Night sky is not a friend of autofocus. The good news is that there are several strategies that have been proven to work. The first is to focus at infinity before dark, tape your focus ring in place, and switch to manual focus. The second is to use Live View, along with the zoom function to focus (manually, again). Both Jupiter and Saturn are in the eastern sky after sunset and provide very good targets to focus on. Third (and I'm not kidding on this one) is to take a couple of kids with you. Give them flashlights and send them out 300 or 400 feet or so and focus on them. That distance will get gou very close to focusing at infinity.

My favorite is the second choice, although it is surprising how much light is available for manual focusing much later than we think, especially if you are using a reasonably fast lens.

The back button will not activate focus if you are in manual mode.

Good luck.

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These are good things to try but one thing I would like to mention is that "Manual Mode" has nothing to do with focus. Manual mode means you set the shutter speed, you set the ISO and the aperture. When you put the camera in "Manual Focus" it is not a camera mode, it is simply turning on auto or off auto focus. If you are in "Manual Focus" the camera will not pay attention to the status of the focus and when you press the shutter button it will simply take the picture. Even in Manual focus most cameras will show the status of focus in the view finder or if using live view in the image. Live view and a remote shutter release (or use time delay) are practically requirements for good Astro-photography.

Night sky is not a friend of autofocus. The good news is that there are several strategies that have been proven to work. The first is to focus at infinity before dark, tape your focus ring in place, and switch to manual focus. The second is to use Live View, along with the zoom function to focus (manually, again). Both Jupiter and Saturn are in the eastern sky after sunset and provide very good targets to focus on. Third (and I'm not kidding on this one) is to take a couple of kids with you. Give them flashlights and send them out 300 or 400 feet or so and focus on them. That distance will get gou very close to focusing at infinity.

My favorite is the second choice, although it is surprising how much light is available for manual focusing much later than we think, especially if you are using a reasonably fast lens.

The back button will not activate focus if you are in manual mode.

Good luck.