Just purchased a Sigma 10-20 f/4-5.6 DC HSM lens. In attempting to capture astrophotography, the lens continuously tries to find a focal point in the sky, and doesn't. So I attempt to turn it to manual focus, and discover that there is no manual focus switch on the lens itself, it being a full-time manual/auto hybrid lens (the Canon mount and the Sigma mount have the af/m switch, not the case with Nikon 5200 [my camera type]). More on the lens here.
So I'm wondering if there is a setting within my camera or a method I can use to get my camera to stop focusing into infinity and allow me to take the shot.
How might this be accomplished?