Making something invisible but have it shown when looking through special glass

HI, I’m looking to make a game where there are several objects that are not visible and have no collision, but when the player brings up a “seeing window” the object is revealed just within the window.

What part of unreal would I have to start modifying?
Is this a post processing event?
is this a special material that needs to be made?

You could render via SceneCapture2D and map the render target on your window. You either render the complete scene including your invisible objects or use a showonly list. To hide the invisible objects from your camera you could set the object property “Only Owner see” and make the scene capture the owner.

Sphere Mask would do it: