question: How many values of r z (arcsecond) correspond with a r y (arcsecond) value of −0.247?
answer: SELECT r_z___arcsecond__ FROM table_15318324_1 WHERE r_y___arcsecond__ = "−0.247"
context: CREATE TABLE table_15318324_1 (r_z___arcsecond__ VARCHAR, r_y___arcsecond__ VARCHAR)