It s an iterative process that involves gradually refining focus until you ve got the best possible focus. Figure 5a-b shows typical star images that are well out of focus. The size of the center dark area will vary with the size of the secondary mirror in the telescope.
TIP Focusing by eye is a challenge.
Always choose a very bright star to do your visual focusing. Exactly how bright a star to choose depends on many factors, however the aperture of your telescope, the sensitivity of your camera, the seeing conditions, etc. You want the brightest star that will not saturate your camera. Examples of very out of focus star images. Figure 6a-c shows a succession of star images, ranging from moderately out of focus at left to accurately focused on the right.