X‐ray–to–optical flux ratio log(f_2–10 keV/f_R) vs. X‐ray hardness ratio HR. Lower limits of log(f_2–10 keV/f_R) are plotted for sources without R‐band counterparts. Dashed lines show location of constant X‐ray–to–optical flux ratio of +1, 0, and ‐1. Dots enclosed by squares are optically extended sources, open circles enclosed by triangles are R‐band nondetected sources, which show up in redder bands (I and z^{﹩\arcmin﹩}), and stars are sources detected only in bluer bands (B_W and V).