M - shaped YGM series鱼骨板 (Fishbone Plate)
Metal structure, brass screws, anti - slip integral rubber sleeve, 1.5MM thickened.
It is used to connect a DSLR camera with an astronomical telescope for quickly focusing on stars. 
 A fishbone plate is equivalent to a set of man - made "diffraction gratings". When light passes through the fishbone plate, it will diffract in 3 different directions, thus producing 3 pairs of "starbursts". As the telescope's focus changes, the starburst pattern will also move until the positions of the 3 peaks are aligned, which indicates that the star has been focused to the appropriate position.
Basically, it is made by cutting a specific pattern on an opaque disk and is installed at the camera lens or the objective lens end of a telescope.
These hollowed - out patterns make them produce a diffraction pattern with 3 peaks when aiming at a point light source. When using it, find relatively bright stars in the sky to increase the visibility of the pattern. As the focus is adjusted, the center line moves left and right at the center of X, and when the center line is completely centered, focusing is achieved.