  The analysis of multiple populations (MPs) in globular clusters (GCs) has
become a forefront area of research in astronomy. Multiple red giant branches
(RGBs), subgiant branches (SGBs), and even main sequences (MSs) have now been
observed photometrically in many GCs. UV photometry has been crucial in
discovering and analyzing these MPs, but the Johnson U and the Stromgren and
Sloan u filters that have generally been used are relatively inefficient and
very sensitive to reddening and atmospheric extinction. In contrast, the
Washington C filter is much broader and redder than these competing UV filters.
Here we investigate the use of the Washington system to uncover MPs using only
a 1-meter telescope. Our analysis of the well-studied GC NGC 1851 finds that
the C filter is both very efficient and effective at detecting its previously
discovered MPs in the RGB and SGB. Remarkably, we have also detected an
intrinsically broad MS best characterized by two distinct but heavily
overlapping populations that cannot be explained by binaries, field stars, or
photometric errors. The MS distribution is in very good agreement with that
seen on the RGB, with ~30% of the stars belonging to the second population.
There is also evidence for two sequences in the red horizontal branch, but this
appears to be unrelated to the MPs in this cluster. Neither of these latter
phenomena have been observed previously in this cluster. The redder MS stars
are also more centrally concentrated than the blue MS. This is the first time
MPs in a MS have been discovered from the ground, and using only a 1-meter
telescope. The Washington system thus proves to be a very powerful tool for
investigating MPs, and holds particular promise for extragalactic objects where
photons are limited.
