In astrophotography, you must always focus manually. You must also tell the camera you want to focus manually, because if the camera is trying to autofocus and can't do so, it will refuse to open the shutter.
On Canon cameras, the manual/autofocus switch is on the lens, but on Nikons, it is on the camera body (Figure 3.3). There may also be a switch on the lens, and if so, the two switches should be set the same way.
Astronomical photographs are hard to focus, but with a DSLR, you have a superpower - you can review your picture right after taking it. I usually focus astrophotos by taking a series of short test exposures and viewing them on the LCD at maximum magnification to see if they are sharp.
Do not trust the infinity (to) mark on the lens. Despite the manufacturer's best intentions, it is probably not perfectly accurate, and anyhow, the infinity setting will shift slightly as the lens expands or contracts with changes in temperature.
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