1. to aim a weapon at s.o. or sth. : X взял Y-a на мушку - X took aim at Y
X got (had) Y (lined up) in X's sights
X drew a bead on Y.
Лейтенант скомандовал взять на прицел два окна с правой стороны дома. The lieutenant ordered his men to take aim at two windows on the right side of the house.
2. to direct one's attention to s.o. or sth. , pay special attention to s.o. or sth. : X возьмёт Y-a на прицел = X will keep an eye on Y
X will take note of Y.
Гроссмейстер заметил незаурядные способности рыжего мальчика и взял его на прицел. The grand master noticed that the red-haired boy had remarkable talent and started keeping an eye on him.
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.