Locus of stars (upper blue contours), z < 2.2 quasars (lower left green contours) and z > 2.2 quasars (lower right red contours) in the c3 vs. c1 color–color plane. The upper solid line corresponds to the color cut c3 < 1.0 − c1/3 (loose, for point sources) and the lower dashed line to c3 < 0.6 − c1/3 (strict, for extended sources).
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