There are many designs of telescopes and they can be created with the use of lenses or curved mirrors. In telescopes created with curved mirrors a larger mirror called the primary mirror captures light from the star that is being viewed and focuses it at the focal plane. A film or digital sensor can be placed directly at the focal point of the mirror to record the image. But doing so can block a large amount of light from reaching the primary mirror.
A way of working around this problem is by using a secondary mirror that reflects the light from the primary mirror to where the sensor is placed. But this comes with its own problems; the secondary mirrors block some the light from the primary mirror which reduces clarity and the supporting structures result in diffraction spikes. The secondary mirror in a telescope is held in place by what are known as spider vanes. Large telescopes have a design that uses a 4 vane spider. It is the diffraction effects of these spider vanes that leads to the formation of a cross in images that are captured of relatively bright stars.