The deflection optics with the eyepiece receiver also accommodate the blocking filter. This filter reduces the brightness of the solar image and is therefore a very important part of the solar telescope, and which must under no circumstances be removed!
The number after the BF in the product name indicates the diameter of the blocking filter in millimetres. For example, BF10 means that the blocking filter is 10 millimeters in diameter. For purely visual observing, a small blocking filter is sufficient, but for photography a larger one should be selected.