour index of Shobbrook
(1984) and the B-V index of S & N is not a straight line, but a
quadratic fit. In fact, the quadratic adjustment gives an 
of 0.866,
numerically better than in the linear case.
Table 5: Compilation of the photometry of the stars with large scatter
in V
In view of this, a B-V versus b-y plot was constructed from the
standard stars that appear in the American Ephemeris and Nautical
Almanac with data in both colour indexes. In this sample a set of 45
stars within a range in b-y between -0.25 and 1.5 is found. A linear
regression with a correlation coefficient 
of 0.9953 was obtained. A
direct comparison of the line obtained from Shobbrook's data to S & N's
was done and it became immediately evident that the slopes were
totally different and that the b-y interval of Shobbrook's data is
very narrow. On the other hand, the comparison of our data to that of
S & N gave a straight line parallel and basically equal to that of the
standard stars; our b-y coverage is much broader, from -0.2 to 1.3.
Hence, since the standard stars of Shobbrook (1984) were
chosen in a similar manner as the observed stars, i.e. with a very narrow
range in b-y, this might be the cause of the differences of the b-y
values in his photometry from all the other photometric sets on NGC 2422.
In view of the systematic difference of his data with all other data
sets, a linear regression with our data was established as 
and all the stars measured by
Shobbrook
(1984) were transformed in the b-y index according to the previous
relation into our system, which, as has been previously proved, is
equivalent to the standard system of Crawford (1970). Finally,
mean values for the stars observed in common were obtained for magnitude
and all the colour indexes. The correctness of the criteria of
considering mean values is established by the fact that the mean
values of the differences between Shobbrook's data and the present
paper's are -0.001,-0.003,-0.004,-0.005, and 0.005 in V,
b-y, 
, 
and 
, respectively. However, there are a few stars, particularly in
V, which show large differences. A brief review of the values in
different photometric compilations, presented in Table 5 (click here), supports the
evidence that the scatter is large and might be due to intrinsic
variability of these stars. Another example of possible variability is that of star 28 whose large difference of 0.1 mag in 
indicates a variable, such as Be star.
The results presented in Table 6 (click here) are the values obtained
from Nissen (1988), Shobbrook (1984), the mean
values of Shobbrook (1984) and our photometry for those stars
observed in common once the b-y values of the former were translated to
our (Crawford's 1970) system and those obtained in the present
research. In Table 6 (click here) the sequence number of the observed stars in
decreasing 
is listed in Col. 1; the remaining columns list the
photometric values obtained; the last column lists the source of the
photometry reported.
Figure 1: Histogram of the distance of the stars in the direction of
NGC 2422
