2 in B, V,
R and I respectively. We used published
aperture photometry data compiled by Prugniel & Heraudeau ([1998]) to test the accuracy of
our values. Synthetic aperture photometry was carried out
on the galaxies for which aperture photometry data exists in the literature and the results of the
comparison are presented in Table 2. Our values agree very well with the values derived by other workers for most of the galaxies
confirming the accuracy of our photometry. Our values for Mrk 14 and Mrk 87
are brighter as compared to the values obtained by Huchra. In case of Mrk 87,
the difference is nearly the same within the limits of the errors and is
most probably a result of an error in the determination of the zero-point for this observation. Mrk 213, Mrk 781 and Mrk 1379 were observed on the same night as Mrk 14. If the discrepancy in our values and those of Huchra would have been the
result of an error in the zero-point determined from the transformations,
the same systematic error would also have appeared for the other galaxies observed on the same night. However, no such effect is seen. Secondly, our values are brighter than those quoted in
the literature which rules out inadequate exposure time
as a cause of the discrepancy. Therefore, the difference appears to be
genuine. One probable explanation for this could be a variability in the
output luminosity of this source. Optical variability may be expected in young
starbursts due to the appearance of supernovae (Terlevich [1992]).
In CCD photometry, the center of the galaxy can be determined with an accuracy of about
one-tenth of an arcsec, while it is not possible to do so in aperture
photometry. Small centering errors can also give rise to a discrepancy between
the magnitudes derived from CCD and from aperture photometry, especially for small apertures.
Copyright The European Southern Observatory (ESO)
