Stars should look like little dots on a star photograph, but often appear as a small luminous cross with four brilliant arms perpendicular to each other. There is a good explanation for it and it drives the astronomers to madness.
If stars are not only small dots of light, but often seen with great rays in four directions on photographs, it is due to the construction of the telescope.
Reflective telescopes are usually used for the shooting of stars, – also called newton telescopes invented by Isaac Newton in 1668.
The incoming light is first reflected by the main mirror at the back of the tubular telescope.
The telescope’s incapabilities create rays
The main mirror sends the light towards the secondary mirror hanging in the middle of the tube and diverts the light toward the camera.
The secondary mirror is typically mounted with four stiffeners that attach the mirror to the inside of the tube.
It is tracked by the four struts spreading star-stared into the special cross form. Astronomers are not happy with the rays as they can hide interesting details in the image.
Effects can be used
Not all star photography takes place through a newton telescope.
With a regular camera pointed at the night sky, the stars will look like little, luminous dots. But some astronomical photographers think the rays make the image more beautiful, so they add the effect on purpose.
This can either be done by crossing a fishing line or thin wire directly in front of the camera lens or by applying the effect through an image processing program on a computer.