How to detect an object that comes in the "Crosshair" field of view?

I’m making an auto turret firing game. It will detect the enemy object seen in the field of crosshair image that I’m using. And it will start firing when the object aligns with the midpoint of the crosshair. Right now I’ve made shooting possible when the enemy objects aligns through the midpoint of crosshair from any distance. But I’m unable to detect any object coming in the field of crosshair.
For shooting I’m using raycast but for detection I’ve tried all method like colliders triggers and raycast. The problem is it will not exactly detect an enemy object, seen or coming in the field of crosshair view.
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How about using Physics.OverlapBox extending from the crosshair to broadly check “is there something within the broad area of the crosshair”, and if that’s the case, then using Physics.OverlapSphere at the intersection position to check if the object is touching exactly the circle of the crosshair?