Graph interpretation
Optical path
The diagram shows the position of the optical components of the telescope and also shows the light path of the light coming from 3 stars. One star is in the image center (light path represented in blue color) and the two other stars are in the image edges (light path represented in red and green color). Remark: the colors are chosen arbitrarily to show the different light paths. They are not representing the color of the light emitted by these 3 stars.
Spot diagram
This analysis by spot diagram shows a lot of light rays coming from 3 stars, going through the optical system of the telescope and ending in the image plane on the camera sensor (or in the eyepiece). The exact places where these rays hit the focal plane are represented by a lot of dots forming the so called spot diagram. The black circle around the spot gives the size of the airy disk. When most of the spot is within the airy disk, the image of the star is diffraction limited, or perfectly imaged by the optics.
One star is in the image center, one at 71% away from the image center and one star at the image edge. This information can be seen at the left of the graph. The field of view on the sky in degrees of half the image can be read also at the left of the graph.
The light rays from these 3 stars are analyzed in 3 different wavelengths: violet (430 nm), green (590 nm) and red (850nm). The last spots representations show a mix of the 3 colors to show a color camera can capture all colors at once in a diffraction limited way in one position of the focuser which is an average optimum for all colors. The spots of the separate colors represents the situation where the telescope is in optimal focus for that color. This is the situation when using a monochrome camera with filters and combining pictures realized in one color at the optimal focuser position for that color.