What should a star look like when perfectly in focus?

Because light is a wave, when light from a star passes through a circular aperture (like a telescope lens) it diffracts -- light waves scraping the sides of the aperture spread out and interfere with each other. Rather than the telescope lens focusing the star to a perfect pinpoint, the best the lens can do is focus the light into a small disk ( called an airy disk) surrounded by faint diffraction rings like the image to the right. The airy disk can be made smaller (more pin-point-like) by using a telescope with a larger diameter objective lens or mirror but...

What does a star actually look like?

But in the real world there are effects that preclude seeing a perfect diffraction-limited airy disk. Some you can't control, some you can.

• What affects focus that I can’t and can control?