Diagnostics for cleaning the RED580 sample using S/N and log (χ²): a) HR diagram colour coded to four subsamples; b) T_eff vs. [M/H] showing four subsamples; c) Cleaned HR diagram; d) Cleaned T_eff vs. [M/H]; e) S/N vs. log (χ²) for metal-poor giant subsample (blue *); f) S/N vs. log (χ²) for metal-rich giant subsample (magenta Λ); g) S/N vs. log (χ²) for hot dwarf subsample (green +); h) S/N vs. log (χ²) for cool dwarf subsample (red x). Rejections are cyan o and final accepted spectra are black •.