CCD $UBV(RI)_{C}$ Photometry of Twenty Open Clusters
  Fundamental astrophysical parameters have been derived for 20 open clusters
(O\!Cs) using CCD~$U\!BV\!(RI)_C$ photometric data observed with the 84~cm
telescope at the San Pedro M\'artir National Astronomical Observatory,
M\'exico.
  The interstellar reddenings, metallicities, distances, and ages have been
compared to the literature values. Significant differences are usually due to
the usage of diverse empirical calibrations and differing assumptions, such as
concerning cluster metallicity, as well as distinct isochrones which correspond
to differing element-abundance ratios, internal stellar physics, and
photometric systems. Different interstellar reddenings, as well as varying
reduction and cluster-membership techniques, are also responsible for these
kinds of systematic differences and errors.
  The morphological ages, which are derived from the morphological indices
($\delta V$ and $\delta 1$) in the CM diagrams, are in good agreement with the
isochrone ages of 12 O\!Cs, those with good red clump (RC) and red giant (RG)
star candidates. No metal abundance gradient is detected for the range $6.82
\leq R_{GC} \leq 15.37$ kpc, nor any correlation between the cluster ages and
metal abundances for these 20 O\!Cs.
  Young, metal-poor O\!Cs, observed here in the third Galactic quadrant, may be
associated with stellar over-densities, such as that in Canis Major (Martin et
al.) and the Monoceros Ring (Newberg et al.), or signatures of past accretion
events, as discussed by Yong et al. and Carraro et al.
