In astrophotography focusing is always important - just how important depends on a number of factors including the photographer's own perception and requirements, the resolution of the telescope or camera lens, and even the size of pixels in the camera's sensor. However, achieving critical focus is arguably more important in this field of photography than in any other. A small error can lead to whole images having a distinctly fuzzy appearance.
Focusing a camera for astrophotography is far from straightforward. The focusing screens in DSLRs are designed to work with relatively high levels of light, such as normal daylight. However astrophotographers have to work with a far lower light level. In cameras with interchangeable focusing screens, the first obvious approach is consequently to obtain a brighter focusing screen more suited to very low light levels.
Modern digital cameras largely solve the normal focusing problems for us with sophisticated autofocus systems but these systems do not help, and in some respects make matters worse, when working with a telescope. Lenses designed for use with autofocus typically focus beyond infinity to prevent the focus-seeking system from hitting its mechanical limit every time it approaches infinity. Consequently, having attached a camera to a telescope, the photographer is unable simply to turn the focusing control to its limit to achieve focus at infinity. Focusing by the unaided eye is difficult and often inaccurate. Another very precise system must be devised. Focusing may also be physically inconvenient with some types of telescope because the camera's viewfinder may be directed towards the ground when the instrument is angled upwards.
Normal DSLR lenses are focused with the aperture diaphragm wide open to give a bright image. Working with the aperture closed, for example at f/16, is much more difficult because the image is much less bright and hence less clearly defined. All modern SLR-type cameras therefore close the aperture diaphragm to the preset position for correct exposure only when the shutter release is pressed and the instant-return mirror rises. When the exposure has been completed, the mirror flips back and the aperture diaphragm returns to its maximum diameter.
Small apertures provide increased depth of field and tend to conceal any small focusing errors. Most of the depth in a typical terrestrial image is rendered acceptably sharp, and if the focus at infinity is less than perfect only the most perceptive eye is likely to notice. In the case of astrophotography, all subjects are at infinity and must be rendered absolutely sharp.
Another problem affecting focusing is that most telescopes have a curved rather than flat focal plane. An image may consequently be sharp at the centre but off-focus towrds its edges. Film planes could be curved to accommodate such characteristics, bur sensors cannot. Optical field flatteners to correct this problem are available, but must be matched to the telescope and positioned precisely the right distance from the sensor.