How do I disable autofocus on my Nikon with a Sigma 10-20, which has no manual focus switch?

Question

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?

Answer 1

From the manual, page 33:

Choose from the following focus modes. [...] Manual focus

followed by some pictures showing how to select a specific focus mode which I'm not going to try to reproduce here, but the steps are basically:

1. Place the cursor in the information display.
2. Display focus mode options.
3. Choose a focus mode.

Answer 2

Use AE-L/AF-L button. go to Custom Setting Menu -> Controls -> Assign AE-L/AF-L button and set it to AF-ON. Now point your camera towards your object and half press the shutter button. Once focused press AE/AF-L button. focus will lock.