  We performed low-resolution spectroscopy for the red giant branch stars in an
intriguing globular cluster (GC) NGC 2808, which hosts subpopulations with
extreme helium and light-element abundances. In order to trace N, C, and Ca
abundance differences among subpopulations, we measured CN, CH, and Ca II H&K
spectral indices, respectively. We identified four subpopulations (G1, G2, G3,
and G4) from CN and CH strength, with CN-weak/CH-strong G1,
CN-intermediate/CH-strong G2, CN-strong/CH-intermediate G3, and
CN-strong/CH-weak G4. Compared to [Na/O] from high-resolution spectroscopy, we
show that CN index can more clearly separate G1 and G2. Since CN traces N
abundance in a GC, it implies that G1 and G2 would show a larger difference in
[N/Fe] compared to [Na/Fe], as predicted by chemical evolution models. Later
generation stars G3 and G4, however, are better separated with high-resolution
spectroscopy. We also found that G4 shows a stronger Ca II H&K line strength
compared to that of G1, but we suspect this to be a result of unusually strong
He enhancement and/or Mg depletion in G4 of this GC. This work illustrates that
combining low- and high-resolution spectroscopic studies can improve the
separation of subpopulations in GCs.
