Manual focusing

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.

Figure 3.1. Display panel of a Canon XTi (400D) set up for deep-sky photography. F00 means the lens is not electronic and the camera cannot determine the f -stop. C.Fn means custom functions are set.
Figure 3.2. For manual control of shutter and aperture, set mode dial to M.

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.

Figure 3.3. Manual/autofocus switch is on camera body, lens, or both.

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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